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More Java GUI Programming

Module by: Stephen Wong, Dung Nguyen. E-mail the authors

Summary: Java GUI programming including basic event handling, adapters, and the command and null-object design patterns.

1. JFrame with JButtons and event handlers (Frame2.java)

Note:

Source code for Frame2.java and the associated Frame2App.java is at the end of this module, or download the entire archive

Command Design Pattern

When a user clicks on a button, an ActionEvent object is delivered to the button. In order for the button to perform an application specific task, we must register an ActionEventListener to the button and program this event listener to perform the task we want. The ActionListener class is an example of what is called a "command" from the Command Design Pattern. The code to add an ActionListener to a button looks like:

myButton.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        // code to perform a task goes here...
    }
});

The example Frame2 (source code at end of the module) shows how to dynamically change the layout manager for a JFrame by calling its setLayout(...) method resulting in a dynamic re-arrangement of the GUI components in the JFrame.

2. JFrame with JButtons and Adapters (Frame3.java)

Note:

Source code for Frame3.java and the associated IView2World.java, Frame3controller.java and Frame3App.java is at the end of this module, or download the entire archive

Frame3 illustrates how to decouple the view from the rest of the world by having the view communicate to an interface called "adapter". A controller object, Frame3Controller connects the view Frame3 to the world by installing a concrete adapter into the view. The adapter is instantiated as anonymous inner object and has access to all of its outer object. The view does not and should not know anything about the world to which it is connected. This adds flexibility to the design.

Figure 1
Figure 1 (ViewAndController.png)

Null-Object Pattern

In much of the current programming practice, the special value null is often used as a flag to represent a gamut of different and often disconnected concepts such as emptiness and falseness. This can result in a lot of confusion and complex control structures in the code. In our view, null should only be used to denote the non-existence of an object. null is not an object and has no "intelligence" (i.e. behavior) for other objects to interact with. Therefore, whenever we work with a union of objects and discover that one or more of these objects may be referencing a null value, we almost always have to write code to distinguish null as a special case. To avoid this problem, we should think of adding a special object, called the null object, to the union with appropriate behavior that will allow us to treat all object references in a consistent and uniform way, devoid of special case consideration. This design pattern is called the null object pattern. We have used the null object pattern in one occasion: the EmptyNode to represent the empty state of a (mutable) list (LRStruct).

In Frame3, the view adapter, _v2wAdapter, is initialized to an (anonymous) IView2World object. It is there to guarantee that _v2wAdapter is always referencing a concrete IView2World object, and , since setV2WAdapter(...) only allows a non-null assignment, we can always call on _v2WAdapter to perform any method without checking for null.

Source code for above classes

Frame2.java


package view;

import java.awt.*;       // to use Container.
import java.awt.event.*; // to use WindowAdpater and WindowEvent.
import javax.swing.*;    // to use JFrame.

/**
 * A subclass of AFrame containing two JButtons that have event handler called
 * "commands" to dynamically switch layout managers when the buttons are clicked
 * upon.
 * @author D.X. Nguyen
 */
public class Frame2 extends AFrame {

    public Frame2(String title) {
        super(title);
    }

    protected void initialize() {
        Container cp = getContentPane();
        cp.setLayout(new FlowLayout());
        JButton jb1 = new JButton("Grid Layout");
        JButton jb2 = new JButton("Flow Layout");
        cp.add(jb1);
        cp.add(jb2);
        pack();

        /**
         * Registers an anonymous event handler for the clicking of button jb1.
         * When jb1 is clicked upon, this event handler will respond by
         * executing its actionPerformed(...) method.
         */
        jb1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                jb1Clicked(e);
            }
        });

        /**
         * Same as the above.
         */
        jb2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                jb2Clicked(e);
            }
        });
    }

    /**
     * Dynamically changes to a GridLayout with 2 rows and 1 column.
     */
    protected void jb1Clicked(ActionEvent e) {
        System.out.println("Set GridLayout...");
        getContentPane().setLayout(new GridLayout(2,1));
        validate();  // forces this frame to re-layout.
    }

