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000-634 - Object Oriented Analysis and Design - Part 2 - Dump Information

Vendor : IBM
Exam Code : 000-634
Exam Name : Object Oriented Analysis and Design - Part 2
Questions and Answers : 72 Q & A
Updated On : November 16, 2018
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000-634 Questions and Answers

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000-634 Object Oriented Analysis and Design - Part 2

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000-634 exam Dumps Source : Object Oriented Analysis and Design - Part 2

Test Code : 000-634
Test Name : Object Oriented Analysis and Design - Part 2
Vendor Name : IBM
Q&A : 72 Real Questions

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IBM IBM Object Oriented Analysis

Analyst firm Positions IBM in Leaders Quadrant of Magic Quadrant file | killexams.com Real Questions and Pass4sure dumps

source: IBM

July 12, 2006 08:00 ET

SOMERS, ny -- (MARKET WIRE) -- July 12, 2006 -- IBM nowadays introduced that Gartner, Inc. has positioned IBM within the leaders quadrant in its Magic Quadrant document of the thing Oriented evaluation and Design equipment space. in keeping with the document*, Gartner estimates that IBM has greater than 50 percent of the market share versus its two nearest competitors who mixed hold 30 % or extra of the market.

model-driven development helps software construction groups have in mind, doc and talk the enterprise manner of application and systems construction to demonstrate architecture resilience just before making full scale building investments, and to define a carrier oriented structure roadmap resulting in business transformation.

IBM's management within the Object Oriented evaluation and Design (OOA&D) equipment marketplace for 2H06-2H07 is the outcome of a finished portfolio of offerings which aid organizations use fashions or patterns to drive their application building, together with:

-- IBM Rational utility Modeler, IBM Rational application Architect, and IBM Rational systems Developer -- IBM's award-profitable mannequin-pushed building tools, based on Eclipse, to support development groups create potent purposes; -- endured guide of Microsoft environments through IBM Rational Rose demonstrates IBM's dedication to presenting a application development platform that goals a wide array of implementation technologies; -- IBM WebSphere business Modeler -- the outcome of IBM's acquisition of Holosofx -- helps enterprise system analysis to shut the gap between an firm's traces of enterprise and their construction corporation's figuring out of the enterprise drivers; -- IBM Rational facts Architect -- an commercial enterprise data modeling and database design tool that also helps clients map information belongings to every different to greater readily create database and integration schemas. "Gartner's assessment of the OOA&D positions IBM within the leaders quadrant which we believe confirms our method round mannequin-pushed construction," noted Danny Sabbah, time-honored supervisor, Rational software, IBM. "IBM's model-driven equipment support geographically distributed utility building groups to talk their designs and requirements visually, casting off the risk of challenge failures as a result of mis-conversation resulting from language and cultural boundaries."

Gartner's Magic Quadrant positions companies alongside two dimensions: completeness of imaginative and prescient, and potential to execute on that vision. For 2006, Object Oriented analysis and Design equipment are smartly into mainstream use attaining 20 p.c to 50 percent of the target viewers: information architects, analysts and builders. in keeping with the file, most agencies have converted to OOA&D strategies and tools in concert with implementing carrier oriented architectures (SOAs), resulting in a speedy explosion of this market from 2000-2004.

modern-day information builds on IBM's management in assisting shoppers govern their utility and systems construction. IBM turned into these days named the worldwide market share chief in the application development and assignment and portfolio management utility market according to total application income for 2005 for the fifth consecutive yr, according to unbiased analyst firm Gartner Inc. based on the impartial record**, IBM become the main market share vendor in complete software profits with 25.four p.c market share, more than double the percentage of its nearest competitor.

For more information, discuss with http://www.ibm.com/software/rational.

concerning the Gartner Magic Quadrant

The Magic Quadrant is copyrighted June 1, 2006 via Gartner, Inc. and is reused with permission. The Magic Quadrant is a graphical representation of a market at and for a selected time duration. It depicts Gartner's analysis of how certain vendors measure in opposition t criteria for that market, as defined with the aid of Gartner. Gartner doesn't propose any dealer, product or carrier depicted within the Magic Quadrant, and doesn't suggest know-how clients to select simplest these vendors placed within the "Leaders" quadrant. The Magic Quadrant is supposed entirely as a analysis tool, and isn't intended to be a particular book to action. Gartner disclaims all warranties, categorical or implied, with recognize to this research, including any warranties of merchantability or health for a specific aim.

* The Gartner Magic Quadrant for Object Oriented evaluation and Design (OOA&D) tools, 2H06-2H07 record changed into authored by means of Michael J. Blechar.

** The Gartner market data record is entitled "Market Share: application construction and venture and Portfolio administration, international, 2005" authored through Laurie Wurster and Fabrizio Biscotti.


net provider market analysis for 2018 purchasable in new document | killexams.com Real Questions and Pass4sure dumps

 world net capabilities Market is expected to develop at a Compound Annual increase rate (CAGR) of +7.46%. the bottom 12 months considered for the study is 2018 and the forecast duration regarded is 2018 To 2025.

 internet technologies, akin to HTTP, initially designed for human-desktop communication, are used to move desktop-to-desktop communications, notably machine-readable file codecs similar to XML and JSON. truly, net services customarily deliver an object-oriented web-primarily based interface to the database server.

as an instance, it will also be used through other web servers, or mobile clients can supply the end person with a consumer interface. Many corporations that provide data with formatted HTML pages deliver this statistics in XML or JSON on the server, and are constantly provided through web services that permit syndication, corresponding to Wikipedia's export.

an additional software offered to conclusion clients can also be a mashup where the net server uses diverse net capabilities on diverse computers and compiles the content right into a single consumer interface.

For pattern reproduction of this report: www.researchnreports.com/request_sample.php?identification=224901

foremost Key gamers during this document

Microsoft

Amazon

IBM

Salesforce

SAP

The international internet service Market research record is a advantageous source of insightful records for business strategists. It offers the net provider business overview with boom evaluation and ancient & futuristic charge, earnings, demand and provide records (as applicable).

The research analysts supply an elaborate description of the value chain and its distributor evaluation. This web provider n market study offers comprehensive records which enhances the realizing, scope and application of this document.

For more enquiry: bit.ly/2OnrVHb 

web capabilities are all utility that can be used on their personal over the internet and that use a standardized XML messaging equipment. XML is used to encode all communications to an internet service.

for instance, the customer sends an XML message to invoke the net service after which waits for the corresponding XML response. because all verbal exchange is completed in XML, internet services don't seem to be tied to a single working equipment or programming language.

