Interface ActionListener
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How to Write an Action Listener
Action listeners are probably the easiest — and most common — event handlers to implement. You implement an action listener to define what should be done when an user performs certain operation.
An action event occurs, whenever an action is performed by the user. Examples: When the user clicks a button, chooses a menu item, presses Enter in a text field. The result is that an actionPerformed message is sent to all action listeners that are registered on the relevant component.
To write an Action Listener, follow the steps given below:
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Declare an event handler class and specify that the class either implements an ActionListener interface or extends a class that implements an ActionListener interface. For example:
In general, to detect when the user clicks an onscreen button (or does the keyboard equivalent), a program must have an object that implements the ActionListener interface. The program must register this object as an action listener on the button (the event source), using the addActionListener method. When the user clicks the onscreen button, the button fires an action event. This results in the invocation of the action listener's actionPerformed method (the only method in the ActionListener interface). The single argument to the method is an ActionEvent object that gives information about the event and its source.
Let us write a simple program which displays how many number of times a button is clicked by the user. First, here is the code that sets up the TextField , button and numClicks variable:
In the above example, the event handler class is AL which implements ActionListener.
We would like to handle the button-click event, so we add an action listener to the button b as below:
In the above code, Button b is a component upon which an instance of event handler class AL is registered.
Now, we want to display the text as to how many number of times a user clicked button. We can do this by writing the code as below:
Now, when the user clicks the Button b, the button fires an action event which invokes the action listener's actionPerformed method. Each time the user presses the button, numClicks variable is appended and the message is displayed in the text field.
Here is the complete program(AL.java):
More Examples: Beeper program example is available in this trail's introduction to events, Introduction to Event Listeners. You can find the entire program in Beeper.java . The other example described in that section, MultiListener.java , has two action sources and two action listeners, with one listener listening to both sources and the other listening to just one.
The Action Listener API
Because ActionListener has only one method, it has no corresponding adapter class.
| Method | Purpose |
|---|---|
| actionPerformed(actionEvent) | Called just after the user performs an action. |
Examples that Use Action Listeners
The following table lists some of the many examples that use action listeners.
SWING — ActionListener Interface
The class which processes the ActionEvent should implement this interface. The object of that class must be registered with a component. The object can be registered using the addActionListener() method. When the action event occurs, that object’s actionPerformed method is invoked.
Interface Declaration
Following is the declaration for java.awt.event.ActionListener interface −
Interface Methods
void actionPerformed(ActionEvent e)
Invoked when an action occurs.
Methods Inherited
This interface inherits methods from the following interfaces −
ActionListener Example
Create the following Java program using any editor of your choice in say D:/ > SWING > com > tutorialspoint > gui >
SwingListenerDemo.java
Compile the program using the command prompt. Go to D:/ > SWING and type the following command.
If no error occurs, it means the compilation is successful. Run the program using the following command.
What does a addActionListener do?
As far as I understand, I create a button on which it is written "Clear". Then I have to associate an action with this button (what will happen if the button is pressed) and it is done by addActionListener . Is it right?
But what I do not understand is where the action is specified. The press of the button should clear text area and, as far as I understand, there should be somewhere a code which clear the text area. But in the given example there is only "this" in the arguments of the addActionListener() .
How does the program know that it should clear the text area when the button is pressed?