20-CS-4003-001 Organization of Programming Languages Fall 2018
Class services

Lambda calculus, Type theory, Formal semantics, Program analysis

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Use Services Provided by Other Classes

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

class java_4Frame {
   JFrame f;
   AListener l;
   JTextArea text;
   JButton go;
   int i;

   public java_4Frame () {
      f = new JFrame();
      l = new AListener(this);
      f.setLayout(new BorderLayout());
            new JScrollPane(text=new JTextArea()));
      JPanel p = new JPanel();
      p.setLayout(new FlowLayout());
      p.add(go = new JButton("Run it"));
      f.add("South", p);
      i = 1;

   public void action (Object evt) {
      if (((ActionEvent)evt).getSource() == go) {
         text.append(i+" ");
         if ((i++ % 10) == 0) text.append("\n");
         int p = text.getDocument().getLength();

public class java_4 
   extends Applet implements ActionListener {
   JButton go;

   public void init () {
      setLayout(new FlowLayout());
      add(go = new JButton("Applet"));

   public void actionPerformed (ActionEvent evt) {
      java_4Frame jf = new java_4Frame();

class AListener implements ActionListener {
   JButton obj;
   java_4Frame cls;

   public AListener (java_4Frame cls) { 
      this.cls = cls; 

   public void addAListener (JButton obj) { 
      this.obj = obj;

   public void actionPerformed (ActionEvent evt) {
 -  In the previous example it is OK to create a JFrame object to provide JFrame services instead of inheriting from JFrame because the JFrame object does not need to send results back to java_3Frame. However, in numerous cases a callback is necessary. For example, when the JButton is clicked, the event it causes must be sent back to java_3Frame to be processed, in that case by actionPerformed. The ActionListener interface provides a guarantee that the appropriate method will be available to receive this callback. But even this is not necessary as the code to the left shows. Class java_4Frame neither extends a class nor implements an interface yet it acts on a button press and it displays a window. To achieve this another class called AListener is written. An object l of this class is instantiated in the constructor of java_4Frame in the same way that JFrame object f is except with this as an argument. Then l.addAlistener(go) is invoked instead of go.addActionListener(this) as in the previous code example. This time java_4Frame provides the guarantee that a callback procedure, namely action, will be available to the AListener class.