Question

I was playing with some code to make a "closure like" construct ( not working btw )

Everything looked fine but when I tried to access a final local variable in the code, the exception InstantiationException is thrown.

If I remove the access to the local variable either by removing it altogether or by making it class attribute instead, no exception happens.

The doc says: InstantiationException

Thrown when an application tries to create an instance of a class using the newInstance method in class Class, but the specified class object cannot be instantiated. The instantiation can fail for a variety of reasons including but not limited to:

– the class object represents an abstract class, an interface, an array class, a primitive type, or void

– the class has no nullary constructor

What other reason could have caused this problem?

Here’s the code. comment/uncomment the class attribute / local variable to see the effect (lines:5 and 10 ).

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
class InstantiationExceptionDemo {
     //static JTextField field = new JTextField();// works if uncommented

    public static void main( String [] args ) {
        JFrame frame = new JFrame();
        JButton button = new JButton("Click");
        final JTextField field = new JTextField();// fails if uncommented

        button.addActionListener( new _(){{
            System.out.println("click " + field.getText());
        }});
    
        frame.add( field );
        frame.add( button, BorderLayout.SOUTH );
        frame.pack();frame.setVisible( true );
    
    }
}
class _ implements ActionListener {
    public void actionPerformed( ActionEvent e ){
        try {
            this.getClass().newInstance();
        } catch( InstantiationException ie ){
            throw new RuntimeException( ie );
        } catch( IllegalAccessException ie ){
            throw new RuntimeException( ie );
        }
    }
}

Is this a bug in Java?

edit

Oh, I forgot, the stacktrace ( when thrown ) is:

Caused by: java.lang.InstantiationException: InstantiationExceptionDemo$1
at java.lang.Class.newInstance0(Class.java:340)
at java.lang.Class.newInstance(Class.java:308)
at _.actionPerformed(InstantiationExceptionDemo.java:25)

Answer 1

Well, that makes sense.

Only your first instance of the _ class has access to the local variable. Subsequent instances can’t, unless you provide them with it (via constructor arg)

Constructor[] constructor = a.getClass().getDeclaredConstructors();
for (Constructor c : constructors) {
     System.out.println(c.getParameterTypes().length);
}

outputs 1. (a is the instance of your anonymous class)

That said, I don’t think this is a good way to implement closures. The initializer block is called at least once, without the need of it. I assume you are just playing around, but take a look at lambdaj. Or wait for Java 7 🙂