Java Language Lambda Expressions Lambda - Listener Example

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Anonymous class listener

Before Java 8, it’s very common that an anonymous class is used to handle click event of a JButton, as shown in the following code. This example shows how to implement an anonymous listener within the scope of btn.addActionListener.

JButton btn = new JButton("My Button");
btn.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        System.out.println("Button was pressed");

Lambda listener

Because the ActionListener interface defines only one method actionPerformed(), it is a functional interface which means there’s a place to use Lambda expressions to replace the boilerplate code. The above example can be re-written using Lambda expressions as follows:

JButton btn = new JButton("My Button");
btn.addActionListener(e -> {
    System.out.println("Button was pressed");

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