Java can confer with Perl. home windows functions can talk with Unix purposes.

net features are self-contained, modular, allotted, dynamic purposes that may also be described, published, discovered, or invoked over the community to create items, techniques, and provide chains. These purposes can also be local, disbursed, or internet-based mostly.

net features are constructed on desirable of open specifications reminiscent of TCP/IP, HTTP, Java, HTML, and XML.  

 desk of Contents:

international web service Market analysis file 2018-2023

Chapter 1: internet carrier Market Overview 

Chapter 2: global economic influence on trade 

Chapter three: web carrier Market competition by using producers 

Chapter four: global construction, profits (price) by area 

Chapter 5: world deliver (construction), Consumption, Export, Import via areas 

Chapter 6: international creation, earnings (value), rate trend by using class 

Chapter 7: global Market evaluation by utility 

Chapter eight: Manufacturing can charge analysis 

Chapter 9: Industrial Chain, Sourcing method and Downstream consumers 

Chapter 10: advertising and marketing method analysis, Distributors/merchants 

Chapter eleven: internet carrier Market effect factors evaluation 

Chapter 12: international net provider Market Forecast

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Innovation: A application That Works | killexams.com Real Questions and Pass4sure dumps

Mary Jo Frederich and Peter Andrews describe IBM's First-of-a-type (FOAK) program, which promotes innovation that gives you actual, profitable business cost.

This chapter is from the publication 

in case you had been tripping over uncut jewels and precious metallic ores, you'd likely locate a method to take capabilities of it. IBM analysis can suppose like that some days. in the hallways, you hear conversations about computers that understand natural language, superior evaluation of streaming statistics, or "green" ideas for reducing power and waste. around you're individuals who suppose for a dwelling, complicated at work—taking over intractable problems of securing financial institution information during disasters, optimizing give chains, or constructing programs that can simulate drug interactions. good issues are going on. unique things.

however for years, achieving out to the real world for innovation partnerships become no longer an evident alternative for IBM analysis. IBM has at all times had decent, artistic minds at work, helping valued clientele and creating the subsequent generation of basic equipment for enterprise and the public sector. but IBM kept the jewels to itself. Researchers (that really expert role emerged in 1945) labored in what appeared to be fabulous isolation. They managed to invent the disk power, random-entry reminiscence, FORTRAN, RISC computing, and dozens of different applied sciences that helped create ultra-modern digital world.

not by the way, IBM made some huge cash during this period. IBM had first-type questions it needed to reply, and it didn't need to look outside for talents. almost everything turned into proprietary, and every little thing that become essential for an entire answer took place in the business. besides, IBM analysis was modeled after Bell Labs, and the notion become that terrifi isolation became each acceptable and integral. The actual world, with its budgets, cut-off dates, and messy issues, would most effective distract the foremost and the brightest. Naturally, there were ideas, issues, and relationships that stored IBM research important. It wasn't a completely closed device, however that changed into the primary standpoint.

IBM research had few formal ties past corporate headquarters unless the Nineteen Seventies. at the moment, different IBM divisions have been facing gigantic challenges, and they grew to become restive about making contributions to IBM analysis once they weren't getting any instant advantages. in accordance with this, so-called "Joint courses" have been based. For the first time, other divisions of IBM, those that developed and bought and struggled with customer problems, all started to at once impact the IBM research agenda and its funding.

instead of securing 100% of its price range during the business enterprise, now IBM research become allocated most effective a portion of its annual funding. IBM analysis essential to relaxed the the rest of its funding at once from the IBM manufacturers. This became meant to align a element of the analysis work with IBM manufacturer options, whereas nevertheless offering IBM research with the liberty to pursue pure, unconstrained exploration.

This funding model still exists nowadays. each year, each of the IBM manufacturers allocates a component of its budget to fund its Joint program with IBM analysis. For every dollar that a manufacturer invests in its Joint program, IBM research matches it. This matching-of-funds approach has ensured that IBM analysis focuses some of its work on areas strategic to the IBM manufacturers. It additionally has supplied an excellent incentive for the manufacturers to make investments of their Joint courses, because it is a mechanism for the brands to enhance the variety of people engaged on their products, whereas proposing only half of the funding. well-nigh, they get added aid at a discount expense.

With the introduction of Joint classes, a substantial and becoming variety of IBMers all started to work shoulder to shoulder with colleagues from throughout IBM. The collaboration become deep, with company division employees working at, and even directing, tasks in the analysis labs. The tasks of researchers extended to the products themselves, and it turned into no longer strange for the researchers to circulate their places of work to a manufacturing or development website. And if a product didn't come off the line with satisfactory best, or a consumer had an issue with an providing that a researcher had a hand in, that researcher may be referred to as in. Firefighting and problem decision became part of the job, and many researchers became widely wide-spread with the resorts in Burlington, Poughkeepsie, Endicott, Hursley, and Markham.

In 1993, IBM research took yet another step toward fitting greater externally focused with the introduction of the services, functions, and options (SAS) software. SAS aimed to carry IBM analysis knowledge and applied sciences to a lots greater variety of shoppers who were fighting company challenges that had no off-the-shelf options.

SAS identified that researchers lived within the state of the art in lots of areas of science and technology. if they could follow the very best of what IBM research had to actual-world issues, they may power massive cost for valued clientele and the IBM supplier.

past producing new income for IBM, SAS led researchers to confront many tricky company challenges. It also forced the researchers to think more deeply and creatively about the expertise influence of their work beyond the laboratory. searching again, possible see how SAS and the Joint programs drove IBM analysis to be greater vital to IBM through guiding the researchers into areas that they may now not have in any other case explored. determine 1.1 suggests the evolution of IBM research from being internally focused to externally focused.

Figure 1.1

determine 1.1 IBM research goes from isolation to ever deeper partnering with different IBM organizations and shoppers.

youngsters IBM analysis did not welcome these adjustments enthusiastically, the cloud had a silver lining. beyond administration questions and financial pressures, it became clear that further and further of the action become going on the place individuals from distinct groups labored collectively. Synergies, new views, and sparkling ideas drove advances equivalent to parallel computing, object-oriented application, and everything that got here with the introduction of the internet. And with few exceptions, success within the industry relied on a complex array of partnerships. brand new competitor is all the time, doubtlessly, the next day's collaborator.


000-634 Object Oriented Analysis and Design - Part 2

Study Guide Prepared by Killexams.com IBM Dumps Experts


Killexams.com 000-634 Dumps and Real Questions

100% Real Questions - Exam Pass Guarantee with High Marks - Just Memorize the Answers



000-634 exam Dumps Source : Object Oriented Analysis and Design - Part 2

Test Code : 000-634
Test Name : Object Oriented Analysis and Design - Part 2
Vendor Name : IBM
Q&A : 72 Real Questions

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Object Oriented Analysis and Design - Part 2

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Object-Oriented Analysis & Design | killexams.com real questions and Pass4sure dumps

Object-Oriented Analysis & DesignJune 2, 3, 9, 10Worcester State CollegeTaught by Jan Bergandy, Computer Science, UMass, Dartmouth

Seminar Objectives:

  • To develop an in-depth understanding of object-oriented paradigm
  • To obtain a working knowledge of object-oriented analysis & design techniques
  • To learn object-oriented modeling using Unified Modeling Language (UML)
  • To learn about basic design patterns and the role of patterns is software development
  • To understand the impact of object paradigm on software development activities
  • To explore synergy between object-oriented design and object-oriented programming
  • To learn about key object technologies
  • Who should attend:This workshop is addressed to faculty involved in teaching programming, software design, and other courses related to software development. It is addressed to those who face a transition to object technology and want to learn about challenges and benefits of this transition. The workshop does not require any prior knowledge of object-oriented programming or knowledge of object paradigm. General computer fluency and general knowledge of issues associated with software and software development are expected.

