这段代码用于在graphic2d中绘制形状，但当我按下绘制按钮时，我有一个错误，我不知道为什么我有一个null指针异常，我想这个错误可能来自于 "向前，向右，向左和移动到 "的方法，我是新的java，我在谷歌搜索，但我没有找到解决方案。
误区
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
ayaaaa
at com.company.Main.actionPerformed(Main.java:101)
at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:269)
at java.desktop/java.awt.Component.processMouseEvent(Component.java:6578)
at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3343)
at java.desktop/java.awt.Component.processEvent(Component.java:6343)
at java.desktop/java.awt.Container.processEvent(Container.java:2259)
at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:4961)
at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2317)
at java.desktop/java.awt.Component.dispatchEvent(Component.java:4793)
at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4904)
at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4539)
at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4480)
at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2303)
at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2758)
at java.desktop/java.awt.Component.dispatchEvent(Component.java:4793)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:766)
at java.desktop/java.awt.EventQueue.access$500(EventQueue.java:97)
at java.desktop/java.awt.EventQueue$3.run(EventQueue.java:717)
at java.desktop/java.awt.EventQueue$3.run(EventQueue.java:711)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:89)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:99)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:739)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:737)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:89)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:736)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:199)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
我的代码。
package com.company;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Main extends JFrame implements ActionListener {
Graphics2D g2d;
Container c;
JLabel label_x, label_y,label_len,label_angle,label_direction,label_shape;
JTextField text_x,text_y,text_len,text_angle,text_direction,text_shape;
JButton draw;
public Main(){
c = getContentPane();
c.setLayout(null);
label_x = new JLabel("x");
label_x.setBounds(20, 20, 50, 30);
text_x = new JTextField();
text_x.setBounds(70, 20, 50, 30);
label_len = new JLabel("Lenght");
label_len.setBounds(130, 20, 50, 30);
text_len = new JTextField();
text_len.setBounds(180, 20, 50, 30);
label_angle = new JLabel("Angle");
label_angle.setBounds(240, 20, 50, 30);
text_angle = new JTextField();
text_angle.setBounds(290, 20, 50, 30);
label_y = new JLabel("y");
label_y.setBounds(20, 60, 50, 30);
text_y = new JTextField();
text_y.setBounds(70, 60, 50, 30);
label_direction = new JLabel("Direction");
label_direction.setBounds(130, 60, 70, 30);
text_direction = new JTextField();
text_direction.setBounds(200, 60, 50, 30);
label_shape = new JLabel("Shape to draw");
label_shape.setBounds(260, 60, 100, 30);
text_shape= new JTextField();
text_shape.setBounds(360, 60, 50, 30);
draw = new JButton("Draw");
draw.setBounds(200, 140, 100, 50);
draw.addActionListener(this);
c.add(label_x);
c. add(label_y);
c. add(text_x);
c. add(text_y);
c. add(label_angle);
c. add(label_direction);
c. add(label_len);
c. add(label_shape);
c. add(draw);
c. add(text_angle);
c. add(text_len);
c. add(text_shape);
c. add(text_direction);
c.setBackground(Color.gray);
setVisible(true);
setSize(600,600);
}
public void drawshape(int x,int y,int len,int dir,double angle,String s){
System.out.println("ayaaaa");
for(int i=0;i<s.length();i++){
char z=s.charAt(i);
if(z=='F'){
forward(len);
}else if(z=='+'){
right(angle);
}else if(z=='-'){
left(angle);
}else if(z=='r'){
g2d.setColor(Color.red); }
else if(z=='b'){
g2d.setColor(Color.blue); }
else if(z=='g'){
g2d.setColor(Color.green);
}
}
}
public void left(double a){
g2d.rotate(-a*Math.PI/180);
}
public void right(double a){
g2d.rotate(a*Math.PI/180);
}
public void forward(int len){
g2d.drawLine(0,0,len,0);
g2d.translate(len,0);
}
public void moveTo(int x,int y){
g2d.translate(x,y);
}
public static void main(String[] args) {
new Main();
}
public void actionPerformed(ActionEvent e) {
if (e.getSource() == draw) {
System.out.println("ayaaaa");
// g2d.drawLine(0,0,50,0);
drawshape(Integer.parseInt(text_x.getText()),Integer.parseInt(text_y.getText()),Integer.parseInt(text_len.getText()),
Integer.parseInt(text_direction.getText()),Double.parseDouble(text_angle.getText()),text_shape.getText());
}
}
}
你从来没有初始化 Graphics2D g2d; 所以它是空的。在许多方法中，例如 drawshape 你调用它的一些方法，例如 g2d.setColor(Color.red);. 这些调用会产生一个 NullPointerException.
见 什么是NullPointerException，如何解决？ 有关NullPointerException的详细解释。