Question

当我从另一个类调用我的refresh（）方法时出现错误。

public void refresh() {
    repaint();
}

JFrame是refresh()，而我是从普通班级调用的。错误是空指针，它位于我的level.refresh()和public void refresh()上。我想要做的就是完全正常，但我收到了这个错误

Player class`package entity.player;

import java.awt.Image;
import java.awt.event.KeyEvent;

import javax.swing.ImageIcon;

import level.Level;

public class Player {
public static Image playerImgLeft = new ImageIcon(
        "resources/Player/left/player_anim1_left.png").getImage();
public static Image playerImgRight = new ImageIcon(
        "resources/Player/right/player_anim1_right.png").getImage();
public static Level levelp;
public int x = 0;
public int y = 0;
public int dy = 0;
public int dx = 0;
// Direction = right
public int direction = 0;

public Player() {
    x = 10;
    y = 10;
}

public int getX() {
    return x;
}

public int getY() {
    return y;
}

public Image getImageLeft() {
    return playerImgLeft;
}

public Image getImageRight() {
    return playerImgRight;
}

public void move() {
    x = x + dx;
    y = y + dy;

}

public void keyPressed(KeyEvent key) {
    if (key.getKeyCode() == KeyEvent.VK_A) {
        dx = -1;
        direction = 1;
        System.out.println("left");

        levelp.refresh();

    }
    if (key.getKeyCode() == KeyEvent.VK_D)
        dx = 1;
    direction = 0;

    if (key.getKeyCode() == KeyEvent.VK_W)
        dy = -1;

    if (key.getKeyCode() == KeyEvent.VK_S)
        dy = 1;

}

public void keyReleased(KeyEvent key) {

    if (key.getKeyCode() == KeyEvent.VK_A)
        dx = 0;

    if (key.getKeyCode() == KeyEvent.VK_D)
        dx = 0;

    if (key.getKeyCode() == KeyEvent.VK_W)
        dy = 0;

    if (key.getKeyCode() == KeyEvent.VK_S)
        dy = 0;

}

}

等级：

package level;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;

import javax.swing.ImageIcon;
import javax.swing.JPanel;
import javax.swing.Timer;

import entity.player.Player;

public class Level extends JPanel implements ActionListener {

private static final long serialVersionUID = 1L;
public static int level = 0;
Player player;
public Timer time;
public Image img;

public Level() {
    this.setVisible(true);
    setFocusable(true);
    player = new Player();
    this.addKeyListener(new LevelKeyListener());
     ImageIcon i = new ImageIcon("");
     img = i.getImage();

    time = new Timer(5, this);
    time.start();
}

public void actionPerformed(ActionEvent e) {
    player.move();

    repaint();
    revalidate();
}

public void refresh () {
    repaint();
}

public void paintComponent(Graphics g) {
    super.paintComponent(g);


    Graphics2D g2d = (Graphics2D) g;

     g.drawImage(img, 0, 0, null);
     if(player.direction == 0) {
    g2d.drawImage(player.getImageRight(), player.getX(), player.getY(), null);
     } else if(player.direction == 1) {
         g2d.drawImage(player.getImageLeft(), player.getX(), player.getY(),    null);

     }

}

private class LevelKeyListener extends KeyAdapter {

    public void keyPressed(KeyEvent e) {
        player.keyPressed(e);
    }

    public void keyReleased(KeyEvent e) {
        player.keyReleased(e);
    }
}
}

`

Answer 1

如所怀疑的那样，您的levelp变量指向任何内容（null）。

当你看到这一行时，你会发现你试图在一个实际上没有引用任何内容的变量上调用refresh()方法：

levelp.refresh();

刚刚声明为

public static Level levelp;

没有任何实际创建Level对象的代码（ergo：你试图在指向null的引用上调用方法。）

尝试创建一个这样的直接对象：

public static Level levelp = new Level();

我还建议您仔细阅读this post，以便了解NullPointerException的发生方式和时间！

从另一个类调用方法时出错

1 个答案: