Question

Possible Duplicate:
Creating an “object” of an interface

I am new to Java. Based on my understanding:

• We cannot instantiate an Interface. We can only instantiate a class which implements an interface.
• The new keyword is used to create an object from a class.

However, when I read the source codes of some Java programs, I found that sometimes an Interface is instantiated. For example:

Example 1:

JButtonObject.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {
        //codes
    }
});

Example 2:

SwingUtilities.invokeLater(new Runnable()
{
    public void run()
    {
        //codes
    }
});

In the example at above, ActionListener and Runnable are both Java interface. May I know why they can be instantiated in these codes?

What is the purpose of instantiating an Interface? Refer to this example, it seems that we should create an instance of a class which implement the interface.

Answer 1

That code does not instantiate an interface, but rather an anonymous class which implements ActionListener or Runnable.

An anonymous class is a local class without a name. An anonymous class
is defined and instantiated in a single succinct expression using the
new operator.

The code is creating an instance of ActionListener anonymously, which means the class does not actually have any name.

After compiling that class, you can see a class YourClass$1.class in the output. The $1 simply means that class is an anonymous class and the number 1 is generated by the compiler. When you have two anonymous classes, it will have something like YourClass$1.class and YourClass$2.class in the compiled classes.

See

• Anonymous Classes