The Anonymous Class Conundrum

问题

I think I understand the basics of Anonymous classes but I'd like to clarify something. when I have a syntax such as this

class A
{
       class AnonymousClass1 Implements ActionListener{}
}
class A
{
     public A()
     {
        JButton btn = new JButton();

       btn.addActionListener( new ActionListener(){} );
     }
}

If the anonymous class is actually an inner class of class A, as in the first example: in theory, is the semantics right?

What happens exactly? I think when the java file is compiled, a .class file is created for the anonymous class so it can be referenced (but I couldn't find it). When an object of A is instantiated it creates a button object, btn then calls the addActionListener() method which actually passes something like this btn.addActionListener(new AnonymousClassOne()) AnonymousClassOne a generic name given by the compiler.

If not what happens? Thanks.

回答1:

Anonymous classes can be recognized by the dollar sign and a number after it - Class$1.class. These classes are just for your own convenience. Imagine this:

class SaveButtonListener implements ActionListener {
  ...
}

class OpenButtonListener implements ActionListener {
  ...
}

This is very tedious. So you can create the implementation right away with an anonymous class. The compiler gives the name prepending the dollar sign and some identifier after it.

What happens behind the scenes is that Java creates a new inner class with an auto-generated name.

Feel free to ask questions if you find my explanation messy. I am tired now.

回答2:

class A
{
    public A()
    {
        JButton btn = new JButton();
        btn.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                // ...
            }
        });
    }
}

is more or less rewritten by the compiler as

class A
{
    private class SomeCuteName implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            // ...
        }
    }

    public A()
    {
        JButton btn = new JButton();
        btn.addActionListener(new SomeCuteName());
    }
}

来源：https://stackoverflow.com/questions/7778556/the-anonymous-class-conundrum