



Inner Classes


Regular Inner Classes:

import java.awt.event.*;
import java.awt.*;
import java.applet.*;

public class tk extends Applet {
 
  TextField inputval, outputval;
  int sum, number;

  public void init(){
    add(new Label("Enter Number: "));
    inputval=new TextField(10);
    add(inputval);
    add(new Label("Total: "));
    outputval=new TextField(10);
    add(outputval);
    outputval.setEditable(false);
    sum=0;
    inputval.addActionListener(new AA());
  }

  class AA implements ActionListener{
    public void actionPerformed(ActionEvent e){
      number = Integer.parseInt(e.getActionCommand());
      sum = sum + number;
      inputval.setText("");
      outputval.setText(""+sum);
    }
  }
}
//<applet code=tk.class height=100 width=210></applet>





The action listener for a TextField is fired only when you press the
"enter" key. The getActionCommand() method simply returns a String which
in the case of textfields is the contents of the textfield.

Anonymous Inner Classes:

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class ev extends Applet{
  Choice b;

  public void init(){
    setBackground(Color.cyan);
    setLayout(null);
    Dimension d = new Dimension();
    d=getSize();
    b=new Choice();
    b.addItem("red");
    b.addItem("green");
    b.addItem("blue");
    b.addItem("yellow");
    b.addItem("magenta");
    b.addItem("black");
    b.setBackground(Color.red);
    b.setForeground(Color.cyan);
    b.addItemListener(new ItemListener(){
      public void itemStateChanged(ItemEvent e){
        String s = b.getSelectedItem();
        Color c = Color.white;
        if(s.equals("red")) c=Color.red;
        else if(s.equals("green")) c=Color.green;
        else if(s.equals("blue")) c=Color.blue;
        else if(s.equals("yellow")) c=Color.yellow;
        else if(s.equals("magenta")) c=Color.magenta;
        else if(s.equals("black")) c=Color.black;
        setBackground(c);
        repaint();
      }
    });
    b.setBounds(d.width/2-30, d.height/2-10, 60, 20);
    add(b);
  }
}
//<applet code=ev.class height=200 width=200></applet>





You will notice that there is no name given to the ItemListener class.
Normally you would see something like:
  b.addItemListener(new IL());



Then somewhere else you'd see a class definition which begins like:
  class IL implements ItemListener{



But using inner classes all this gets skipped or abbreviated. You will
notice that the parenthesis which is opened in the addItemListener line
isn't closed until thirteen lines later. The ItemListener interface contains
only one method: itemStateChanged().