如何在内部嵌套方法中访问外部变量？

Question

I am aware that this question may already exist, and this is my first question on the site, so please bear with me.I still have difficulty understanding this in my case.

So here is the thing: I have a method that I call , and I have a button that will change the value of x. Depending on the value of x, I want the program to do something. The program below is not very complete but you will get the idea:

public class foo{
        private void do(){
        int x=0;
        JButton changeValue= new JButton("Change the value of x");
        changeValue.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent e){
                       x=10; //change the x value when the button is clicked
                             //Here the user may also change the value of x
                             //by inputting some other number
                 }
             });
        //Something happens depending on x
        //But nothing happens here because when I get the value of x,
        //it reverts back to 0.
        }
}

However, no matter where I declare my x in do(), I keep getting an error telling me that inner class cannot access outer class variables and that they must be declared final. But I can't declare it final because I need to change it later. I have tried putting the values in a new class. I have also tried declaring x as a member of foo() but that results to x being 0 or null because for some reason once it exits the button click method it takes x to back to its old value: 0 or null.

What I need : when the button is pressed, the value of x is changed (Assuming that the user can change the value of x to some other number) Thanks for any answers in advance.

Answer 1

You need to create a final reference to your x variable.

Since it is primitive type, create a class to wrap it:

private static class MyX {
    int x;
    // + getter and setter
}

And then:

final MyX myX = new MyX(x);
JButton changeValue= new JButton("Change the value of x");
changeValue.addActionListener(new ActionListener(){
    @Override
    public void actionPerformed(ActionEvent e){
        myX.setX(10);
    });
// get the value of MyX back if necessary

Answer 2

The problem is: when you create that ActionListener you are declaring an actionPerformed that will be executed sometime later. You cannot change x from actionPerformed because x exists only inside that method call.

class Foo {
    void doSomething() {

        int x = 0; // x inside doSomething

        JButton btn = new Button("Do Something");

        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                /*
                 * This does not happen now:
                 * it happens later.
                 *
                 * By the time this happens
                 * the method will have exited
                 * and x will be gone.
                 *
                 */
            }
        });

        /* That x disappears forever here.
         * (Or a little bit later but basically here.)
         */
    }
}

You can declare x as final and then the anonymous class is given the value of it but you are wanting to change it. Probably what you are wanting is to make it a field:

class Foo {

    int x = 0; // x inside Foo

    JButton btn = new Button("Do Something");

    Foo() {
        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                /* do something with x as needed */
            }
        });
    }
}

Anonymous classes can also have fields:

btn.addActionListener(new ActionListener() {
    int x = 0; // x inside anonymous ActionListener

    @Override
    public void actionPerformed(ActionEvent ae) {
        /* use x only in actionPerformed */
    }
});

"I have also tried declaring x as a member of foo() but that results to x being 0 or null because for some reason once it exits the button click method it takes x to back to its old value: 0 or null."

Unfortunately, this probably means you're doing something else wrong. I'm not sure what that might be from your description and code. For example, you're creating a new JButton locally inside a method which seems unusual. Unless it's one of those "createAndShowGUI" methods (as seen all over the Swing tutorials ), you might be creating multiple buttons, multiple listeners, etc.