Objects of Local Classes as Caches in Java

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Example 811
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class Account { int balance; /** (1) Method makes a deposit into an account */ void deposit(final int amount) { /** (2) Local class to save the necessary data and to check that the transaction was valid */ class Auditor { /** (3) Stores the old balance */ private int balanceAtStartOfTransaction = balance; /** (4) Checks the postcondition */ void check() { assert balance - balanceAtStartOfTransaction == amount; } }
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85: ANONYMOUS CLASSES Auditor auditor = new Auditor(); // (5) Save the data balance += amount; // (6) Do the transaction auditorcheck(); // (7) Check the postcondition } public static void main(String[] args) { Account ac = new Account(); acdeposit(250); } }
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85 Anonymous Classes
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Classes are usually first defined and then instantiated using the new operator Anonymous classes combine the process of definition and instantiation into a single step Anonymous classes are defined at the location they are instantiated, using additional syntax with the new operator As these classes do not have a name, an instance of the class can only be created together with the definition An anonymous class can be defined and instantiated in contexts where a reference value can be used (ie, as expressions that evaluate to a reference value denoting an object) Anonymous classes are typically used for creating objects on the fly in contexts such as the value in a return statement, an argument in a method call, or in initialization of variables Typical uses of anonymous classes are to implement event listeners in GUI-based applications, threads for simple tasks (see examples in 13, p 613), and comparators for providing a total ordering of objects (see Example 1511, p 774) Like local classes, anonymous classes can be defined in static or non-static context The keyword static is never used
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Extending an Existing Class
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The following syntax can be used for defining and instantiating an anonymous class that extends an existing class specified by <superclass name>:
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<superclass name> (<optional argument list>)
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<member declarations>
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Optional arguments can be specified, which are passed to the superclass constructor Thus, the superclass must provide a constructor corresponding to the arguments passed No extends clause is used in the construct Since an anonymous class cannot define constructors (as it does not have a name), an instance initializer can be used to achieve the same effect as a constructor Only non-static members and final static fields can be declared in the class body
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CHAPTER 8: NESTED TYPE DECLARATIONS
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Example 812 Defining Anonymous Classes interface IDrawable { // (1) void draw(); } //_____________________________________________________________________________ class Shape implements IDrawable { // (2) public void draw() { Systemoutprintln("Drawing a Shape"); } } //_____________________________________________________________________________ class Painter { // (3) Top-level Class public Shape createShape() { // (4) Non-static Method return new Shape(){ // (5) Extends superclass at (2) public void draw() { Systemoutprintln("Drawing a new Shape"); } }; } public static IDrawable createIDrawable() { // (7) Static Method return new IDrawable(){ // (8) Implements interface at (1) public void draw() { Systemoutprintln("Drawing a new IDrawable"); } }; } } //_____________________________________________________________________________ public class AnonClassClient { public static void main(String[] args) { // (9) IDrawable[] drawables = { // (10) new Painter()createShape(), // (11) non-static anonymous class PaintercreateIDrawable(), // (12) static anonymous class new Painter()createIDrawable() // (13) static anonymous class }; for (int i = 0; i < drawableslength; i++) // (14) drawables[i]draw(); Systemoutprintln("Anonymous Class Names:"); Systemoutprintln(drawables[0]getClass());// (15) Systemoutprintln(drawables[1]getClass());// (16) } }
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Output from the program:
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Drawing a new Shape Drawing a new IDrawable Drawing a new IDrawable Anonymous Class Names: class Painter$1 class Painter$2
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Class declarations from Example 810 are adapted to use anonymous classes in Example 812 The non-static method createShape() at (4) defines a non-static anony-
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85: ANONYMOUS CLASSES
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mous class at (5), which extends the superclass Shape The anonymous class overrides the inherited method draw()
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// class Shape implements IDrawable { // (2) public void draw() { Systemoutprintln("Drawing a Shape"); } } class Painter { // (3) Top-level Class
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public Shape createShape() { // (4) Non-static Method return new Shape() { // (5) Extends superclass at (2) public void draw() { Systemoutprintln("Drawing a new Shape"); } }; } // } //
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As we cannot declare references of an anonymous class, the functionality of the class is only available through superclass references Usually it makes sense to override methods from the superclass Any other members declared in an anonymous class cannot be accessed directly by an external client