anon inner class

[stuckonjava]

public class MyApp2{
public MyApp2(){

JFrame frame = new JFrame ("My App");
JTextField tf = new JTextField();
JButton button = new JButton("ok");
button.addActionListener(
new ActionListener(){
public void actionPerformed(ActionEvent event){
System.out.println(tf.getText());
}
}


);
}
}


Assume everything has been imported etc.

Can someone explain why the textfield tf has to be declared as final, I don't understand why.

Thanks for your time

[Tolls]

This ensures the reference tf does not change, and so is not accidentally reflected in the inner class.
Something along the lines of:

JTextField tf = new JTextField();
myPanel.add(tf);
JButton button = new JButton("ok");
button.addActionListener(
    new ActionListener(){
        public void actionPerformed(ActionEvent event){
            System.out.println(tf.getText());
        }
    }
);
tf = new JTextField();
myPanel.add(tf);

It's possible that the action listener could be referring to the wrong text field.
So the desgners way back when in the 90s decided to avoid that whole question and simply insist that these references are final.

I wish I had the documentation on it that I read years ago, so I wouldn't have to rely on pure memory here...

[stuckonjava]

This ensures the reference tf does not change, and so is not accidentally reflected in the inner class.
Something along the lines of:

JTextField tf = new JTextField();
myPanel.add(tf);
JButton button = new JButton("ok");
button.addActionListener(
    new ActionListener(){
        public void actionPerformed(ActionEvent event){
            System.out.println(tf.getText());
        }
    }
);
tf = new JTextField();
myPanel.add(tf);

It's possible that the action listener could be referring to the wrong text field.
So the desgners way back when in the 90s decided to avoid that whole question and simply insist that these references are final.

I wish I had the documentation on it that I read years ago, so I wouldn't have to rely on pure memory here...

Thanks!!!

[Norm]

The reason could be because the variable is defined locally to method/constructor. The compiler wants a way to keep its value past the exiting of the method.

[stuckonjava]

The compiler wants a way to keep its value past the exiting of the method.

What do you mean by this?

[JosAH]

Local variables are gone when the enclosing method terminates but that class object survices and so everything that is used by that object still has to be there; marking local variables final ensures that they survice the termination of the method.

kind regards,

Jos

[stuckonjava]

Local variables are gone when the enclosing method terminates but that class object survices and so everything that is used by that object still has to be there; marking local variables final ensures that they survice the termination of the method.

kind regards,

Jos

Does this mean we always have to declare them as final???

[kjkrum]

No, you could make them member (aka instance) variables by simply changing where you declare them:

public class MyApp2{
   JFrame frame;
   JTextField tf;
   JButton button;

public MyApp2(){
   frame = new JFrame ("My App");
   tf = new JTextField();
   button = new JButton("ok");
   button.addActionListener(
      new ActionListener(){
         public void actionPerformed(ActionEvent event){
            System.out.println(tf.getText());
         }
      }
   );
}
}

[stuckonjava]

But why did my intial one require the final declaration on it?

[Norm]

The variable's value would be gone when the method exited. Did you miss the explanations given earlier?

[stuckonjava]

But if the new method cant see the variable tf, how does declaring as final make a difference?

[Norm]

See the previous posts.

[stuckonjava]

So if we declare it as final it kind of becomes like an instance variable?

Sorry for being so slow but I'm just finding it confusing as there are so many different ways of doing things in java for the most simple cases.

Thanks for your patience

[Norm]

final makes its value available to the method in the anonymous class.

[kjkrum]

Variables (The Java™ Tutorials > Learning the Java Language > Language Basics)