java：将匿名内部类转换为具体类

Question

I have the following anonymous class

box = new JComboBox(filename);
...
    box.addItemListener(
                    new ItemListener() {
                        public void itemStateChanged(ItemEvent event )
                        {
                            if (event.getStateChange() == ItemEvent.SELECTED)
                                picture.setIcon(pics[box.getSelectedIndex()]);



                        }
                    }
               );

I'm trying to convert this to a concrete class with the following:

import java.awt.*; 
import java.awt.Event.*;
import javax.swing.*; 
    public class Gui extends JFrame { 
    ...

    public Gui () {
    box.addItemListener(new Handler());
    ...        
    add(box);    
    }

        class Handler extends ItemListener {

            public void itemStateChanged (ItemEvent e)  {
                if (e.getStateChange() == ItemEvent.SELECTED) {
                    picture.setIcon(pics[box.getSelectedIndex()]);

                }//end if 

            }//end itemstatechanged 
        }

Any ideas on what I'm doing wrong? the error i get says it can't find the symbol ItemListener

Answer 1

Since ItemListener is an interface , you need to use the keyword implements .

class Handler implements ItemListener {

Also, when I create my listeners I usually make them a private member variable in case I want to reuse them.

private ItemListener setIconHandler = new ItemListener() {
    public void itemStateChanged (ItemEvent e)  {
        if (e.getStateChange() == ItemEvent.SELECTED) {
            picture.setIcon(pics[box.getSelectedIndex()]);

        }//end if 

    }//end itemstatechanged 
};

This way you can reuse the listener, and you don't have to define a class that you are only going to use once or twice.

Also, for completeness, you need to fix your imports. This was not caught by me , but by other people answering the question. You should use import java.awt.event.ItemListener; instead of import java.awt.Event.*; .

Answer 2

您还需要为ItemListener和ItemEvent添加import语句。

Answer 3

Shouldn't it be implements ItemListener ?

And you'll need to import it via:

import java.awt.event.ItemListener;

import java.awt.Event.*; imports the inner classes of the class Event in java.awt.

Answer 4

ItemListener是一个接口，因此您需要implement它，而不是从它extend