Final keyword

[Pino]

Hello!

I have a small question about the 'final' keyword. I just now used it to solve a certain problem, but I'm not sure how it works (Eclipse told me to ).
It's the following:
In a constructor of a class, I loop through an enum with this: for(Direction dir : Direction.getValues()).
In this loop, I create an instance of some other class for each direction. This instance overrides a method
from its class, and uses the 'dir' value in this method.

Now I've had to use the final modifier on 'dir'. But I don't really understand why. To be completely honest, my instinct told me what I was trying to do the way I was doing it was logically impossible. That's why I was completely stumped when the 'final' keyword actually solved it. Is this because the value is an enum?

[pbrockway2]

Now I've had to use the final modifier on 'dir'. But I don't really understand why.

If you were using dir as an argument to some method and that method was declared to take a final argument, then you have to declare dir as final. If a method is declared to take an int[] argument, you must declare the argument as int[]: if it is declared to take final Direction, then final Direction it must be.

If the method in question is not declared to take a final argument, or it is and you don't see why, post the relevant code (or documentation if this other class in a library).

[awinston]

If you were using dir as an argument to some method and that method was declared to take a final argument, then you have to declare dir as final. If a method is declared to take an int[] argument, you must declare the argument as int[]: if it is declared to take final Direction, then final Direction it must be.

If the method in question is not declared to take a final argument, or it is and you don't see why, post the relevant code (or documentation if this other class in a library).

I didn't realize that a method could be declared to take in a final argument. Does this apply to accessibility descriptions such as 'public' and 'private' as well? It would be great if you could link to a tutorial if one exists.

[pbrockway2]

No public/private only make sense for declarations of instance or class methods and variables. But final (== "I won't go changing this value") can occur anywhere including local variables and the parameters of methods.

Not a tutorial, but Roedy Green discusses final in his Glossary.

[Edit] Green declares himself a fan of final, but doesn't actually talk about final method parameters as such. They don't seem to occur much in the standard Java API (do they?) so I wouldn't use them.

// untested
public void setMessage(String msg) {
    final String theMsg = msg;
    myButton.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            System.out.println(theMsg);
        }
    });
}

In that code I could have declared the method as setMessage(final String msg) and done away with the other variable, but I don't think I would want to impose this requirement on the method's caller.

[awinston]

No public/private only make sense for declarations of instance or class methods and variables. But final (== "I won't go changing this value") can occur anywhere including local variables and the parameters of methods.

Not a tutorial, but Roedy Green discusses final in his Glossary.

[Edit] Green declares himself a fan of final, but doesn't actually talk about final method parameters as such. They don't seem to occur much in the standard Java API (do they?) so I wouldn't use them.

// untested
public void setMessage(String msg) {
    final String theMsg = msg;
    myButton.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            System.out.println(theMsg);
        }
    });
}

In that code I could have declared the method as setMessage(final String msg) and done away with the other variable, but I don't think I would want to impose this requirement on the method's caller.

Okay, thank you. I thought you meant that a method could be declared like so:

public void setMessage(final String msg)
{
   ...
}

[Fubarable]

Okay, thank you. I thought you meant that a method could be declared like so:

public void setMessage(final String msg)
{
   ...
}

Yes, he did mean that just that is possible and is used -- try it yourself, and you'll see that it compiles fine. Some recommend that most variables and parameters should be declared final unless you the coder are sure that they will need to be re-assigned within their scope.

[pbrockway2]

Yes, I meant that the alternative would compile.

But forget what I said about marking the parameter as final as imposing any condition on the caller. Of course it doesn't. I don't know what I was thinking.

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;

public class Test {
    
    private JButton myButton;
    
    public void setMessage(final String msg) {
        // msg in this method is final
        // it has to be  to be used by actionPerformed()
        myButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                System.out.println(msg);
            }
        });
    }
    
    void foo() {
            // msg in this method is a different variable and need not be final
        String msg = "asd";
        setMessage(msg);
    }
}

[awinston]

Yes, I meant that the alternative would compile.

But forget what I said about marking the parameter as final as imposing any condition on the caller. Of course it doesn't. I don't know what I was thinking.

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;

public class Test {
    
    private JButton myButton;
    
    public void setMessage(final String msg) {
        // msg in this method is final
        // it has to be  to be used by actionPerformed()
        myButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                System.out.println(msg);
            }
        });
    }
    
    void foo() {
            // msg in this method is a different variable and need not be final
        String msg = "asd";
        setMessage(msg);
    }
}

Okay, thanks pbrockway2 and Furarable. So in this example (please correct me if I am wrong): If "Hello!" is the argument of setMessage(), a final local variable named msg will hold the value of "Hello!".

[Fubarable]

Okay, thanks pbrockway2 and Furarable.

You're welcome

So in this example (please correct me if I am wrong): If "Hello!" is the argument of setMessage(), a final local variable named msg will hold the value of "Hello!".

Yes, that is correct. It behaves no different from any other parameter other than once assigned it cannot be re-assigned.

[Pino]

Well, thank you everyone for the input!

If the method in question is not declared to take a final argument.

I don't think it is. During the entire project I haven't had to use the final keyword anywhere yet, and I don't usually.
Here's the constructor of my class where what I was talking about happened:

/**
* Creates a new DirectionSelectDialog.
*/
public DirectionSelectDialog()
{
	super(null);
	setLocation(Game.DISPLAY.getWidth() / 2 - 48, Game.DISPLAY.getHeight() / 2 - 48);
	setDimension(96, 96);
	
	for(final Direction dir : Direction.values())
	{
		Button btnDir = new Button(this)
		{
			@Override
			public void activate()
			{
				buffer.setDirection(dir);
				if(buffer.getType() == ActionType.ENGRAVE)
					((GameScreen) Game.ACTIVESCREEN).requestString(buffer);
				else
					Campaign.getPlayer().execute(buffer);
				getParent().hide();
			}
		};
		btnDir.setLocation(43 + dir.getDeltaX() * 32, 43 + dir.getDeltaY() * 32);
		btnDir.setDimension(10, 10);
		btnDir.setImage(Campaign.getImage("BTN_DIR"));
		addComponent(btnDir);
	}
}

Fair warning: The buttons & the like you see here are NOT native java classes. I've written my own Button classes & such from scratch, to optimize for games and fullscreen drawing. Where I got the warning was at 'buffer.setDirection(dir)'; which is nothing more than a simple setter. Buffer is an instance of a simple class called GameAction, containing only getters and setters for its fields, one of them being a Direction.

[pbrockway2]

In that example dir has to be declared as final because it is used in the implementation of the activate() method. It is similar in that regard to the example I gave where the string argument had to be final because it was used within the actionPerformed() method.

The idea is that you are promising not to change the value of dir within the for loop. And that allows an anonymous subclass of Button to be declared with a copy of the known value of dir being passed to the instance of the subclass of Button you create. Other languages don't have this restriction - eg JavaScript where variables in the scope of a function you are defining are part of the context of that function and can change. In Java, however, variables you "import" into a method in this way must be final.

[Pino]

That makes sense, thank you.

[pbrockway2]

You're welcome.