Desk Checked

Java, “this” and Inner/Anonymous Classes

June 12th, 2007

Brushing up on my Java for the first time in a while and I came across something fairly trivial but new to me. If you’re a seasoned Javanaut, you’ll have to indulge me for a second .

When writing a method within an anonymous/inner class in Java, the “this” keyword obviously refers to the anonymous/inner class itself. What if we needed a reference to the instance of the outer class?

public class OuterJFrame extends JFrame {
    public OuterJFrame() {
        super("Outer/Inner/Anonymous Classes Test");
        JButton button = new JButton("Click me!");
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JFileChooser chooser = new JFileChooser();
                // type error! "this" refers to our ActionListener instance
                chooser.showOpenDialog(this);
            }
        });

    }
}

I stumbled across the answer to this little dilemma in the Java SE documentation: it is possible to refer to refer to the instance of the outer class using OuterClass.this:

public class OuterJFrame extends JFrame {
    public OuterJFrame() {
        super("Outer/Inner/Anonymous Classes Test");
        JButton button = new JButton("Click me!");
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JFileChooser chooser = new JFileChooser();
                // fixed! this works as expected
                chooser.showOpenDialog(OuterJFrame.this);
            }
        });

    }
}

Obviously this neat little syntactic tango won’t work with static inner classes, because they are not associated with instances of the outer class. Still useful, yeah?

Categories: Java, Software Development |