what is the difference between an inner class and a local inner class in java?

问题

If a class is a local inner class, does this mean that it is inside a method of another class or does it mean that it is just defined in another method somewhere.

For example, in the code below, is the MenuListener considered an inner local class?

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class MenuDemo extends JFrame{
private Container c;
private ImageIcon[] images = new ImageIcon[5];
private JLabel picture;
private JPanel mainPanel;
private JMenuBar menuBar;
private JMenu menu;
private JMenuItem bird,cat,dog,rabbit,pig;

public MenuDemo() {
    super("Menu Demo");

    c = getContentPane();
    c.setLayout(new BorderLayout());

    mainPanel = new JPanel();

    // Store the animal pictures in an array.
    for (int i=0; i<5; i++){
        images[i] = new ImageIcon("image" + i + ".gif");
    }
    //Set up the picture label.
    picture = new JLabel(images[0]);
    picture.setPreferredSize(new Dimension(177,122));
    mainPanel.add(picture, BorderLayout.CENTER);
    c.add(mainPanel);
    buildMenuBar();
    this.setJMenuBar(menuBar);
    pack();
    setVisible(true);
}

private void buildMenuBar(){
    MenuListener listener = new MenuListener();
    menuBar = new JMenuBar();

    menu = new JMenu("Animals");
    menu.setMnemonic(KeyEvent.VK_A);
    menuBar.add(menu);

    bird = new JMenuItem("Bird", KeyEvent.VK_B);
    bird.addActionListener(listener);
    menu.add(bird);

    cat = new JMenuItem("Cat", KeyEvent.VK_C);
    cat.addActionListener(listener);
    menu.add(cat);

    dog = new JMenuItem("Dog", KeyEvent.VK_D);
    dog.addActionListener(listener);
    menu.add(dog);

    rabbit = new JMenuItem("Rabbit", KeyEvent.VK_R);
    rabbit.addActionListener(listener);
    menu.add(rabbit);

    pig = new JMenuItem("Pig", KeyEvent.VK_P);
    pig.addActionListener(listener);
    menu.add(pig);
}

private class MenuListener implements ActionListener {
    public void actionPerformed(ActionEvent e){
        if (e.getSource() == bird)
            picture.setIcon(images[0]);
        else if (e.getSource() == cat)
            picture.setIcon(images[1]);
        else if (e.getSource() == dog)
            picture.setIcon(images[2]);
        else if (e.getSource() == rabbit)
            picture.setIcon(images[3]);
        if (e.getSource() == pig)
            picture.setIcon(images[4]);
    }
}

public static void main(String[] args){
    MenuDemo m = new MenuDemo();
    m.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}   
}

回答1:

According to some official Oracle Java tutorial, a local inner class is a class that is defined in a code block rather than a part of another class' definition. This means a class defined inside a method or in an initializer block.

Following that definition, MenuListener is not a local inner class -- it is just an inner class.

So:

public class Outer {

    public class Inner1 {
    }

    public static class Inner2 {
    }

    public void method(){

        class LocalInner{
        }

        LocalInner yesThisActuallyCompiles = new LocalInner();
    }
}

回答2:

If a class is a local inner class, does this mean that it is inside a method

Yes.

of another class

No. Of a class, but any method has to be in a class.

or does it mean that it is just defined in another method somewhere.

Yes.

There not much difference between these, except for the difference between 'in another class' and 'somewhere'. It's not really clear what you're actually asking.

回答3:

Inner Class/ Member Class

A non-static class that is created inside a class but outside a method is called member inner class.

class TestMemberOuter1{  
 private int data=30;  
 class Inner{  
  void msg(){System.out.println("data is "+data);}  
 }  
 public static void main(String args[]){  
  TestMemberOuter1 obj=new TestMemberOuter1();  
  TestMemberOuter1.Inner in=obj.new Inner();  
  in.msg();  
 }  
}

Local Inner Class

A class i.e. created inside a method is called local inner class in java. If you want to invoke the methods of local inner class, you must instantiate this class inside the method

   public class localInner1{  
     private int data=30; 
     void display(){  
      class Local{  
       void msg(){System.out.println(data);}  
      }  
      Local l=new Local();  
      l.msg();  
     }  
     public static void main(String args[]){  
      localInner1 obj=new localInner1();  
      obj.display();  
     }  
    }

来源：https://stackoverflow.com/questions/10392725/what-is-the-difference-between-an-inner-class-and-a-local-inner-class-in-java