    Seminar Organization:The course will be conducted as a project with instructor giving short presentations pertaining to a specific stage of the analysis and design process. During this course the participants will construct an analysis model for a selected problem. This model will be refined in to the detailed design level providing an opportunity for discussion about the relationship between object-oriented design and object-oriented programming. Each student will receive a copy of the course materials and the textbook.

    Tools & Platforms:Rational-Rose CASE toolThe CASE tool is used exclusively to expedite the process of model construction. The students spend no more than half an hour of their time during the entire class on learning how to use the tool. Not using the CASE toll will make it almost impossible to experience hands-on all the elements of the object-oriented analysis and design process.

    Textbooks:M. Fowler, ÒUML DistilledÓ, Addison-Wesley, ISBN 0-201-32563-2 (additional/optional )

    E. Gamma, R. Helm, R. Johnson, J. Vlissides, ÒDesign PatternÓ, Addison-Wesley, ISBN 0-201-63361-2

    Outline:

    June 2, 2001, 9:00 - 5:00Topics to be addressed:Object paradigm top-down - analysis & design perspectiveObject paradigm bottom-up - programming perspectiveBasic concepts: abstraction, encapsulation, information hiding, modularityResponsibility view of the requirementsClasses and objects emerging from responsibilitiesComparison of procedural and object-oriented paradigmsClasses and relationships as the building blocks of software architectureCriteria of class qualityIntroduction to Unified Modeling Language (UML)Static & dynamic modelActors and use casesTransitioning from functional requirements to objects - introduction

    Project:Analysis of the requirements for the selected projectIdentifying actors and use casesConstructing use case diagrams

    June 3, 2001, 9:00 - 2:00Topics to be addressed:Transitioning from functional requirements to objectsIdentifying the first group of classesClass specificationClass as an encapsulation of a responsibilityClass, Utility Class, Parameterized Class and its instantiationClass diagram - introductionIdentifying relationships between classesAssociation relationshipsAssociation classesRepresenting relationships with cardinalityAggregation versus compositionRepresenting aggregation and composition relationshipsRepresenting generalization/ specialization (inheritance)PolymorphismAbstract classes and interfacesSpecification of relationshipsImplementing classes & relationships (bottom-up view of relationships)Class diagram

    Project:Identifying first group of classes based on responsibilitiesPreliminary class diagramIdentifying relationships between classesDefining cardinalitiesClass diagram

    June 10, 2001, 9:00 - 5:00 (part I)Topics to be addressed:Static versus dynamic modelIdentifying scenarios through refinement of use casesModeling scenarios using object-interaction and sequence diagrams

    Project:Refining use casesDeveloping and modeling scenariosIdentifying methodsRefining class specifications

    June 10, 2001 (part II)Topics to be addressed:Events, states and actionsState diagramCriteria for using state diagramsConcurrency, active objectsMutual exclusion problemSequential, guarded, and synchronous objectsModeling concurrencyConcurrent state diagramsActivity diagrams

    Project:Evaluating classes for the need of state diagramsConstructing state diagrams for selected classes(Constructing activity diagrams)Refining class specifications

    June 10, 2001, 9:00 - 2:00Topics to be addressed:Introduction to design patterns: Creational patterns, Abstract Factory, Builder, Prototype, Singleton, Virtual Constructor

    Structural Patterns: Adapter, Bridge, Composite, Decorator, Façade, Proxy

    Behavioral Patterns: Chain of Responsibility, Command, Iterator, Mediator, Memento

    Other important topics to be covered in this course:What to expect from an object-oriented languageDynamic nature of object-oriented systems and the issues of garbage collectionEffective use of inheritance and polymorphism and their impact on software qualitySingle versus multiple inheritancePolymorphism versus genericsClass design and data normalization (attribute dependence issues)


    Object-oriented design patterns in the kernel, part 2 | killexams.com real questions and Pass4sure dumps

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    June 7, 2011

    This article was contributed by Neil Brown

    In the first part of this analysis we looked at how the polymorphic side of object-oriented programming was implemented in the Linux kernel using regular C constructs. In particular we examined method dispatch, looked at the different forms that vtables could take, and the circumstances where separate vtables were eschewed in preference for storing function pointers directly in objects. In this conclusion we will explore a second important aspect of object-oriented programming - inheritance, and in particular data inheritance.

    Data inheritance

    Inheritance is a core concept of object-oriented programming, though it comes in many forms, whether prototype inheritance, mixin inheritance, subtype inheritance, interface inheritance etc., some of which overlap. The form that is of interest when exploring the Linux kernel is most like subtype inheritance, where a concrete or "final" type inherits some data fields from a "virtual" parent type. We will call this "data inheritance" to emphasize the fact that it is the data rather than the behavior that is being inherited.

    Put another way, a number of different implementations of a particular interface share, and separately extend, a common data structure. They can be said to inherit from that data structure. There are three different approaches to this sharing and extending that can be found in the Linux kernel, and all can be seen by exploring the struct inode structure and its history, though they are widely used elsewhere.

    Extension through unions

    The first approach, which is probably the most obvious but also the least flexible, is to declare a union as one element of the common structure and, for each implementation, to declare an entry in that union with extra fields that the particular implementation needs. This approach was introduced to struct inode in Linux-0.97.2 (August 1992) when

    union { struct minix_inode_info minix_i; struct ext_inode_info ext_i; struct msdos_inode_info msdos_i; } u;

    was added to struct inode. Each of these structures remained empty until 0.97.5 when i_data was moved from struct inode to struct ext_inode_info. Over the years several more "inode_info" fields were added for different filesystems, peaking at 28 different "inode_info" structures in 2.4.14.2 when ext3 was added.

    This approach to data inheritance is simple and straightforward, but is also somewhat clumsy. There are two obvious problems. Firstly, every new filesystem implementation needs to add an extra field to the union "u". With 3 fields this may not seem like a problem, with 28 it was well past "ugly". Requiring every filesystem to update this one structure is a barrier to adding filesystems that is unnecessary. Secondly, every inode allocated will be the same size and will be large enough to store the data for any filesystem. So a filesystem that wants lots of space in its "inode_info" structure will impose that space cost on every other filesystem.

    The first of these issues is not an impenetrable barrier as we will see shortly. The second is a real problem and the general ugliness of the design encouraged change. Early in the 2.5 development series this change began; it was completed by 2.5.7 when there were no "inode_info" structures left in union u (though the union itself remained until 2.6.19).

    Embedded structures

    The change that happened to inodes in early 2.5 was effectively an inversion. The change which removed ext3_i from struct inode.u also added a struct inode, called vfs_inode, to struct ext3_inode_info. So instead of the private structure being embedded in the common data structure, the common data structure is now embedded in the private one. This neatly avoids the two problems with unions; now each filesystem needs to only allocate memory to store its own structure without any need to know anything about what other filesystems might need. Of course nothing ever comes for free and this change brought with it other issues that needed to be solved, but the solutions were not costly.

    The first difficulty is the fact that when the common filesystem code - the VFS layer - calls into a specific filesystem it passes a pointer to the common data structure, the struct inode. Using this pointer, the filesystem needs to find a pointer to its own private data structure. An obvious approach is to always place the struct inode at the top of the private inode structure and simply cast a pointer to one into a pointer to the other. While this can work, it lacks any semblance of type safety and makes it harder to arrange fields in the inode to get optimal performance - as some kernel developers are wont to do.

    The solution was to use the list_entry() macro to perform the necessary pointer arithmetic, subtracting from the address of the struct inode its offset in the private data structure and then casting this appropriately. The macro for this was called list_entry() simply because the "list.h lists" implementation was the first to use this pattern of data structure embedding. The list_entry() macro did exactly what was needed and so it was used despite the strange name. This practice lasted until 2.5.28 when a new container_of() macro was added which implemented the same functionality as list_entry(), though with slightly more type safety and a more meaningful name. With container_of() it is a simple matter to map from an embedded data structure to the structure in which it is embedded.

    The second difficulty was that the filesystem had to be responsible for allocating the inode - it could no longer be allocated by common code as the common code did not have enough information to allocate the correct amount of space. This simply involved adding alloc_inode() and destroy_inode() methods to the super_operations structure and calling them as appropriate.

    Void pointers

    As noted earlier, the union pattern was not an impenetrable barrier to adding new filesystems independently. This is because the union u had one more field that was not an "inode_info" structure. A generic pointer field called generic_ip was added in Linux-1.0.5, but it was not used until 1.3.7. Any file system that does not own a structure in struct inode itself could define and allocate a separate structure and link it to the inode through u.generic_ip. This approach addressed both of the problems with unions as no changes are needed to shared declarations and each filesystem only uses the space that it needs. However it again introduced new problems of its own.

    Using generic_ip, each filesystem required two allocations for each inode instead of one and this could lead to more wastage depending on how the structure size was rounded up for allocation; it also required writing more error-handling code. Also there was memory used for the generic_ip pointer and often for a back pointer from the private structure to the common struct inode. Both of these are wasted space compared with the union approach or the embedding approach.

    Worse than this though, an extra memory dereference was needed to access the private structure from the common structure; such dereferences are best avoided. Filesystem code will often need to access both the common and the private structures. This either requires lots of extra memory dereferences, or it requires holding the address of the private structure in a register which increases register pressure. It was largely these concerns that stopped struct inode from ever migrating to broad use of the generic_ip pointer. It was certainly used, but not by the major, high-performance filesystems.

    Though this pattern has problems it is still in wide use. struct super_block has an s_fs_info pointer which serves the same purpose as u.generic_ip (which has since been renamed to i_private when the u union was finally removed - why it was not completely removed is left as an exercise for the reader). This is the only way to store filesystem-private data in a super_block. A simple search in the Linux include files shows quite a collection of fields which are void pointers named "private" or something similar. Many of these are examples of the pattern of extending a data type by using a pointer to a private extension, and most of these could be converted to using the embedded-structure pattern.

    Beyond inodes

    While inodes serve as an effective vehicle to introduce these three patterns they do not display the full scope of any of them so it is useful to look further afield and see what else we can learn.

    A survey of the use of unions elsewhere in the kernel shows that they are widely used though in very different circumstances than in struct inode. The particular aspect of inodes that is missing elsewhere is that a wide range of different modules (different filesystems) each wanted to extend an inode in different ways. In most places where unions are used there are a small fixed number of subtypes of the base type and there is little expectation of more being added. A simple example of this is struct nfs_fattr which stores file attribute information decoded out of an NFS reply. The details of these attributes are slightly different for NFSv2 and NFSv3 so there are effectively two subtypes of this structure with the difference encoded in a union. As NFSv4 uses the same information as NFSv3 this is very unlikely to ever be extended further.

    A very common pattern in other uses of unions in Linux is for encoding messages that are passed around, typically between the kernel and user-space. struct siginfo is used to convey extra information with a signal delivery. Each signal type has a different type of ancillary information, so struct siginfo has a union to encode six different subtypes. union inputArgs appears to be the largest current union with 22 different subtypes. It is used by the "coda" network file system to pass requests between the kernel module and a user-space daemon which handles the network communication.

    It is not clear whether these examples should be considered as the same pattern as the original struct inode. Do they really represent different subtypes of a base type, or is it just one type with internal variants? The Eiffel object-oriented programming language does not support variant types at all except through subtype inheritance so there is clearly a school of thought that would want to treat all usages of union as a form of subtyping. Many other languages, such as C++, provide both inheritance and unions allowing the programmer to make a choice. So the answer is not clear.

    For our purposes it doesn't really matter what we call it as long as we know where to use each pattern. The examples in the kernel fairly clearly show that when all of the variants are understood by a single module, then a union is a very appropriate mechanism for variants structures, whether you want to refer to them as using data inheritance or not. When different subtypes are managed by different modules, or at least widely separate pieces of code, then one of the other mechanisms is preferred. The use of unions for this case has almost completely disappeared with only struct cycx_device remaining as an example of a deprecated pattern.

    Problems with void pointers

    Void pointers are not quite so easy to classify. It would probably be fair to say that void pointers are the modern equivalent of "goto" statements. They can be very useful but they can also lead to very convoluted designs. A particular problem is that when you look at a void pointer, like looking at a goto, you don't really know what it is pointing at. A void pointer called private is even worse - it is like a "goto destination" command - almost meaningless without reading lots of context.

    Examining all the different uses that void pointers can be put to would be well beyond the scope of this article. Instead we will restrict our attention to just one new usage which relates to data inheritance and illustrates how the untamed nature of void pointers makes it hard to recognize their use in data inheritance. The example we will use to explain this usage is struct seq_file used by the seq_file library which makes it easy to synthesize simple text files like some of those in /proc. The "seq" part of seq_file simply indicates that the file contains a sequence of lines corresponding to a sequence of items of information in the kernel, so /proc/mounts is a seq_file which walks through the mount table reporting each mount on a single line.

    When seq_open() is used to create a new seq_file it allocates a struct seq_file and assigns it to the private_data field of the struct file which is being opened. This is a straightforward example of void pointer based data inheritance where the struct file is the base type and the struct seq_file is a simple extension to that type. It is a structure that never exists by itself but is always the private_data for some file. struct seq_file itself has a private field which is a void pointer and it can be used by clients of seq_file to add extra state to the file. For example md_seq_open() allocates a struct mdstat_info structure and attaches it via this private field, using it to meet md's internal needs. Again, this is simple data inheritance following the described pattern.

    However the private field of struct seq_file is used by svc_pool_stats_open() in a subtly but importantly different way. In this case the extra data needed is just a single pointer. So rather than allocating a local data structure to refer to from the private field, svc_pool_stats_open simply stores that pointer directly in the private field itself. This certainly seems like a sensible optimization - performing an allocation to store a single pointer would be a waste - but it highlights exactly the source of confusion that was suggested earlier: that when you look at a void pointer you don't really know what is it pointing at, or why.

    To make it a bit clearer what is happening here, it is helpful to imagine "void *private" as being like a union of every different possible pointer type. If the value that needs to be stored is a pointer, it can be stored in this union following the "unions for data inheritance" pattern. If the value is not a single pointer, then it gets stored in allocated space following the "void pointers for data inheritance" pattern. Thus when we see a void pointer being used it may not be obvious whether it is being used to point to an extension structure for data inheritance, or being used as an extension for data inheritance (or being used as something else altogether).

    To highlight this issue from a slightly different perspective it is instructive to examine struct v4l2_subdev which represents a sub-device in a video4linux device, such as a sensor or camera controller within a webcam. According to the (rather helpful) documentation it is expected that this structure will normally be embedded in a larger structure which contains extra state. However this structure still has not just one but two void pointers, both with names suggesting that they are for private use by subtypes:

    /* pointer to private data */ void *dev_priv; void *host_priv;

    It is common that a v4l sub-device (a sensor, usually) will be realized by, for example, an I2C device (much as a block device which stores your filesystem might be realized by an ATA or SCSI device). To allow for this common occurrence, struct v4l2_subdev provides a void pointer (dev_priv), so that the driver itself doesn't need to define a more specific pointer in the larger structure which struct v4l2_subdev would be embedded in. host_priv is intended to point back to a "parent" device such as a controller which acquires video data from the sensor. Of the three drivers which use this field, one appears to follow that intention while the other two use it to point to an allocated extension structure. So both of these pointers are intended to be used following the "unions for data inheritance" pattern, where a void pointer is playing the role of a union of many other pointer types, but they are not always used that way.

    It is not immediately clear that defining this void pointer in case it is useful is actually a valuable service to provide given that the device driver could easily enough define its own (type safe) pointer in its extension structure. What is clear is that an apparently "private" void pointer can be intended for various qualitatively different uses and, as we have seen in two different circumstances, they may not be used exactly as expected.

    In short, recognizing the "data inheritance through void pointers" pattern is not easy. A fairly deep examination of the code is needed to determine the exact purpose and usage of void pointers.

    A diversion into struct page

    Before we leave unions and void pointers behind a look at struct page may be interesting. This structure uses both of these patterns, though they are hidden somewhat due to historical baggage. This example is particularly instructive because it is one case where struct embedding simply is not an option.

    In Linux memory is divided into pages, and these pages are put to a variety of different uses. Some are in the "page cache" used to store the contents of files. Some are "anonymous pages" holding data used by applications. Some are used as "slabs" and divided into pieces to answer kmalloc() requests. Others are simply part of a multi-page allocation or maybe are on a free list waiting to be used. Each of these different use cases could be seen as a subtype of the general class of "page", and in most cases need some dedicated fields in struct page, such as a struct address_space pointer and index when used in the page cache, or struct kmem_cache and freelist pointers when used as a slab.

    Each page always has the same struct page describing it, so if the effective type of the page is to change - as it must as the demands for different uses of memory change over time - the type of the struct page must change within the lifetime of that structure. While many type systems are designed assuming that the type of an object is immutable, we find here that the kernel has a very real need for type mutability. Both unions and void pointers allow types to change and as noted, struct page uses both.

    At the first level of subtyping there are only a small number of different subtypes as listed above; these are all known to the core memory management code, so a union would be ideal here. Unfortunately struct page has three unions with fields for some subtypes spread over all three, thus hiding the real structure somewhat.

    When the primary subtype in use has the page being used in the page cache, the particular address_space that it belongs to may want to extend the data structure further. For this purpose there is a private field that can be used. However it is not a void pointer but is an unsigned long. Many places in the kernel assume an unsigned long and a void * are the same size and this is one of them. Most users of this field actually store a pointer here and have to cast it back and forth. The "buffer_head" library provides macros attach_page_buffers and page_buffers to set and get this field.

    So while struct page is not the most elegant example, it is an informative example of a case where unions and void pointers are the only option for providing data inheritance.

    The details of structure embedding

    Where structure embedding can be used, and where the list of possible subtypes is not known in advance, it seems to be increasingly the preferred choice. To gain a full understanding of it we will again need to explore a little bit further than inodes and contrast data inheritance with other uses of structure embedding.

    There are essentially three uses for structure embedding - three reasons for including a structure within another structure. Sometimes there is nothing particularly interesting going on. Data items are collected together into structures and structures within structures simply to highlight the closeness of the relationships between the different items. In this case the address of the embedded structure is rarely taken, and it is never mapped back to the containing structure using container_of().

    The second use is the data inheritance embedding that we have already discussed. The third is like it but importantly different. This third use is typified by struct list_head and other structs used as an embedded anchor when creating abstract data types.

    The use of an embedded anchor like struct list_head can be seen as a style of inheritance as the structure containing it "is-a" member of a list by virtue of inheriting from struct list_head. However it is not a strict subtype as a single object can have several struct list_heads embedded - struct inode has six (if we include the similar hlist_node). So it is probably best to think of this sort of embedding more like a "mixin" style of inheritance. The struct list_head provides a service - that of being included in a list - that can be mixed-in to other objects, an arbitrary number of times.

    A key aspect of data inheritance structure embedding that differentiates it from each of the other two is the existence of a reference counter in the inner-most structure. This is an observation that is tied directly to the fact that the Linux kernel uses reference counting as the primary means of lifetime management and so would not be shared by systems that used, for example, garbage collection to manage lifetimes.

    In Linux, every object with an independent existence will have a reference counter, sometimes a simple atomic_t or even an int, though often a more explicit struct kref. When an object is created using several levels of inheritance the reference counter could be buried quite deeply. For example a struct usb_device embeds a struct device which embeds struct kobject which has a struct kref. So usb_device (which might in turn be embedded in a structure for some specific device) does have a reference counter, but it is contained several levels down in the nest of structure embedding. This contrasts quite nicely with a list_head and similar structures. These have no reference counter, have no independent existence and simply provide a service to other data structures.

    Though it seems obvious when put this way, it is useful to remember that a single object cannot have two reference counters - at least not two lifetime reference counters (It is fine to have two counters like s_active and s_count in struct super_block which count different things). This means that multiple inheritance in the "data inheritance" style is not possible. The only form of multiple inheritance that can work is the mixin style used by list_head as mentioned above.

    It also means that, when designing a data structure, it is important to think about lifetime issues and whether this data structure should have its own reference counter or whether it should depend on something else for its lifetime management. That is, whether it is an object in its own right, or simply a service provided to other objects. These issues are not really new and apply equally to void pointer inheritance. However an important difference with void pointers is that it is relatively easy to change your mind later and switch an extension structure to be a fully independent object. Structure embedding requires the discipline of thinking clearly about the problem up front and making the right decision early - a discipline that is worth encouraging.

    The other key telltale for data inheritance structure embedding is the set of rules for allocating and initializing new instances of a structure, as has already been hinted at. When union or void pointer inheritance is used the main structure is usually allocated and initialized by common code (the mid-layer) and then a device specific open() or create() function is called which can optionally allocate and initialize any extension object. By contrast when structure embedding is used the structure needs to be allocated by the lowest level device driver which then initializes its own fields and calls in to common code to initialize the common fields.

