Object lifetime in Java in Java

Printer DataMatrix in Java Object lifetime in Java
3
Data Matrix 2d Barcode Printer In Java
Using Barcode maker for Java Control to generate, create Data Matrix image in Java applications.
In this case, the Observable class provides some real functionality It has a method (addObserver) that builds a list of all Observers When its notifyObservers method is called, it uses that list to call the update method of each Observer On the other hand, the only function of the Observer definition is to specify that there will be an update method provided for the Observable to call Since the behavior of each Observer is likely to be quite different, there is no need for any functionality in the Observer specification Thus, Observer is defined as a Java interface that specifies a single method, update Furthermore, it is likely that some of the classes that implement Observer objects will be derived for other classes Thus, the fact that Observer is an interface means it will serve as a simple mix-in to the actual classes that define Observers, and without the need for multiple inheritance While multiple inheritance can be useful for some programming problems, it does in fact lead to some significant problems for the language compiler and run time implementation (These issues are beyond the scope of this book, but are real, although solvable at some cost) The Java interface provides a similar functionality to multiple inheritance, but avoids the big complications that multiple inheritance brings to a language implementation Thus, it gives most of the benefits without the added cost of true multiple inheritance
Bar Code Maker In Java
Using Barcode generation for Java Control to generate, create barcode image in Java applications.
Object lifetime in Java
Recognizing Bar Code In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
As we discussed in 2, all objects have a lifetime They are created, carry out their responsibilities, and usually go away - either when the program is finished using an object, or when the program terminates (It is possible to have persistent objects that "live" from one invocation of the program to another, but such persistent objects are a different matter)
DataMatrix Printer In Visual C#.NET
Using Barcode encoder for VS .NET Control to generate, create Data Matrix 2d barcode image in Visual Studio .NET applications.
file:///C|/oobook/3html (23 of 36) [13/03/2003 02:55:14 }
DataMatrix Printer In .NET
Using Barcode creation for ASP.NET Control to generate, create Data Matrix image in ASP.NET applications.
3
Data Matrix 2d Barcode Encoder In .NET
Using Barcode generator for .NET framework Control to generate, create Data Matrix 2d barcode image in .NET applications.
No Java object exists until it has been created with the Java new operator Because a program has to start by creating new objects, a Java program requires a static component, eg, public static main(String args[]) The main method exists statically, and creates new objects to get the program running Constructors When a Java object is created with the new operator, it begins its lifetime by having its constructor automatically called by the runtime system The constructor is really a special purpose method The constructor has the same name as the class, with no other type attribute The constructor can be declared to be public, private, or protected The goal of a constructor is to initialize the class It should set all class attributes to a known state It can create other subobjects that are a part of the class When the constructor is done, the newly created object should be ready to use A constructor does not return a value, but it can throw an exception The new operator returns a reference to the newly created object Java allows you to define multiple constructors for a class Each constructor has a different signature determined by the order and type of the constructor's arguments The default constructor has no arguments It is often useful to define several constructors to initialize objects to different values If a class is derived from another class, the constructor of the parent should be called This is done with the Java super() statement Java will automatically call the default constructor of the parent class if you don't explicitly use a super() statement If you want to use a parent constructor other than the default one, you can use super() with an appropriate set of arguments Almost every Java example shown in this book will have constructors that you can study to see the different ways constructors can be used Garbage collection After an object has carried out its purpose, it is often not needed by the program any longer All objects consume at least some system resources, and it is important to free these resources when an object is no longer needed Whenever an object no longer has any references to it, the Java runtime system will consider the object unused, and therefore available for garbage collection Automatic garbage collection is a great simplification for the programmer Unlike in other object-oriented languages such as C++, the Java programmer is not responsible for explicitly freeing resources used by an object The garbage collector handles all this (usually - see the next section, "Memory leaks")
DataMatrix Maker In Visual Basic .NET
Using Barcode printer for .NET Control to generate, create ECC200 image in VS .NET applications.
file:///C|/oobook/3html (24 of 36) [13/03/2003 02:55:14 }
Making Barcode In Java
Using Barcode drawer for Java Control to generate, create barcode image in Java applications.
Code 128 Code Set B Printer In Java
Using Barcode drawer for Java Control to generate, create Code 128B image in Java applications.
UCC-128 Creation In Java
Using Barcode creator for Java Control to generate, create USS-128 image in Java applications.
International Standard Serial Number Creation In Java
Using Barcode creator for Java Control to generate, create ISSN - 10 image in Java applications.
Encode GS1 128 In .NET Framework
Using Barcode creation for ASP.NET Control to generate, create GS1 128 image in ASP.NET applications.
Encode DataMatrix In .NET
Using Barcode generation for ASP.NET Control to generate, create Data Matrix ECC200 image in ASP.NET applications.
Read ECC200 In Visual Studio .NET
Using Barcode reader for Visual Studio .NET Control to read, scan read, scan image in .NET applications.