Java 8 Lambdas

Chapter 2. Lambda Expressions

The biggest language change in Java 8 is the introduction of lambda expressions—a compact way of passing around behavior. They are also a pretty fundamental building block that the rest of this book depends upon, so let’s get into what they’re all about.

Your First Lambda Expression

Swing is a platform-agnostic Java library for writing graphical user interfaces (GUIs). It has a fairly common idiom in which, in order to find out what your user did, you register an event listener. The event listener can then perform some action in response to the user input (see Example 2-1).

Example 2-1. Using an anonymous inner class to associate behavior with a button click

button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent event) {
        System.out.println("button clicked");
    }
});

In this example, we’re creating a new object that provides an implementation of the ActionListener class. This interface has a single method, actionPerformed, which is called by the button instance when a user actually clicks the on-screen button. The anonymous inner class provides the implementation of this method. In Example 2-1, all it does is print out a message to say that the button has been clicked.

Note

This is actually an example of using code as data—we’re giving the button an object that represents an action.

Anonymous inner classes were designed to make it easier for Java programmers to pass around code as data. Unfortunately, they don’t ...