    /**
     * Dynamically changes to FlowLayout.
     */
    protected void jb2Clicked(ActionEvent e) {
        System.out.println("Set FlowLayout...");
        getContentPane().setLayout(new FlowLayout());
        validate();  // forces this frame to re-layout.
    }
}

Frame2App.java



package app;

import javax.swing.*;
import view.*;

public class Frame2App {

    public static void main(String[] args) {
        JFrame f = new Frame2("A JFrame with 2 JButtons with event listeners");
        f.setVisible(true);
        f.setSize(300, 100);  // Guess what this does!
        f.setLocation(200, 200);  // Guess what this does!

        // Forces the frame to re-layout its components:
        f.validate();
    }
}

Frame3.java



package view;

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

/**
 * Same functionality as Frame2 but uses an adapter interface to communicate
 * with the outside world instead.  This design allows varying the communication
 * with the outside world.  How the outside world reacts to the events that
 * happen to the view is a variant behavior!
 * Uses the Null-Object Pattern to initialize the adapter avoiding checking for
 * null.
 * @author DXN
 */
public class Frame3 extends Frame2 {

    // Initializes the adapter to a null object:
    private IView2World _v2wAdapter = new IView2World() {
        public Object button1Clicked (ActionEvent e) {
            return null; // does nothing!
        }
        public Object button2Clicked (ActionEvent e) {
            return null; // does nothing!
        }
    };

    public Frame3(String title) {
        super(title);
    }

    public void setV2WAdapter(IView2World v2w) {
        if (null == v2w) {
            throw new IllegalArgumentException("Argument cannot be null!");
        }
        _v2wAdapter = v2w;
    }

    /**
     * Tells _v2wAdapter the button click event happens on jb1.
     */
    protected void jb1Clicked(ActionEvent e) {
        _v2wAdapter.button1Clicked(e);
    }

    /**
     * Tells _v2wAdapter the button click event happens on jb2.
     */
    protected void jb2Clicked(ActionEvent e) {
        _v2wAdapter.button2Clicked(e);
    }

}

IView2World.java



package view;

import java.awt.event.*;

/**
 * Adapter connecting Frame3 (the View) to the outside world.
 */
public interface IView2World {
    /**
     * Called by Frame3 when its button 1 is clicked.
     */
    public Object button1Clicked (ActionEvent e);

    /**
     * Called by Frame3 when its button 2 is clicked.
     */
    public Object button2Clicked (ActionEvent e);
}

Frame3Controller.java



package controller;

import view.*;
import java.awt.event.*;
import javax.swing.JFrame;
import java.awt.*; // For layout managers.

/**
 * The controller that instantiates a concrete IView2World object and connects
 * the world outside of Frame3 (the view) to Frame3.
 * The concrete adapter is implemented as an anonymous inner classe.
 */
public class Frame3Controller {

    public JFrame constructFrame()  {
        // Local variable needs to be final so that the local inner class
        // can access it:
        final Frame3 frame = new Frame3("View and Controller");

        /**
         * Install as concrete anonymous IView2World adapter in frame, without
         * frame knowing what the adapter does.
         */
        frame.setV2WAdapter(new IView2World()  {

            public Object button1Clicked (ActionEvent e) {
                frame.getContentPane().setLayout(new GridLayout(2,1));
                frame.validate();
                return null;
            }

            public Object button2Clicked (ActionEvent e) {
                frame.getContentPane().setLayout(new FlowLayout());
                frame.validate();
                return null;
            }
        });
        frame.setVisible(true);
        return frame;
    }
}

Frame3App.java



package app;

import view.*;
import controller.*;

/**
 * Instantiates the controller and builds the frame!
 */
public class Frame3App {

    public static void main(String[] args) {
        new Frame3Controller().constructFrame().validate();
    }
}

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What are tags? tag icon

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