inner class and static

[jim]

I was told that the variable accessed inside a anonymous inner class
needs to be static.
However the followign code from Thinking in java 3 works fine.
Why?



import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Button2b extends JApplet {
private JButton
b1 = new JButton("Button 1"),
b2 = new JButton("Button 2");
private JTextField txt = new JTextField(10);
private ActionListener bl = new ActionListener() {
public void actionPerformed(ActionEvent e) {
String name = ((JButton)e.getSource()).getText();
txt.setText(name); ------question? the txt is not final
but no complain from compiler
}
};
public void init() {
b1.addActionListener(bl);
b2.addActionListener(bl);
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(b1);
cp.add(b2);
cp.add(txt);
}
public static void main(String[] args) {
Console.run(new Button2b(), 200, 75);
}
} ///:~

 

[Matt Humphrey]

I was told that the variable accessed inside a anonymous inner class
needs to be static.

An inner class (whether anonymous or not) may access the instance variables
of the outer class and it can access the local variables of the method that
instantiates it, provided they are final.

Cheers,
Matt Humphrey (e-mail address removed) http://www.iviz.com/

 

[Mike Schilling]

An inner class (whether anonymous or not) may access the instance
variables of the outer class and it can access the local variables of the
method that instantiates it, provided they are final.

Though a *static* inner class can only access static field of its enclosing
class (and can only call static methods of that class); pehshps that's what
the OP was thinking of.

 

[jim]

<Quote from TIJ3 chapter 8>
If you're defining an anonymous inner class and want to use an object
that's defined outside the anonymous inner class, the compiler
requires that the argument reference be final, like the argument to
dest( ). If you forget, you'll get a compile-time error message.
Feedback
</Quote>

The txt mentioned in the original post is an object that's defined
outside the anonymous inner class, but still the compiler didn't
complain, why?

 

[Stefan Ram]

<Quote from TIJ3 chapter 8>
If you're defining an anonymous inner class and want to use an object
that's defined outside the anonymous inner class, the compiler
requires that the argument reference be final, like the argument to
dest( ).

First, objects are not "defined": Objects are created at
run-time and identifiers are declared at compile-time in Java.
Also, finality is not an attribute of references but of
identifiers.

I can not understand why people recommend TIJ from this
quotation (I never read more of it).

 

[Mike Schilling]

<Quote from TIJ3 chapter 8>
If you're defining an anonymous inner class and want to use an object
that's defined outside the anonymous inner class, the compiler
requires that the argument reference be final, like the argument to
dest( ). If you forget, you'll get a compile-time error message.
Feedback
</Quote>

Is that really what it says? That's awful. What it means to say is that
local and anonymous classes can access local variables and formal parameters
that are visible in their enclosing block. For this to work, these variables
must be declared final. (Paraphrased from JLS 3, sec 8.1.3)

The txt mentioned in the original post is an object that's defined
outside the anonymous inner class, but still the compiler didn't
complain, why?

It's a field in the enclosing class, not a local variable, so final isn't
necessary.

 

[IchBin]

First, objects are not "defined": Objects are created at
run-time and identifiers are declared at compile-time in Java.
Also, finality is not an attribute of references but of
identifiers.

I can not understand why people recommend TIJ from this
quotation (I never read more of it).

Yes, drop "TIJ". You should be reading a book like say: 'Effective Java:
Programming Language Guide' by Joshua Bloch.

Thanks in Advance...
IchBin, Pocono Lake, Pa, USA http://weconsultants.phpnet.us
__________________________________________________________________________

'If there is one, Knowledge is the "Fountain of Youth"'
-William E. Taylor, Regular Guy (1952-)

 

[Tony Morris]

Though a *static* inner class can only access static field of its enclosing

class (and can only call static methods of that class); pehshps that's what
the OP was thinking of.

FWIW, "static inner class" is an oxymoron. You might mean "static nested
class". http://jqa.tmorris.net/GetQAndA.action?qids=67&showAnswers=true

 

[Mike Schilling]

FWIW, "static inner class" is an oxymoron. You might mean "static nested
class". http://jqa.tmorris.net/GetQAndA.action?qids=67&showAnswers=true

Yes, that distinction is made at the top level. But what about a local or
anonymous class defined in a static method? I've never seen one of those
called a "nested class".

In other words, the terminology is fouled up. There are really six
categories of classes defined within other classes, defined along two axes:

Instance Static
Class-level 1 2
Local 3 4
Anonymous 5 6

For those defined at the class-level, "static" is a keyword. For those
defined withing methods, it depends on whether they occur withing instance
or static methods. The odd-numbered ones have encosing instances; the
even do not.

Now, what would you call 4 and 6, if not "static inner classes"? For that
matter, why call everything besides 2 an "inner class", when the real
distinction is odd vs. even?

 

[Oliver Wong]

First, objects are not "defined": Objects are created at
run-time and identifiers are declared at compile-time in Java.
Also, finality is not an attribute of references but of
identifiers.

I'm guessing you mean "Also, finality is not an attribute of objects,
but of identifiers/references".

- Oliver