Introduction
This article explains listeners supported by Swing components. The NetBeans IDE is used to create "ComponentListener".
Common Listeners
The following listeners are common to all Swing components.
- KeyListener
fires a key event when a key is pressed by the user.
- ComponentListener
changes the size, visibility or position of components.
- HierarchyBoundsListener
occurs when the hierarchy of a component changes in size or movement.
- mousemotion Listener
shows an alert when the mouse cursor changes its position.
- FocusListener
provides a focus to the components.
- HierarchyListener
occurs when the hierarchy of components change.
- MouseListener
generates an event when the mouse is moved, clicked, released or pressed.
- mousewheel listener
shows an alert when the mouse wheel position changes.
Component Listener in Java
This listener works on components of Swing. A component is an object that has a graphical representation that's shown on the display. Components are like buttons, checkboxes, textfields, etcetera.
The following procedure is required to write a Component Listener as described below:
- First declare a class that implements its.
- Now define the component on which you want to fire this listener. For example: frame, panel, label, checkbox, and so on.
- Now add a component to your defined components. For example:
...
frm.addComponentListener(this);
- Finally, catch the various events of these components by using four methods of Component Listener as shown below:
public void componentShown(ComponentEvent ey)
{
showMessage(ey.getClass().getName() + ">Component Visible");
}
public void componentHidden(ComponentEvent ey)
{
showMessage(ey.getComponent().getClass().getName() + ">Component Hidden");
}
public void componentResized(ComponentEvent ey)
{
showMessage(ey.getComponent().getClass().getName() + ">Component Resized ");
}
public void componentMoved(ComponentEvent ey)
{
showMessage(ey.getComponent().getClass().getName() + ">Component Moved");
}
Let's use an example
In this example; we show the use of component listener. In this program we create a window that contains a panel that has a label and a check box. The visibility of the label is controlled by a check box. A text area is used to display the message when any component fires an event.
The following procedure is included in this example.
Step 1
Open the NetBeans IDE.
Step 2
Now go to the File Menu and choose "New Project", a new window appears like this.
Step 3
Choose "Java"-> "Java application" as shown below:
Step 4
Click on "Next" and then type your project name. I specified "ComponentDemo" as shown below:
Step 5
Click on "Finish"; your project is set up with a default Java file "ComponentDemo.java" as shown below.
Step 6
Now change the code and write the following code in the "ComponentDemo.java" file.
ComponentDemo.java
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class ComponentDemo extends JPanel implements ItemListener,ComponentListener {
static JFrame frm;
JTextArea print;
JLabel labl;
String blankline = "\n";
public ComponentDemo() {
super(new BorderLayout());
print = new JTextArea();
print.setEditable(false);
JScrollPane scrll = new JScrollPane(print);
scrll.setPreferredSize(new Dimension(350, 200));
JPanel pnl = new JPanel(new BorderLayout());
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