Java - How to assign a variable from an inner class to a variable in the outer class/ or any other class?

Question

I have a class which contains a textfield and within that class an inner class containg the action performed method. On entering text and pressing enter, I would like the text to be assigned to a String "s" outside of the inner class that implements Action Listener. I want to use that string in another class.

public class Peaches extends JFrame {
    private JTextField item1;

    public Peaches() {
        super("the title");
        setLayout(new FlowLayout());        
        item1 = new JTextField(10);
        add(item1);

    thehandler handler = new thehandler(); //object of thehandler inner class
    item1.addActionListener(handler);      

    // I want to use the textfield content here, or in other classes, updated
    // with the new text everytime i hit enter after entering new text in the textfield 

    //String s = the content of the textfield
}

private class thehandler implements ActionListener { // inner class
    @Override
    public void actionPerformed(ActionEvent event) {
        String string = "";           
        string = String.format(event.getActionCommand());            
        JOptionPane.showMessageDialog(null, string);           
    }
}

Is there any way of doing this? It must be possible to use the input of a text field in other parts of your program. I hope I make myself clear, thanks all.

EDIT Thanks for the replies, so making the variable string a class variable instead of declaring it in the constructor allows the whole class to access it, simple but something i just didnt understand. Thanks Marko

A comment on the edit: how does that solve your question ... you already had access to the string by simply calling item1.getText() — Robin
– Robin, Commented Apr 25, 2012 at 21:38

Marko Topolnik · Accepted Answer · 2012-04-25 21:22:24Z

4

I suggest you do it like this:

public class Peaches extends JFrame {
  private JTextField item1;
  private String string;

  public Peaches() {
    super("the title");
    setLayout(new FlowLayout());
    item1 = new JTextField(10);
    add(item1);
    item1.addActionListener(new ActionListener() {
      @Override public void actionPerformed(ActionEvent event) {
        string = String.format(event.getActionCommand() );
        JOptionPane.showMessageDialog(null, string);
    }});
  }
}

This uses an anonymous class, simpler than having dozens of inner classes around. Plus with these you can make closures (use local variables declared outside the anonymous class). And your string is there outside, so you can access it.

answered Apr 25, 2012 at 21:22

Marko Topolnik

201k31 gold badges337 silver badges455 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Robin Over a year ago

And how does this help ? There is no reason to make it a member variable, as I can call item1.getText() so there was already access to that String throughout the whole class

Robin Over a year ago

Java still does not support closures, and an anonymous class does not solve that. You cannot access local variables, only final ones. This is different to what can achieved when real support for closures would be available

Marko Topolnik Over a year ago

Being able to access a final var is much better than not being able to access any. The main point of a closure is closing over lexical scope -- and that's what we do have in Java. Mutability can be achieved with the standard trick of a one-element array. BTW closures come from FP languages like Scheme or Clojure. Do you think those languages allow access to non-final variables?

Robin · Accepted Answer · 2012-04-25 21:34:49Z

I want to use the textfield content here, or in other classes, updated with the new text everytime i hit enter after entering new text in the textfield

The short answer here is that if you want code to be executed each time the value in the textfield is changed, you will have to attach a listener to the textfield and trigger that code through your listener. That is what the listener is for ... the only thing that knows the text has been changed is the textfield, and he will inform its listeners about the fact a change has been made (Observer pattern)

If you want to be informed of this change in several classes you can either

provide API to attach listeners to this particular textfield from outside the current class
provide API to give direct access to the textfield from outside the current class
attach one listener to the textfield which updates some kind of model, and share the model between the different classes. Of course the model should be able to fire events and inform listeners about changes, or you will not be able to tell when a value has been changed.

Whether you use inner classes, anonymous class, full-fledged classes as listener doesn't really matter.

ControlAltDel · Accepted Answer · 2012-04-25 21:14:18Z

0

If you move s to be a class variable rather than a method variable (as it is currently, commented out in your constructor), it will work.

answered Apr 25, 2012 at 21:14

ControlAltDel

35.3k10 gold badges60 silver badges86 bronze badges

1 Comment

Marko Topolnik Over a year ago

An instance variable, I'd say.

11684 · Accepted Answer · 2012-04-25 21:14:40Z

0

you could pass the outer class as parameter in the constructor of the inner class.

answered Apr 25, 2012 at 21:14

11684

7,52812 gold badges53 silver badges74 bronze badges

Comments

Tharwen · Accepted Answer · 2012-04-25 21:37:35Z

0

You shouldn't need an inner class at all. Just implement an ActionListener in Peaches and have it listen for the ActionEvent:

public class Peaches extends JFrame implements ActionListener
{
    private JTextField item1;

    public Peaches()
    {
        super("the title");
        setLayout(new FlowLayout());        
        item1 = new JTextField(10);
        add(item1);

        item1.addActionListener(this);      
    }

    public void actionPerformed(ActionEvent event)
    {
        if (event.getSource() == item1){
            String string = "";           
            string = String.format(event.getActionCommand() );            
            JOptionPane.showMessageDialog(null, string);   
        }        
    }
}

answered Apr 25, 2012 at 21:37

Tharwen

3,1072 gold badges26 silver badges37 bronze badges

3 Comments

Robin Over a year ago

By implementing the ActionListener interface directly you suggest this class can attached to other classes (since it is an ActionListener itself). However, that makes no sense at all. I strongly prefer the inner class or the anonymous class compared to this solution

Tharwen Over a year ago

@Robin Is that a problem if the author knows it will only be used in this context?

Robin Over a year ago

It is a matter of personal taste, but I prefer a clean design, so I would use an ActionListener and not being an ActionListener for the reason stated above. Is it a problem ... not really

Collectives™ on Stack Overflow

Java - How to assign a variable from an inner class to a variable in the outer class/ or any other class?

5 Answers 5

3 Comments

Comments

1 Comment

Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

3 Comments

Comments

1 Comment

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related