Swing

Handling Events

In the previous chapter, we learn how to create frames with labels, buttons and panels. However, when the button is clicked it does nothing. In this chapter we will learn how to handle events for buttons when it is clicked. The technique we use is called event listening, and it's one of the most important aspect of writing swing programs.
An event is an object that's generated when the user does something with one of the user interface components. For examples, a click of a button, selection of an item, a drag of the mouse over a label and so on, all those are events. Then those event objects are passed to a special method we create called event listener. Hence, the event listener determine exactly what type of event occurred, and respond accordingly.

Look at the link (" Swing Method & constructor Table " ) below for a list of event and listener class with description.

When working with the swing class, we need to import javax.swing.*; for the swing components, import java.awt.*; for the color class, and import java.awt.event.*; for the event class.

Lets see a program that will demonstrate a click of a button changes the background and text colors. The method for changing the background color is
setBackground(name of color) and the method for changing the text color is setForeground(name of color).

Here is a list of the color:

Color.BLACK
Color.RED
Color.PINK
Color.YELLOW
Color.GREEN
Color.ORNAGE
Color.WHITE
Color.CYAN
        .
        .
        .
        .

Example)
 

/*
    The program demonstrates handling of events
    It has 3 buttons, each buttons represent a different color
    A push of a button changes the window background color

*
*/

import javax.swing.*; // Needed for the swing class
import java.awt.*; // Needed for color class
import java.awt.event.*; // Needed for event listener interface

public class ChangeBackground extends JFrame
{
    JLabel msg;
    JButton blackButton;
    JButton yellowButton;
    JButton greenButton;
    JButton pinkButton;
    JPanel panel;

public ChangeBackground()
{
    // Sets the title of the window

    setTitle("Change Background Window");

    // Sets the size of the window

    setSize(230, 400);

    // Sets the action when the window is close

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    // Creates a label that print on the window

    msg = new JLabel("Click a button to change the color");

    // Creates the 4 button with names associate to it

    blackButton = new JButton("Black");
    yellowButton = new JButton("Yellow");
    greenButton = new JButton("Green");
    pinkButton = new JButton("Pink");

    // Register an event listener with all the buttons

    blackButton.addActionListener(new blackButtonListener());
    yellowButton.addActionListener(new yellowButtonListener());
    greenButton.addActionListener(new greenButtonListener());
    pinkButton.addActionListener(new pinkButtonListener());

    /*
        Now we will create a panel
        Then adds all the buttons to the panel & the label

    *
    */

    panel = new JPanel();
    panel.add(msg);
    panel.add(blackButton);
    panel.add(yellowButton);
    panel.add(greenButton);
    panel.add(pinkButton);

    // Adds the panel to the content pane ( The JFrame window )

    add(panel);

    // Sets the window to be visible

    setVisible(true);

    }

    /*
        Here handles the event when the user clicks the button
        The background color will changes to black

    *
    */

    public class blackButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            // Sets the panel's background to black

            panel.setBackground(Color.BLACK);

            // Sets the label's text to blue

            msg.setForeground(Color.BLUE);

        }
    }

    // The background color changes to yellow

    public class yellowButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            // Sets the panel's background to yellow

            panel.setBackground(Color.YELLOW);

            // Sets the label's text to red

            msg.setForeground(Color.RED);

        }
    }

    // The background color changes to green

    public class greenButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            // Sets the panel's background to green

            panel.setBackground(Color.GREEN);

            // Sets the label's text to black

            msg.setForeground(Color.BLACK);

        }
    }

    // The background color changes to pink

    public class pinkButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            // Sets the panel's background to pink

            panel.setBackground(Color.PINK);

            // Sets the label's text to red

            msg.setForeground(Color.RED);

        }
    }

    public static void main(String[] args)
    {
        new ChangeBackground();
    }

}
 

ClickHere to download ChangeBackground.java

The above example created 3 buttons, each button has its own event. A click of a button calls the event listener, then inside the event listener it changes the text and background color. Although, sometimes it would be a lot of work to write each event listener out hence we can use a else-if statement to determine which button is clicked with the help of two unique methods:

e.getSource(); --- Returns a reference to the object that generated this event

e.getActionCommand(); --- Returns the action command for this event as a String

Example)

