在Java中，内部类有四种不同的类型。这篇文章用四个简单的例子来阐明它们。

1.静态嵌套类

class Outer {
 static class Inner {
  void go() {
   System.out.println("Inner class reference is: " + this);
  }
 }
}
 
public class Test {
 public static void main(String[] args) {
  Outer.Inner n = new Outer.Inner();
  n.go();
 }
}

输出： Inner class reference is: Outer$Inner@19e7ce87

2.成员内部类

成员类是特定于实例的。 它可以访问所有的方法，字段和其他的外部类的引用。

public class Outer {
    private int x = 100;
 
    public void makeInner(){
        Inner in = new Inner();
        in.seeOuter();
    }
 
    class Inner{
        public void seeOuter(){
            System.out.println("Outer x is " + x);
            System.out.println("Inner class reference is " + this);
            System.out.println("Outer class reference is " + Outer.this);
        }
    }
 
    public static void main(String [] args){
     Outer o = new Outer();
        Inner i = o.new Inner();
        i.seeOuter();
    }
}

输出：

Outer x is 100
Inner class reference is Outer$Inner@4dfd9726
Outer class reference is Outer@43ce67ca

3.方法内部类

public class Outer {
 private String x = "outer";
 
 public void doStuff() {
  class MyInner {
   public void seeOuter() {
    System.out.println("x is " + x);
   }
  }
 
  MyInner i = new MyInner();
  i.seeOuter();
 }
 
 public static void main(String[] args) {
  Outer o = new Outer();
  o.doStuff();
 }
}

输出：

x is outer

public class Outer {
 private static String x = "static outer";
 
 public static void doStuff() {
  class MyInner {
   public void seeOuter() {
    System.out.println("x is " + x);
   }
  }
 
  MyInner i = new MyInner();
  i.seeOuter();
 }
 
 public static void main(String[] args) {
  Outer.doStuff();
 }
}

4.匿名内部类

当你在一个GUI程序中，为一个widget添加一个动作监听器的时候常被用到。

button.addActionListener(new ActionListener(){
     public void actionPerformed(ActionEvent e){
         comp.setText("Button has been clicked");
     }
});