ActionListener（actionPerformed）返回什么？

Question

I'm currently trying to learn new things in Java and decided to start GUI programming. I created a GUI with an ActionListener and with the method "actionPerformed".

My question is if I can return anything from this method (actionPerformed) and where will it land? Because the Method is called when I do something specific in this GUI.

Or how can I give this this actionPerformed method an other parameter?

Thanks in advance ;)

Answer 1

As you probably already know, the actionPerformed method is declared as returning void or nothing , and so no, you cannot return anything from it, but you can change the state of any mutable field within the scope of this method, for instance the text displayed in a JLabel.

For example:

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

public class ActionListenerTest extends JPanel {
    private static final String[] DAYS = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday"};
    private int dayIndex = 0;
    private JLabel label = new JLabel("", SwingConstants.CENTER);
    private JButton button = new JButton("Press Me Please!");

    public ActionListenerTest() {
        label.setText(DAYS[dayIndex]);
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                dayIndex++; // advance the index
                dayIndex %= DAYS.length; // if index >= the length of the array, make it 0
                label.setText(DAYS[dayIndex]);
            }
        });

        setLayout(new GridLayout(2, 1));
        add(label);
        add(button);
    }

    private static void createAndShowGui() {
        ActionListenerTest mainPanel = new ActionListenerTest();

        JFrame frame = new JFrame("ActionListenerTest");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGui();
            }
        });
    }
}

While actionPerformed returns nothing, it advances the dayIndex by 1, or sets it to 0 if the index is the same as the length of the String array. The method then sets the text of the JLabel using the String array data.

Answer 2

No actionPerformed is callback, so you can handle action event by implementing actionPerformed method. Here you can: 1) update some GUI elemenets in your app 2) save results data in private field of your View class to process it in future.

For example:

private Map<String,Object> data = ...;

public void showElements(){
    ...
    button1.addActionListener (new ActionListener()
    {
          public void actionPerformed (ActionEvent e){
              Object info = "Somebody clicked on my button!"//or you can use ActionEvent to extract mode information
              data.put("button1",info)
          }
    })
}