    Continuing the struct inode example from above which has an alloc_inode() method in the super_block to request allocation, we find that initialization is provided for with inode_init_once() and inode_init_always() support functions. The first of these is used when the previous use of a piece of memory is unknown, the second is sufficient by itself when we know that the memory was previously used for some other inode. We see this same pattern of an initializer function separate from allocation in kobject_init(), kref_init(), and device_initialize().

    So apart from the obvious embedding of structures, the pattern of "data inheritance through structure embedding" can be recognized by the presence of a reference counter in the innermost structure, by the delegation of structure allocation to the final user of the structure, and by the provision of initializing functions which initialize a previously allocated structure.

    Conclusion

    In exploring the use of method dispatch (last week) and data inheritance (this week) in the Linux kernel we find that while some patterns seem to dominate they are by no means universal. While almost all data inheritance could be implemented using structure embedding, unions provide real value in a few specific cases. Similarly while simple vtables are common, mixin vtables are very important and the ability to delegate methods to a related object can be valuable.

    We also find that there are patterns in use with little to recommend them. Using void pointers for inheritance may have an initial simplicity, but causes longer term wastage, can cause confusion, and could nearly always be replaced by embedded inheritance. Using NULL pointers to indicate default behavior is similarly a poor choice - when the default is important there are better ways to provide for it.

    But maybe the most valuable lesson is that the Linux kernel is not only a useful program to run, it is also a useful document to study. Such study can find elegant practical solutions to real problems, and some less elegant solutions. The willing student can pursue the former to help improve their mind, and pursue the latter to help improve the kernel itself. With that in mind, the following exercises might be of interest to some.

    Exercises
  • As inodes now use structure embedding for inheritance, void pointers should not be necessary. Examine the consequences and wisdom of removing "i_private" from "struct inode".

  • Rearrange the three unions in struct page to just one union so that the enumeration of different subtypes is more explicit.

  • As was noted in the text, struct seq_file can be extended both through "void pointer" and a limited form of "union" data inheritance. Explain how seq_open_private() allows this structure to also be extended through "embedded structure" data inheritance and give an example by converting one usage in the kernel from "void pointer" to "embedded structure". Consider submitting a patch if this appears to be an improvement. Contrast this implementation of embedded structure inheritance with the mechanism used for inodes.

  • Though subtyping is widely used in the kernel, it is not uncommon for a object to contain fields that not all users are interested in. This can indicate that more fine grained subtyping is possible. As very many completely different things can be represented by a "file descriptor", it is likely that struct file could be a candidate for further subtyping.

    Identify the smallest set of fields that could serve as a generic struct file and explore the implications of embedding that in different structures to implement regular files, socket files, event files, and other file types. Exploring more general use of the proposed open() method for inodes might help here.

  • Identify an "object-oriented" language which has an object model that would meet all the needs of the Linux kernel as identified in these two articles.

  • (Log in to post comments)

    Java and Object-Oriented Programming | killexams.com real questions and Pass4sure dumps

    This chapter is from the book 

    Many seasoned Java developers will scoff at the fact that this section even exists in this book. It is here for two very important reasons. The first is that I continually run across Java applications built with a procedural mind-set. The fact that you know Java doesn't mean that you have the ability to transform that knowledge into well-designed object-oriented systems. As both an instructor and consultant, I see many data-processing shops send COBOL and/or Visual Basic developers to a three-day class on UML and a five-day class on Java and expect miracles. Case in point: I was recently asked to review a Java application to assess its design architecture and found that it had only two classes—SystemController and ScreenController—which contained over 70,000 lines of Java code.

    The second reason for the emphasis on how the language maps to object-oriented principles is that people like language comparisons and how they stack up to their counterparts. To appease those that live and die by language comparisons, let's put Java under the scrutiny of what constitutes an object-oriented language.

    No definitive definition of what makes a language object-oriented is globally accepted. However, a common set of criteria I personally find useful is that the language must support the following:

  • Classes
  • Complex types (Java reference types)
  • Message passing
  • Encapsulation
  • Inheritance
  • Polymorphism
  • These are discussed in the next subsections.

    Java and Classes

    Java allows classes to be defined. There are no stray functions floating around in Java. A class is a static template that contains the defined structure (attributes) and behavior (operations) of a real-world entity in the application domain. At runtime, the class is instantiated, or brought to life, as an object born in the image of that class. In my seminars, when several folks new to the object world are in attendance, I often use the analogy of a cookie cutter. The cookie cutter is merely the template used to stamp out what will become individually decorated and unique cookies. The cookie cutter is the class; the unique blue, green, and yellow gingerbread man is the object (which I trust supports a bite operation).

    Java exposes the class to potential outside users through its public interface. A public interface consists of the signatures of the public operations supported by the class. A signature is the operation name and its input parameter types (the return type, if any, is not part of the operation's signature).

    Good programming practice encourages developers to declare all attributes as private and allow access to them only via operations. As with most other languages, however, this is not enforced in Java. Figure 2-1 outlines the concept of a class and its interface.

    FIGURE 2-1 Public interface of a class

    The figure uses a common eggshell metaphor to describe the concept of the class's interface, as well as encapsulation. The internal details of the class are hidden from the outside via a well-defined interface. In this case, only four operations are exposed in the classes interface (Operation_A, B, C, and D). The other attributes and operations are protected from the outside world. Actually, to the outside world, it's as if they don't even exist.

    Suppose you want to create an Order class in Java that has three attributes—orderNumber, orderDate, and orderTotal—and two operations—calcTotalValue() and getInfo(). The class definition could look like this:

    /** * Listing 1 * This is the Order class for the Java/UML book */ package com.jacksonreed; import java.util.*; public class Order { private Date orderDate; private long orderNumber; private long orderTotal; public Order() { } public boolean getInfo() { return true; } public long calcTotalValue() { return 0; } public Date getOrderDate() { return orderDate; } public void setOrderDate(Date aOrderDate) { orderDate = aOrderDate; } public long getOrderNumber() { return orderNumber; } public void setOrderNumber(long aOrderNumber) { orderNumber = aOrderNumber; } public long getOrderTotal() { return orderTotal; } public void setOrderTotal(long aOrderTotal) { orderTotal = aOrderTotal; } public static void main(String[] args) { Order order = new Order(); System.out.println("instantiated Order"); System.out.println(order.getClass().getName()); System.out.println(order.calcTotalValue()); try { Thread.currentThread().sleep(5*1000); } catch(InterruptedException e) {} } }

    A few things are notable about the first bit of Java code presented in this book. Notice that each of the three attributes has a get and a set operation to allow for the retrieval and setting of the Order object's properties. Although doing so is not required, it is common practice to provide these accessor-type operations for all attributes defined in a class. In addition, if the Order class ever wanted to be a JavaBean, it would have to have "getters and setters" defined in this way.

    Some of the method code in the main() operation does a few things of note. Of interest is that a try block exists at the end of the operation that puts the current thread to sleep for a bit. This is to allow the console display to freeze so that you can see the results.

    If you type in this class and then compile it and execute it in your favorite development tool or from the command prompt with

    javac order.java //* to compile it java order //* to run it

    you should get results that look like this:

    instantiated Order com.jacksonreed.Order 0

    NOTE

    Going forward, I promise you will see no code samples with class, operation, or attribute names of foo, bar, or foobar.

    More on Java and Classes

    A class can also have what are called class-level operations and attributes. Java supports these with the static keyword. This keyword would go right after the visibility (public, private, protected) component of the operation or attribute. Static operations and attributes are needed to invoke either a service of the class before any real instances of that class are instantiated or a service that doesn't directly apply to any of the instances. The classic example of a static operation is the Java constructor. The constructor is what is called when an object is created with the New keyword. Perhaps a more business-focused example is an operation that retrieves a list of Customer instances based on particular search criteria.

    A class-level attribute can be used to store information that all instances of that class may access. This attribute might be, for example, a count of the number of objects currently instantiated or a property about Customer that all instances might need to reference.

    Java and Complex Types (Java Reference Types)

    A complex type, which in Java is called a reference type, allows variables typed as something other than primitive types (e.g., int and boolean) to be declared. In Java, these are called reference types. In object-oriented systems, variables that are "of" a particular class, such as Order, Customer, or Invoice, must be defined. Taken a step further, Order could consist of other class instances, such as OrderHeader and OrderLine.

    In Java, you can define different variables that are references to runtime objects of a particular class type:

    Public Order myOrder; Public Customer myCustomer; Public Invoice myInvoice;

    Such variables can then be used to store actual object instances and subsequently to serve as recipients of messages sent by other objects. In the previous code fragment, the variable myOrder is an instance of Order. After the myOrder object is created, a message can be sent to it and myOrder will respond, provided that the operation is supported by myOrder's interface.

    Java and Message Passing

    Central to any object-oriented language is the ability to pass messages between objects. In later chapters you will see that work is done in a system only by objects that collaborate (by sending messages) to accomplish a goal (which is specified in a use-case) of the system.

    Java doesn't allow stray functions floating around that are not attached to a class. In fact, Java demands this. Unfortunately, as my previous story suggested, just saying that a language requires everything to be packaged in classes doesn't mean that the class design will be robust, let alone correct.

    Java supports message passing, which is central to the use of Java's object-oriented features. The format closely resembles the syntax of other languages, such as C++ and Visual Basic. In the following code fragment, assume that a variable called myCustomer, of type Customer, is defined and that an operation called calcTotalValue() is defined for Customer. Then the calcTotalValue() message being sent to the myCustomer object in Java would look like this:

    myCustomer.calcTotalValue();

    Many developers feel that, in any other structured language, this is just a fancy way of calling a procedure. Calling a procedure and sending a message are similar in that, once invoked, both a procedure and a message implement a set of well-defined steps. However, a message differs in two ways:

  • There is a designated receiver, the object. Procedures have no designated receiver.

  • The interpretation of the message—that is, the how-to code (called the method) used to respond to the message—can vary with different receivers. This point will become more important later in the chapter, when polymorphism is reviewed.

  • The concepts presented in this book rely heavily on classes and the messaging that takes place between their instances, or objects.

    Java and Encapsulation

    Recall that a class exposes itself to the outside world via its public interface and that this should be done through exposure to operations only, and not attributes. Java supports encapsulation via its ability to declare both attributes and operations as public, private, or protected. In UML this is called visibility.

    Using the code from the previous Order example, suppose you want to set the value of the orderDate attribute. In this case, you should do so with an operation. An operation that gets or sets values is usually called a getter or a setter, respectively, and collectively such operations are called accessors. The local copy of the order date, orderDate, is declared private. (Actually, all attributes of a class should be declared private or protected, so that they are accessible only via operations exposed as public to the outside world.)

    Encapsulation provides some powerful capabilities. To the outside world, the design can hide how it derives its attribute values. If the orderTotal attribute is stored in the Order object, the corresponding get operation defined previously looks like this:

    public long getOrderTotal() { return orderTotal; }

    This snippet of code would be invoked if the following code were executed by an interested client:

    private long localTotal; private Order localOrder; localOrder = New Order(); localTotal = localOrder.getOrderTotal()

    However, suppose the attribute orderTotal isn't kept as a local value of the Order class, but rather is derived via another mechanism (perhaps messaging to its OrderLine objects). If Order contains OrderLine objects (declared as a Vector or ArrayList of OrderLine objects called myOrderLines) and OrderLine knows how to obtain its line totals via the message getOrderLineTotal(), then the corresponding get operation for orderTotal within Order will look like this:

    public long getOrderTotal() { long totalAmount=0; for (int i=0; i < myOrderLines.length; i++) { totalAmount = totalAmount + myOrderLines[i].getOrderLineTotal(); } return totalAmount; }

    This code cycles through the myOrderLines collection, which contains all the Orderline objects related to the Order object, sending the getOrderLineTotal() message to each of Order's OrderLine objects. The getOrderTotal() operation will be invoked if the following code is executed by an interested client:

    long localTotal; Order myOrder; myOrder = new Order(); localTotal = localOrder.getOrderTotal()

    Notice that the "client" code didn't change. To the outside world, the class still has an orderTotal attribute. However, you have hidden, or encapsulated, just how the value was obtained. This encapsulation allows the class's interface to remain the same (hey, I have an orderTotal that you can ask me about), while the class retains the flexibility to change its implementation in the future (sorry, how we do business has changed and now we must derive orderTotal like this). This kind of resiliency is one of the compelling business reasons to use an object-oriented programming language in general.

    Java and Inheritance

    The inclusion of inheritance is often the most cited reason for granting a language object-oriented status. There are two kinds of inheritance: interface and implementation. As we shall see, Java is one of the few languages that makes a clear distinction between the two.

    Interface inheritance (Figure 2-2) declares that a class that is inheriting an interface will be responsible for implementing all of the method code of each operation defined in that interface. Only the signatures of the interface are inherited; there is no method or how-to code.

    FIGURE 2-2 Interface inheritance

    Implementation inheritance (Figure 2-3) declares that a class that is inheriting an interface may, at its option, use the method code implementation already established for the interface. Alternatively, it may choose to implement its own version of the interface. In addition, the class inheriting the interface may extend that interface by adding its own operations and attributes.

    FIGURE 2-3 Implementation inheritance

    Each type of inheritance should be scrutinized and used in the appropriate setting. Interface inheritance is best used under the following conditions:

  • The base class presents a generic facility, such as a table lookup, or a derivation of system-specific information, such as operating-system semantics or unique algorithms.

  • The number of operations is small.

  • The base class has few, if any, attributes.

  • Classes realizing or implementing the interface are diverse, with little or no common code.

  • Implementation inheritance is best used under the following conditions:

  • The class in question is a domain class that is of primary interest to the application (i.e., not a utility or controller class).

  • The implementation is complex, with a large number of operations.

