使用键盘（“ AWT-EventQueue-0”）

Question

having a little difficulty. I ask you to look at the code:

1 class (MyCanvas.java)

package Game;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;
import javax.swing.JComponent;
import javax.swing.JFrame;

class MyCanvas extends JComponent {

    private static final long serialVersionUID = 1L;

    private static final int WIDTH = 490;
    private static final int HEIGHT = 470;
    public static InputKey input = new InputKey();
    private int x = 10;
    private int y = 10;

    // public MyCanvas() {
    // addKeyListener(input);
    // }

    public void move() {
        if (x == 0) {
            x = 10;
        }
        if (y == 0) {
            y = 10;
        }

        if (input.left) {
            x--;
        }
        if (input.right) {
            x++;
        }
    }

    public void paint(Graphics g) {
        Image img1 = Toolkit.getDefaultToolkit().getImage(
                "C:\\Users\\дНМ\\workspace\\Game\\image\\Peopl.png");

        int width = img1.getWidth(this);
        int height = img1.getHeight(this);

        int scale = 4;
        int w = scale * width;
        int h = scale * height;
        g.drawImage(img1, x, y, (int) w, (int) h, this);

    }

    public static void main(String[] a) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(WIDTH, HEIGHT);
        frame.getContentPane().add(new MyCanvas());
        frame.getContentPane().setBackground(Color.BLACK);
        frame.setVisible(true);
        frame.setFocusable(true);
        frame.requestFocusInWindow();
        frame.addKeyListener(input);
    }

}

The second class (InputKey.java)

package Game;

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

import javax.swing.JComponent;

public class InputKey extends JComponent implements KeyListener {

    private static final long serialVersionUID = 1L;

    public boolean left;
    public boolean right;

    public  MyCanvas cv;

    void FBool() {
        left = right = false;
    }

    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_LEFT) {
            left = true;
        }
        if (e.getKeyCode() == KeyEvent.VK_RIGHT) {
            right = true;
        }
        cv.move();
        repaint();
    }

    public void keyReleased(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_LEFT) {
            left = false;
        }
        if (e.getKeyCode() == KeyEvent.VK_RIGHT) {
            right = false;
        }
        //cv.move();
        //repaint();
    }

    public void keyTyped(KeyEvent e) {
        // bla...bla..bla
    }
}

First class works perfectly and the picture frame is displayed in it too.But when I press the button (the left arrow or the right), I get an error:

 Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Game.InputKey.keyPressed(InputKey.java:28)
    at java.awt.Component.processKeyEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Window.processEvent(Unknown Source)

Please tell me what to fix in the code to make it work)

Thanks in advance, sorry for the bad design.

UPD

Still have not figured out a bit easy, but I guess not. If someone writes a ready solution to the problem - I will be happy)

Answer 1

The problem causing the NullPointerException was that you didn't assigned a value to cv.

I changed the code so it works now:

InputKey

package help.stackoverflow.keyboard_awt;

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

import javax.swing.JComponent;

public class InputKey extends JComponent implements KeyListener {

    private static final long serialVersionUID = 1L;

    public boolean left;
    public boolean right;

    private MyCanvas cv;

    public InputKey(MyCanvas cv){
        this.cv = cv;
    }

    void FBool() {
        left = right = false;
    }

    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_LEFT) {
            left = true;
        }
        if (e.getKeyCode() == KeyEvent.VK_RIGHT) {
            right = true;
        }
        cv.move();
        repaint();
    }

    public void keyReleased(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_LEFT) {
            left = false;
        }
        if (e.getKeyCode() == KeyEvent.VK_RIGHT) {
            right = false;
        }
        //cv.move();
        //repaint();
    }

    public void keyTyped(KeyEvent e) {
        // bla...bla..bla
    }
}

MyCanvas

package help.stackoverflow.keyboard_awt;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;
import javax.swing.JComponent;
import javax.swing.JFrame;

class MyCanvas extends JComponent {

    private static final long serialVersionUID = 1L;

    private static final int WIDTH = 490;
    private static final int HEIGHT = 470;
    private InputKey input = new InputKey(this);
    private int x = 10;
    private int y = 10;

    // public MyCanvas() {
    // addKeyListener(input);
    // }

    public void move() {
        if (x == 0) {
            x = 10;
        }
        if (y == 0) {
            y = 10;
        }

        if (input.left) {
            x--;
        }
        if (input.right) {
            x++;
        }
        repaint();
    }

    public void paint(Graphics g) {
        Image img1 = Toolkit.getDefaultToolkit().getImage(
                "C:\\Users\\дНМ\\workspace\\Game\\image\\Peopl.png");

        int width = img1.getWidth(this);
        int height = img1.getHeight(this);

        int scale = 4;
        int w = scale * width;
        int h = scale * height;
        g.drawImage(img1, x, y, (int) w, (int) h, this);

    }

    public static void main(String[] a) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(WIDTH, HEIGHT);
        MyCanvas myCanvas = new MyCanvas();
        frame.getContentPane().add(myCanvas);
        frame.getContentPane().setBackground(Color.BLACK);
        frame.setVisible(true);
        frame.setFocusable(true);
        frame.requestFocusInWindow();
        frame.addKeyListener(myCanvas.input);
    }

}

There was also a repaint() missing which I added.