如何从类中添加匿名实例作为ActionListener

Question

Im working on a lab that requires me to make a JFrame with 2 inner classes. One that entends JPanel, has a text area and a jbutton. And another that implements action listener. How do i add an anonymouse instance of the second class to my JButton that is already in an inner class. Here is the brief to get a better understanding.

here is the code i have written so far. I can get the Frame to appear, but the JPanel doesnt appear, nor does the JButtons or JTextArea.

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;



@SuppressWarnings("serial")
public class FormFrame extends JFrame
{

public static void main(String[] args)
{
    JFrame frame = new FormFrame();
    frame.setLocationRelativeTo(null);
}   
public FormFrame()
{
    Container contentPane = getContentPane();

    RegisterPanel p = new RegisterPanel();
    p.button.addActionListener(new SubmitResponder());
    //
    // Here is where im lost...
    //


    contentPane.add(p);
    setSize(300, 200);
    setVisible(true);
}

class RegisterPanel extends JPanel
{

    JPanel panel = new JPanel();
    JTextField text = new JTextField();
    JButton button = new JButton("Submit");     
}

class SubmitResponder implements ActionListener
{

    @Override
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource()== RegisterPanel.button) //Asks me to make button static here
        {

            //Shows "No enclosing instance of the type FormFrame.RegisterPanel is accessible in scope"
                RegisterPanel.this.text.setText("Submit Complete");

        }
    }
}
}

Any help with this would be much appreciated

Answer 1

You could pass the RegisterPanel instance to the action listener:

class SubmitResponder implements ActionListener {

    private final RegisterPanel rp;

    public SubmitResponder(RegisterPanel rp) {
        this.rp = rp;
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        rp.text.setText("Submit Complete");
    }
}

There's no need to check the source btw. The AL is only listening to 1 source.

RegisterPanel p = new RegisterPanel();
p.button.addActionListener(new SubmitResponder(p));

Answer 2

p.button.addActionListener(new SubmitResponder());

Here the SubmitResponder is already an anonymous instance, quite literally, because it has no name.

Your error about "no enclosing instance" is unrelated. Since SubmitResponder is not an inner class of RegisterPanel (it's a sibling) it doesn't belong to an instance of RegisterPanel and so it cannot logically refer to RegisterPanel.this . How would it know which instance that is? There might be many, or even zero, depending on how many the parent FormFrame decides to create. It so happens that there's only one, but that's not the point. On the other hand if you said FormFrame.this there would be no doubt what that meant no matter the code did, unless RegisterPanel stopped being an inner class or it became static. Does that make sense?

To do what you want, the SubmitResponder needs to talk to RegisterPanel via a method in FormFrame . Incidentally you don't actually need to say FormFrame.this.doSomething() unless SubmitResponder also has a method called doSomething .

Answer 3

The instructions tell you that the RegisterPanel should be a field in the FormFrame class, which you haven't done. Something like this:

public class FormFrame extends JFrame
{
    RegisterPanel panel;
    ...

    public FormFrame()
    {
        panel = new RegisterPanel();
        ...
    }

    ...

    class SubmitResponder implements ActionListener
    {
        @Override
        public void actionPerformed(ActionEvent e)
        {
            if (e.getSource() == panel.button)
            {
                panel.text.setText(...);
            }
        }
    }
}

Now you can access panel from inside the SubmitResponder class.

---

As a side note, the instructions are using some terminology in an ambiguous and incorrect way:

• "Anonymous instance" is not an official term with a precise meaning.
• Using official definitions, "class field" would imply the static modifier. Given the context of the assignment, I doubt that's correct. It should probably have said "instance field".