  • Many attributes and operations are common across specialized implementations of the base class.

  • Some practitioners contend that implementation inheritance leads to a symptom called the fragile base class problem. Chiefly, this term refers to the fact that over time, what were once common code and attributes in the superclass may not stay common as the business evolves. The result is that many, if not all, of the subclasses, override the behavior of the superclass. Worse yet, the subclasses may find themselves overriding the superclass, doing their own work, and then invoking the same operation again on the superclass. These practitioners espouse the idea of using only interface inheritance. Particularly with the advent of Java and its raising of the interface to a first-class type, the concept and usage of interface-based programming have gained tremendous momentum.

    As this book evolves, keeping in mind the pointers mentioned here when deciding between the two types of inheritance will be helpful. Examples of both constructs will be presented in the theme project that extends throughout this book.

    Implementation Inheritance

    Java supports implementation inheritance with the extends keyword. A class wanting to take advantage of implementation inheritance simply adds an extendsClassName statement to its class definition. To continue the previous example, suppose you have two different types of orders, both warranting their own subclasses: Commercial and Retail. You would still have an Order class (which isn't instantiated directly and which is called abstract). The previous fragment showed the code for the Order class. Following is the code for the Commercial class.

    package com.jacksonreed; public class Commercial extends Order { public Commercial() { } /* Unique Commercial code goes here */ }

    Implementation inheritance allows the Commercial class to utilize all attributes and operations defined in Order. This will be done automatically by the Java Virtual Machine (JVM) in conjunction with the language environment. In addition, implementation inheritance has the ability to override and/or extend any of Order's behavior. Commercial may also add completely new behavior if it so chooses.

    Interface Inheritance

    Java supports interface inheritance with the implements keyword. A class wanting to realize a given interface (actually being responsible for the method code) simply adds an implements InterfaceName statement. However, unlike extension of one class by another class, implementation of an interface by a class requires that the interface be specifically defined as an interface beforehand.

    Looking again at the previous example with Order, let's assume that this system will contain many classes—some built in this release, and some built in future releases—that need the ability to price themselves. Remember from earlier in this chapter that one of the indicators of using interface inheritance is the situation in which there is little or no common code but the functional intent of the classes is the same. This pricing functionality includes three services: the abilities to calculate tax, to calculate an extended price, and to calculate a total price. Let's call the operations for these services calcExtendedPrice(), calcTax(), and calcTotalPrice(), respectively, and assign them to a Java interface called IPrice. Sometimes interface names are prefixed with the letter I to distinguish them from other classes:

    package com.jacksonreed; interface IPrice { long calcExtendedPrice(); long calcTax(); long calcTotalPrice(); }

    Notice that the interface contains only operation signatures; it has no implementation code. It is up to other classes to implement the actual behavior of the operations. For the Order class to implement, or realize, the IPrice interface, it must include the implements keyword followed by the interface name:

    public class Order implements IPrice { }

    If you try to implement an interface without providing implementations for all of its operations, your class will not compile. Even if you don't want to implement any method code for some of the operations, you still must have the operations defined in your class.

    One very powerful aspect of interface inheritance is that a class can implement many interfaces at the same time. For example, Order could implement the IPrice interface and perhaps a search interface called ISearch. However, a Java class may extend from only one other class.

    Java and Polymorphism

    Polymorphism is one of those $50 words that dazzles the uninformed and sounds really impressive. In fact, polymorphism is one of the most powerful features of any object-oriented language.

    Roget's II: The New Thesaurus cross-references the term polymorphism to the main entry of variety. That will do for starters. Variety is the key to polymorphism. The Latin root for polymorphism means simply "many forms." Polymorphism applies to operations in the object-oriented context. So by combining these two thoughts, you could say that operations are polymorphic if they are identical (not just in name but also in signatures) but offer variety in their implementations.

    Polymorphism is the ability of two different classes each to have an operation that has the same signature, while having two very different forms of method code for the operation. Note that to take advantage of polymorphism, either an interface inheritance or an implementation inheritance relationship must be involved.

    In languages such as COBOL and FORTRAN, defining a routine to have the same name as another routine will cause a compile error. In object-oriented languages such as Java and C++, several classes might have an operation with the same signature. Such duplication is in fact encouraged because of the power and flexibility it brings to the design.

    As mentioned previously, the implements and extends keywords let the application take advantage of polymorphism. As we shall see, the sample project presented later in this book is an order system for a company called Remulak Productions. Remulak sells musical equipment, as well as other types of products. There will be a Product class, as well as Guitar, SheetMusic, and Supplies classes.

    Suppose, then, that differences exist in the fundamental algorithms used to determine the best time to reorder each type of product (called the economic order quantity, or EOQ). I don't want to let too much out of the bag at this point, but there will be an implementation inheritance relationship created with Product as the ancestor class (or superclass) and the other three classes as its descendants (or subclasses). The scenario that follows uses implementation inheritance with a polymorphic example. Note that interface inheritance would yield the same benefits and be implemented in the same fashion.

    To facilitate extensibility and be able to add new products in the future in a sort of plug-and-play fashion, we can make calcEOQ() polymorphic. To do this in Java, Product would define calcEOQ() as abstract, thereby informing any inheriting subclass that it must provide the implementation. A key concept behind polymorphism is this: A class implementing an interface or inheriting from an ancestor class can be treated as an instance of that ancestor class. In the case of a Java interface, the interface itself is a valid type.

    For example, assume that a collection of Product objects is defined as a property of the Inventory class. Inventory will support an operation, getAverageEOQ(), that needs to calculate the average economic order quantity for all products the company sells. To do this requires that we iterate over the collection of Product objects called myProducts to get each object's unique economic order quantity individually, with the goal of getting an average:

    public long getAverageEOQ() { long totalAmount=0; for (int i=0; i < myProducts.length; i++) { totalAmount = totalAmount + myProducts[i].calcEOQ(); } return totalAmount / myProducts.length; }

    But wait! First of all, how can Inventory have a collection of Product objects when the Product class is abstract (no instances were ever created on their own)? Remember the maxim from earlier: Any class implementing an interface or extending from an ancestor class can be treated as an instance of that interface or extended class. A Guitar "is a" Product, SheetMusic "is a" Product, and Supplies "is a" Product. So anywhere you reference Guitar, SheetMusic, or Supplies, you can substitute Product.

    Resident in the array myProducts within the Inventory class are individual concrete Guitar, SheetMusic, and Supplies objects. Java figures out dynamically which object should get its own unique calcEOQ() message. The beauty of this construct is that later, if you add a new type of Product—say, Organ—it will be totally transparent to the Inventory class. That class will still have a collection of Product types, but it will have four different ones instead of three, each of which will have its own unique implementation of the calcEOQ() operation.

    This is polymorphism at its best. At runtime, the class related to the object in question will be identified and the correct "variety" of the operation will be invoked. Polymorphism provides powerful extensibility features to the application by letting future unknown classes implement a predictable and well-conceived interface without affecting how other classes deal with that interface.



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    IBM 000-634 Exam (Object Oriented Analysis and Design - Part 2) Detailed Information



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