从匿名类检索数据

Question

I was wondering how to retrieve the string textOperandValue,from this code :

final JTextField textOperand = new JTextField();
textOperand.setBounds(200,100,75,25);

//textOperand action Listener
textOperand.addActionListener( new ActionListener () {
  public void actionPerformed(ActionEvent e) {
    String textOperandValue = textOperand.getText();    
  }
});

So I can take it, and then parse it into a double to be used later in the program. I tried setting it equal to a another string String Input = " "; but it said I had to initialize the string to a final String Input = " "; which I learned is something like a constant in C++.

Answer 1

Any variables you declare within an ActionListener won't be visible to the rest of your code. Either you need to set a variable (from within the listener) that has wider scope:

public class Listen
{
    String usefulResult = null;

    public Listen()
    {
        final JTextField textOperand = new JTextField();
        textOperand.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                Listen.this.usefulResult = textOperand.getText();
            }
        });
    }
}

Here we use the "OuterClass.this" trick to access the surrounding scope without needing a final variable.

or you need to perform all the necessary work from within the listener itself (ie you don't "retrieve" the value, you just use the value):

    public void doSomethingUseful(String usefulValue) { /* add code here */ }

    textOperand.addActionListener( new ActionListener ()
    {
        public void actionPerformed(ActionEvent e) 
        {
            doSomethingUseful(textOperand.getText());    
        }
    });

Or you could use this second technique to call a setter method that changes the value of a variable, avoiding the problems of accessing final variables within event listeners:

public class Listen
{
    String usefulResult = null;

    public void setUseful(String usefulValue){
        usefulResult = usefulValue;
    }

    public Listen()
    {
        final JTextField textOperand = new JTextField();
        textOperand.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                setUseful(textOperand.getText());
            }
        });
    }
}

It depends what you want to do with the value from the TextField.

Answer 2

You can't access the members of anonymous classes. In this case it's even a local variable in a method, those are never accessible.

Either, you'll have to set the value of a member of the outer class, but beware of synchronization trouble.

class MyClass {
  final JTextField textOperand = new JTextField();
  public String textOperandValue;

  public MyClass() {
    textOperand.setBounds(200,100,75,25);

    //textOperand action Listener
    textOperand.addActionListener( new ActionListener () {
      public void actionPerformed(ActionEvent e) {
        textOperandValue = textOperand.getText();   
      }
    });
  }

  public someOtherMethod() {
    // textOperandValue available here
    if (textOperandValue != null) 
      //is set
  }
}

Or, you'll have to call a method somewhere else that can store the value.

class MyClass {
  final JTextField textOperand = new JTextField();

  public MyClass() {
    textOperand.setBounds(200,100,75,25);

    //textOperand action Listener
    textOperand.addActionListener( new ActionListener () {
      public void actionPerformed(ActionEvent e) {
        someOtherMethod(textOperand.getText()); 
      }
    });
  }

  public someOtherMethod(String value) {
    System.out.println(value);
  }
}

Or, you create a (named) class that is an ActionListener and that can store the value in a retrievable form.

class MyClass {
  final JTextField textOperand = new JTextField();
  public String textOperandValue;

  private class MyActionListener implements ActionListener {
    String value;

    public void actionPerformed(ActionEvent e) {
      value =textOperand.getText(); 
    }
  }

  MyActionListener l = new MyActionListener();

  public MyClass() {
    textOperand.setBounds(200,100,75,25);

    //textOperand action Listener
    textOperand.addActionListener(l);
  }

  public someOtherMethod() {
    if (l.value != null) 
      //is set
  }
}

Or, you just do what you need to do in the action method:

class MyClass {
  final JTextField textOperand = new JTextField();

  public MyClass() {
    textOperand.setBounds(200,100,75,25);

    //textOperand action Listener
    textOperand.addActionListener( new ActionListener () {
      public void actionPerformed(ActionEvent e) {
        System.out.println(textOperand.getText());
      }
    });
  }
}

Answer 3

it can work if textOperandValue is global(if it defined in class globally) variable. not inside ActionListener, not inside method where you wrote this code.