Java 从其他类调用方法时出错
从另一个类调用refresh()方法时出错Java 从其他类调用方法时出错,java,swing,Java,Swing,从另一个类调用refresh()方法时出错 public void refresh() { repaint(); } refresh()位于JFrame中,我从普通类调用它。错误是一个空指针,它位于我的级别。refresh(),以及public void refresh()。 我试图做的是工作完全正常,但我得到了这个错误 Player class`package entity.player; import java.awt.Image; import java.awt.event.K
public void refresh() {
repaint();
}
refresh()
位于JFrame
中,我从普通类调用它。错误是一个空指针,它位于我的级别。refresh()
,以及public void refresh()
。
我试图做的是工作完全正常,但我得到了这个错误
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;
}
}
级别类别:
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);
}
}
}
`正如所怀疑的那样,您的
levelp
变量没有指向任何内容(即null
)
当您查看这一行时,您将看到您正试图对一个实际上没有引用任何内容的变量调用refresh()
方法:
levelp.refresh();
它刚刚被宣布为
public static Level levelp;
没有任何实际创建级别
对象的代码(因此:您试图对指向null
的引用调用方法)
尝试创建一个立即对象,如下所示:
public static Level levelp = new Level();
我还鼓励您通读一遍,以便了解
NullPointerException
s是如何以及何时发生的 如果看不到更多的代码,我们无法判断。堆栈跟踪也会有帮助。如果堆栈跟踪发生在level.refresh()
的行上,则您的level
对象很可能为空。如果堆栈跟踪可能是重复的,请向我们展示更多,这样我们就可以知道如何帮助您了——但不要太多,让我们淹没在不相关的代码中。是的,在这里提问是一项习得的技能,因此你最好开始改进你的技能。我刚刚添加了代码。*Epic fail*我想当我调用新级别()时,它会创建一个新的JPanel。我不敢苟同。levelp字段绝对不应该是静态字段@user3234092:您应该进行此更改,将其更改为实例字段。