visual basic barcode printing 8: Inner Classes in Java

Printing PDF 417 in Java 8: Inner Classes

8: Inner Classes
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Let s look at what s in the preceding code:
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We define two classes, Popcorn and Food. Popcorn has one method, pop(). Food has one instance variable, declared as type Popcorn. That s it for Food. Food has no methods.
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And here s the big thing to get:
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The Popcorn reference variable refers not to an instance of Popcorn, but
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to an instance of an anonymous (unnamed) subclass of Popcorn. Let s look at just the anonymous class code:
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2. Popcorn p = new Popcorn() { 3. public void pop() { 4. System.out.println("anonymous popcorn"); 5. } 6. };
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Line 2 Line 2 starts out as an instance variable declaration of type Popcorn. But instead of looking like this:
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Popcorn p = new Popcorn(); //
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notice the semicolon at the end
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there s a curly brace at the end of line 2, where a semicolon would normally be
Popcorn p = new Popcorn() { //
a curly brace rather than semicolon
You can read line 2 as saying, Declare a reference variable, p, of type Popcorn. Then declare a new class which has no name, but which is a subclass of Popcorn. And here s the curly brace that opens the class definition
Anonymous Inner Classes
Line 3 Line 3, then, is actually the first statement within the new class definition. And what is it doing Overriding the pop() method of the superclass Popcorn. This is the whole point of making an anonymous inner class to override one or more methods of the superclass! (Or to implement methods of an interface, but we ll save that for a little later.) Line 4 Line 4 is the first (and in this case only) statement within the overriding pop() method. Nothing special there. Line 5 Line 5 is the closing curly brace of the pop() method. Again, nothing special there. Line 6 Here s where you have to pay attention: line 6 includes a curly brace closing off the anonymous class definition (it s the companion brace to the one on line 2), but there s more! Line 6 also has the semicolon that ends the statement started on line 2. The statement where it all began. The statement declaring and initializing the Popcorn reference variable. And what you re left with is a Popcorn reference to a brand new instance of a brand new, just-in-time, anonymous (no name) subclass of Popcorn.
The closing semicolon is often hard to spot. So you might see code on the exam like this:
2. Popcorn p = new Popcorn() { 3. public void pop() { 4. System.out.println( anonymous popcorn ); 5. } 6. } // Missing the semicolon needed to end statement on 2!! 7. Foo f = new Foo();
You ll need to be especially careful about the syntax when inner classes are involved, because the code on Line 6 looks perfectly natural. We re not used to seeing semicolons following curly braces (the only other time it happens is with shortcut array initializations).
8: Inner Classes
Polymorphism is in play when anonymous inner classes are involved. Remember that, as in the preceding Popcorn example, we re using a superclass reference variable type to refer to a subclass object. What are the implications You can only call methods on an anonymous inner class reference that are defined in the reference variable type! This is no different from any other polymorphic references, for example,
class Horse extends Animal{ void buck() { } } class Animal { void eat() { } } class Test { public static void main (String[] args) { Animal h = new Horse(); h.eat(); // Legal, class Animal has an eat() method h.buck(); // Not legal! Class Animal doesn t have buck() } }
So on the exam, you must be able to spot an anonymous inner class that rather than overriding a method of the superclass defines its own new method. The method definition isn t the problem, though; the real issue is how do you invoke that new method The reference variable type (the superclass) won t know anything about that new method (defined in the anonymous subclass), so the compiler will complain if you try to invoke any method on an anonymous inner class reference that is not in the superclass class definition.
Check out the following, illegal code:
class Popcorn { public void pop() { System.out.println("popcorn"); } } class Food { Popcorn p = new Popcorn() { public void sizzle() { System.out.println("anonymous sizzling popcorn"); } public void pop() {
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