FIVE in Java

Paint QR Code in Java FIVE
CHAPTER
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FIVE
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Classes in Java
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AIMS
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To show how Java defines objects using the class facility To describe the concepts of instance variable, static variable, instance method and static method To show how inheritance can be used within Java To briefly introduce the Java class library
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51 Introduction
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The previous chapter has described the processing facilities and data types that are available within Java A casual reader who has just read that chapter without reading the rest of the book would be forgiven for assuming that Java was not substantially different from other programming languages; in particular the programming language C You could say that it had a somewhat strange syntax for calling what looked like subroutines, but apart from that there is little that differentiates Java from other programming languages The aim of this chapter is to dispel this notion It is not difficult to write Java
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programs in a way resembling those of conventional procedural programming languages However, to derive the most power from Java it is necessary to develop code which is object-oriented By the end of this chapter you should be able to do this
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52 Objects
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It is worth carrying out a little revision before looking at how Java implements objects An object is something which has a state: it is associated with some stored data value or data values Everything can be regarded as an object The user of an email system is an object whose state may contain data that describes his or her location and name A bank account is an object whose state may be the name of the account holder, the current balance, the overdraft limit and a list of recent transactions A plane in an air traffic control system is an object; its state might consist of data which identifies the plane, its position and its eventual destination A pull-down menu in the user interface of an applet can be regarded as an object; its state might consist of a list of the commands that make up the pull-down menu, the type of box that encloses the menu when it is instantiated on a screen and the colour of the pull-down menu Thus, the first thing that needs saying is that the object paradigm is universal Another important point is that objects can receive messages By now you will have seen plenty of examples of messages in Java and should be totally familiar with the dot notation that is used to indicate that a message is being sent to an object The expression: objmess(arg1, arg2argn); has the interpretation that the message mess is sent to the object obj The message consists of n arguments which are used when the code corresponding to the message is executed (n could be zero) An applet or application can be regarded as a set or collection of objects which communicate with each other by means of messages in order to achieve some processing demanded by the world outside the applet For example, assume that we have an application which is a simple word processor and a user wishes to count the number of words in the document The user will pull down a menu which contains the word count command, instantiate this command and the count will be displayed on the screen This seemingly simple piece of processing would contain quite a large amount of processing which involves messages being sent to receiver objects First, the mouse object would send a message to the pull-down menu object; this would result in the display of the pull-down menu, and the mouse would then send a message to the pull-down menu when it has selected the word count command This will then generate a message sent from the pull-down menu to the object which represents the document whose word count is to be found This document will then send a series of messages to the words in the document with a count being incremented each time a word is found When this processing has finished a message would be sent to a window asking it to display the word count on the user s computer monitor Java contains facilities for defining an object, naming an object and creating an object Naming an object requires a simple declaration of the form: ObjectType objectname; Here ObjectType is the name of an object type and objectname is the identifier which is used to refer to the object Thus, if the user had defined an object emailUser the code: emailUser john, jim, angela, forbiddenUsers[]; would declare three objects john, jim, angela and an array of user objects identified by forbiddenUsers It is important to point out that such a declaration does not create the objects, but just informs the Java system that the names will be used to designate the objects In order to create an object you will need to use a facility known as new This creates an object and binds a name to the object For example, the code:
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emailUser john = new emailUser(); results in an emailUser object being created which is identified by the name john As you will see later in this chapter there are a number of variants which can be used when constructing new objects The one above is the simplest: it has no arguments and results in a new object being created with its instance variables uninitialized As you will see in the next section of this chapter there are a number of ways of writing code which creates objects with a wide variety of initial values One keyword which you will sometimes encounter in a Java program is null This represents an empty object It is similar to zero in arithmetic An example of the use of null is shown below: User John = null; This creates a User variable named John but does not put any data in it
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Barcode Encoder In Java
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Painting Bar Code In Visual C#
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Painting Universal Product Code Version A In VS .NET
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Read DataMatrix In Visual Studio .NET
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