Java: Defining a generic method inside an anonymous class

Question

The following java code works fine.

public static void main(String[] arg){
    JPanel p = (new JPanel());
    p.add( new Object(){
        JButton f(JButton x){
            x.setEnabled(false);
            return x;
        }}.f(new JButton("B")) );
    JFrame w = new JFrame("W");
    w.add(p);
    w.pack();
    w.setVisible(true);
}

If the method is changed to its generic form the program fails.

public static void main(String[] arg){
    JPanel p = (new JPanel());
    p.add( new Object(){
        <T> T f(T x){
            x.setEnabled(false);
            return x;
        }}.f(new JButton("B")) );
    JFrame w = new JFrame("W");
    w.add(p);
    w.pack();
    w.setVisible(true);
}

Why does it fail? How can you define generic methods within an anonymous class?

This question is for learning purposes.

Solo

Please post the exact error message. – Dagney

Why do you think you want to do this? – Tweezers

This is the error (in Intellij): Error:(11, 18) java: cannot find symbol symbol: method setEnabled(boolean) location: variable x of type T. – Solo

A good first step would have been to copy the method into a normal, named class and ensure it compiles there. – Fu

Answer 1

The T generic doesn't derive explicitly from a class or an interface. So it derives from Objectand Object has no setEnabled() method.

If you want to use generic, you could specify a java.swing base type that has this method. For example : javax.swing.AbstractButton.

public static void main(String[] arg){
    JPanel p = (new JPanel());
    p.add( new Object(){
        <T extends AbstractButton> T f(T x){
            x.setEnabled(false);
            return x;
        }}.f(new JButton("B")) );
    JFrame w = new JFrame("W");
    w.add(p);
    w.pack();
    w.setVisible(true);
}

San

Answer 2

T's erasure is Object, which does not have setEnabled. It works if you give it a bound of JComponent, which definessetEnabled:

public static void main(String[] arg) {
    JPanel p = (new JPanel());
    p.add( new Object(){
        <T extends JComponent> T f(T x){
            x.setEnabled(false);
            return x;
        }}.f(new JButton("B")) );
    JFrame w = new JFrame("W");
    w.add(p);
    w.pack();
    w.setVisible(true);
}

Anaglyph