Java：如何将变量从JButton ActionListener传递给主类？

Question

我试图制作一个程序，当单击一个按钮然后在屏幕上显示输出时，它将生成一个随机数。但是，我无法使用JLabel将保存随机数的变量传递给类，以便可以在该类中使用它。我编写了一个程序，以使其易于演示：

import java.awt.BorderLayout;
import java.awt.Panel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Main{
    public static void main(String[] args){

    JFrame mainFrame = new JFrame("Experiment");
    mainFrame.setSize(500,500);
    mainFrame.setVisible(true);

    Panel panel = new Panel();
    mainFrame.getContentPane().add(panel);

    JLabel output = new JLabel("This is where the result from the num variable in the numGenerator class would go"); 
    panel.add(output); 

    JButton numGenerator = new JButton("Generate Number");
    panel.add(numGenerator);
    numGenerator.addActionListener(new numGenerator());

    }

static class numGenerator implements ActionListener{
    public void ActionPerfomed (ActionEvent e){

        int num; //This is the variable I want to be passed to the
                 //Main class so it can be displayed in the "output" Jlabel.

        Random dice = new Random();
        num = dice.nextInt(3);

    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        // TODO Auto-generated method stub

    }
}
}

I saw other help online to create an object of the class and then use that in the class you want the variable to be in, but I couldn't get that to work in this situation.

Answer 1

There are a number of methods that you could use to accomplish this...

You Could...

Make a class, instance variable available for the numGenerator to access directly...

public class Main{
    public static void main(String[] args){
        new Main();
    }

    // This variable will be visible to the inner class numGenerator
    private JLabel output;

    public Main() {
        JFrame mainFrame = new JFrame("Experiment");
        mainFrame.setSize(500,500);
        mainFrame.setVisible(true);

        Panel panel = new Panel();
        mainFrame.getContentPane().add(panel);

        output = new JLabel("This is where the result from the num variable in the numGenerator class would go"); 
        panel.add(output); 

        JButton numGenerator = new JButton("Generate Number");
        panel.add(numGenerator);
        numGenerator.addActionListener(new numGenerator());
    }

    public class numGenerator implements ActionListener{
        public void actionPerformed(ActionEvent e){

            Random dice = new Random();
            int num = dice.nextInt(3);
            output.setText(Integer.toString(num));

        }
    }
}

This tightly couples you action with your label, making the code less reusable...

You could...

Pass a reference of the label to be changed to the numGenerator...

public class Main{
    public static void main(String[] args){
        new Main();
    }

    public Main() {
        JFrame mainFrame = new JFrame("Experiment");
        mainFrame.setSize(500,500);
        mainFrame.setVisible(true);

        Panel panel = new Panel();
        mainFrame.getContentPane().add(panel);

        JLabel output = new JLabel("This is where the result from the num variable in the numGenerator class would go"); 
        panel.add(output); 

        JButton numGenerator = new JButton("Generate Number");
        panel.add(numGenerator);
        numGenerator.addActionListener(new numGenerator(output));
    }

    public class numGenerator implements ActionListener{
        private JLabel label;

        public numGenerator(JLabel label) {
            this.label = label;
        }

        public void actionPerformed(ActionEvent e){

            Random dice = new Random();
            int num = dice.nextInt(3);
            if (label != null) {
                label.setText(Integer.toString(num));
            }

        }
    }
}

This makes the numGenerator more reusable, as it's not reliant on a single instance of the JLabel

You Could...

Use a Observer style pattern that could tell some interested party that a new number has being generated...

public class Main{
    public static void main(String[] args){
        new Main();
    }

    public Main() {
        JFrame mainFrame = new JFrame("Experiment");
        mainFrame.setSize(500,500);
        mainFrame.setVisible(true);

        Panel panel = new Panel();
        mainFrame.getContentPane().add(panel);

        final JLabel output = new JLabel("This is where the result from the num variable in the numGenerator class would go"); 
        panel.add(output); 

        JButton numGenerator = new JButton("Generate Number");
        panel.add(numGenerator);
        numGenerator.addActionListener(new numGenerator(new NumberGeneratorListener() {
            public void numberGenerated(int number) {
                output.setText(Integer.toString(number));
            }
        }));
    }

    public interface NumberGeneratorListener {
        public void numberGenerated(int number);
    }

    public class numGenerator implements ActionListener{
        private NumberGeneratorListener listener;

        public numGenerator(NumberGeneratorListener listener) {
            this.listener = listener;
        }

        public void actionPerformed(ActionEvent e){

            Random dice = new Random();
            int num = dice.nextInt(3);
            if (listener != null) {
                listener.numberGenerated(num);
            }

        }
    }
}

这不仅使numGenerator代码与其余代码脱钩，因为它不依赖于代码的任何其他部分，而且还使其具有极高的可重用性，因为它不在乎数字的位置或使用方式，这取决于观察员/听众决定...

旁注...

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