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Understanding Inner Classes in Java: Static and Non-Static Examples

DESCRIPTION

This guide provides an overview of inner classes in Java, including static inner classes, member inner classes, local inner classes, and anonymous inner classes. It demonstrates how each type of inner class interacts with its enclosing class fields, including challenges such as accessing non-static fields from a static context. Sample code snippets highlight the functionality of each class type, errors associated with improper access, and best practices for using inner classes effectively.

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Understanding Inner Classes in Java: Static and Non-Static Examples

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  1. Inner class

  2. Static inner class 範例程式1: public class InnerClass1 {   static int i;   final int j = 1; int k;   public static void main(String[] args) { Nested n = new Nested(); n.amethod();   } static class Nested {     public void amethod() { System.out.println("i = " + i); System.out.println("j = " + j); System.out.println("k = " + k);     }   } } • Static inner class: • 只能存取外部類別的static field • Inner class 有static 關鍵字 可以如何修改?? • "InnerClass1.java": Error #: 308 : non-static variable j cannot be referenced from a static context • "InnerClass1.java": Error #: 308 : non-static variable k cannot be referenced from a static context

  3. MEMBER INNER CLASSES 範例程式2: public class InnerClass2 {   static int i = 10;   final int j = 1; int k;   public static void main(String[] args) { Nested c = new InnerClass2().new Nested(); c.amethod();   }  class Nested {     public void amethod() { System.out.println("i = " + i); System.out.println("j = " + j); System.out.println("k = " + k);     }   } } • Member inner classes: • 能存取外部類別的所有field • Inner class 沒有static 關鍵字 執行結果: i = 10 j = 1 k = 0

  4. LOCAL INNER CLASSES public class InnerClass3 {    public static void main(String[] args) {     InnerClass3 in = new InnerClass3(); in.amethod();   }   public void amethod() {     final staticint i = 100; finalint j = 10;     class NestedC {       public void bmethod() { System.out.println("i = " + i); System.out.println("j = " + j);       }     }     new NestedC().bmethod();   } } • Local inner classes • 宣告在method中的類別，故Local inner classes 不能有static，也不能有modifier(public、protected 、private) • 可以存取外部類別的所有變數，但是不能存取method的local變數 "InnerClass3.java": Error #: 480 : local variable j is accessed from within inner class; needs to be declared final

  5. ANONYMOUS INNER CLASSES • 沒有名稱的類別，匿名類別最常用在event的處理上

  6. import java.awt.*; import java.awt.event.*; public class InnerClass4 extends Frame {   public InnerClass4() {     Button b = new Button("Click me!"); b.addActionListener(       new ActionListener() {         public void actionPerformed(ActionEvent e) { System.out.println("Click me!");         }       }     ); this.add(b); this.setSize(200, 100); this.show(); }   public static void main(String[] args) {     InnerClass4 in = new InnerClass4();   } } • anonymous inner class : • overriding methods • 存取外部類別的所有變數

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