/*
    The program gets a name from the user using e.getSource
    Then it outputs the name to the screen

*
*/

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

public class NameFrame extends JFrame
{
    JPanel panel;
    JButton okbutton;
    JLabel msg;
    JTextField txtfield;

    public NameFrame()
    {
        // Sets the title of the frame

        setTitle("Name Window");

        // Sets the size of the window

        setSize(300, 400);

        // Sets the action when window is close

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Sets the visibility of the window

        setVisible(true);

        // Creates the panel

        panel = new JPanel();

        // Creates a label

        msg = new JLabel("Enter a name: ");

        // Creates the text field

        txtfield = new JTextField(15);

        // Creates the button

        okbutton = new JButton("OK");

        // Adds to the panel

        panel.add(msg);
        panel.add(txtfield);
        panel.add(okbutton);

        // Add the panel to the content pane

        add(panel);

        // Creates the action listener

        okbutton.addActionListener(new ButtonListener());

   }

    // Handles the event if a button is clicked

    public class ButtonListener implements ActionListener
   {
        public void actionPerformed(ActionEvent e)
        {
            // If ok button is clicked, gets the input name

            if(e.getSource()==okbutton)
            {
                // Gets the name from the text field

                String name = txtfield.getText();

                // If the text field is empty return a message

                if(name.length() == 0)
                {
                    JOptionPane.showMessageDialog(null, "You didn't enter anything");
                }
                else
                {
                    JOptionPane.showMessageDialog(null, "Hello!! " + name);

                }
                txtfield.requestFocus();
            }
        }
    }

    public static void main(String[] args)
    {   
        new NameFrame();

    }
}

ClickHere to download NameFrame.java

The above program used the requestFocus() method, what it does is when the user enters a name, clicks the ok button, then the JOptionPane pops out and prints out a message. When the use clicks the ok button on the JOptionPane again, with the requestFocus() method, the focus will automatically go back to the text field. However, if we didn't put the requestFocus() method, the focus will go back to the ok button. It might be a little confusing just reading it, thus download/copy the program run it and test it out for yourself with the focus method then run once again without the focus method.

Creating JTextArea

A text area simply is a text field that allows the user to enter more than one line of text. The text area can use the scroll bar to help the user to see the entire text. To create a text area we must use two classes. First, we use the JTextArea class to create the text area, then we would use the JScrollPane class to create scroll bars for the text area. Lets see a basic example on how to create a JTextArea.

Note: For more GUI examples please refer to the Java Example, GUI on the home page or simply click here. As for the JTextArea & JScrollPane methods and constructors click on the link below " Swing Method & Constructor Table. "

/*
    The program creates a JTextArea with scroll bars

*
*/

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

public class TextArea extends JFrame
{
    JPanel panel;
    JButton ok;
    JTextArea txt;
    JScrollPane scroll;

    public TextArea()
    {
        // Sets the title

        setTitle("Text Area");

        // Sets the size

        setSize(300, 400);

        // Sets the event when the window close

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Creates a panel

        panel = new JPanel();

        // Creates an empty Text Area with 10 rows and 20 columns

        txt = new JTextArea("", 10, 20);

        // Creates a button

        ok = new JButton("OK");

        // Create a scroll pane in the text area

        scroll = new JScrollPane(txt, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);

        // Adds the scroll to the panel

        panel.add(scroll);

        // Adds the button to the panel

        panel.add(ok);

        // Adds the panel to the content pane

        add(panel);

        // Sets the window to be visible

        setVisible(true);

        // Creates an action listener

        ok.addActionListener(new ButtonListener());

    }

    public class ButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            if(e.getSource()==ok)
            {
                // Appends Hello World to the text area whenever ok button is clicked

                txt.append("Hello World!!!\n");
            }

        }
    }

    public static void main(String[] args)
    {
        new TextArea();
    }
}

ClickHere to download TextArea.java