每次按下空格键时在 Java 中增加一个变量

Question

I have been trying to increment a variable every time the space bar is pressed in Java.

The code I wrote is below but it seems to be wrong as it increments without pressing the key.

import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class TestKeyPress {
    public static void main(String[] args) {
        
        System.out.println(keyPressed(null));
        
    }
    public static int keyPressed(KeyEvent e) {
        int var = 0;
        while (var <6) {
            var ++;
            System.out.println(hi);
        }
        System.out.println("max reached");
        return var;
    }
}

Answer 1

You need to bind the listener into an event. You cannot expect anything happens when the passed KeyEvent is null . Moreover, you don't specify anywhere the space bar is pressed. You also miss the context to which is the key-press event registered. Is it a console or a desktop application? As long as you use java.awt.event I suspect the latter. Here is a minimal sample:

Java Desktop application using Swing

public static void main(String[] argv)  {

    JTextField textField = new JTextField();
    textField.addKeyListener(new KeyAdapter() {

        int counter = 0;
 
        @Override
        public void keyPressed(KeyEvent e) {
            if (e.getKeyCode() == KeyEvent.VK_SPACE) {
                if (counter < 6) {
                    counter++;
                    System.out.println("hi");
                } else {
                    System.out.println("max reached");
                }
            }
        }
    });

    JFrame jframe = new JFrame();
    jframe.add(textField);
    jframe.setSize(640, 320);
    jframe.setVisible(true);
}

Console application

In that case, you cannot use anything from the java.awt.* or javax.swing.* libraries and you have to stick with classes from java.io.* .

Answer 2

Because when the keyPressed called the var variable will be zero, you should declare outside the keyPressed .

Also if you want to bind with the progress bar you should implement the listener.

import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class TestKeyPress {

    private static int var = 0;

    public static void main(String[] args) {
        System.out.println(keyPressed(null));
    }

    public static int keyPressed(KeyEvent e) {
        while (var < 6) {
            var ++;
            System.out.println(hi);
        }
        System.out.println("max reached");
        return var;
    }
}