我正在制作一个 Java 程序，它允许用户绘制选定的几何对象。
我想知道当我按下绘图按钮时，哪些特定的代码块会产生这个错误。
该代码片段基本上将接收用户输入的值，并将其传递给用于绘制形状/对象的四个类之一。
编辑：这是actionPerforemed全班
public void actionPerformed (ActionEvent e)
{
if (e.getSource() == exitButton)
{
System.exit(0);
}
if (e.getSource() == clearButton)
{
field1.setText("");
field2.setText("");
field3.setText("");
field4.setText("");
field5.setText("");
field6.setText("");
field7.setText("");
field8.setText("");
clearButton.setEnabled(false);
drawButton.setEnabled(false);
}
if (e.getSource() == drawButton)
{
if (combo.getSelectedIndex() == 1)
{
x1 = Integer.parseInt(field1.getText());
y1 = Integer.parseInt(field4.getText());
Dot.paintPoint(g, x1, y1);
}
if (combo.getSelectedIndex() == 2)
{
x1 = Integer.parseInt(field1.getText());
y1 = Integer.parseInt(field4.getText());
x2 = Integer.parseInt(field2.getText());
y2 = Integer.parseInt(field5.getText());
Line.paintLine(g,x1,y1,x2,y2);
}
if (combo.getSelectedIndex() == 3)
{
x1 = Integer.parseInt(field1.getText());
y1 = Integer.parseInt(field4.getText());
w = Integer.parseInt(field7.getText());
h = Integer.parseInt(field8.getText());
Oval.paintOval(g,x1,y1,w,h);
}
if (combo.getSelectedIndex() == 4)
{
x1 = Integer.parseInt(field1.getText());
y1 = Integer.parseInt(field4.getText());
x2 = Integer.parseInt(field2.getText());
y2 = Integer.parseInt(field5.getText());
x3 = Integer.parseInt(field3.getText());
y3 = Integer.parseInt(field6.getText());
Traingle.paintTriangle(g,x1,y1,x2,y2,x3,y3);
}
}
}
以下是四门课。
为了提出一个观点：
import java.awt.*;
public class Dot
{
protected static int x1,y1;
public Dot()
{
setPoint(0,0);
}
public Dot(int x1, int y1)
{
setPoint(x1,y1);
}
public void setPoint(int x1, int y1)
{
this.x1 = x1;
this.y1 = y1;
}
public static int getX1()
{
return x1;
}
public static int getY1()
{
return y1;
}
public static void paintPoint(Graphics g, int x1, int y1)
{
g.drawString(".", x1,y1);
}
}
制作一条线：
import java.awt.*;
public class Line extends Dot
{
protected static int x2,y2;
public Line()
{
setPoint(0,0,0,0);
}
public Line(int x1, int y1, int x2, int y2)
{
setPoint(x1,y1,x2,y2);
}
public void setPoint(int x1, int y1, int x2, int y2)
{
this.x1 = x1;
this.y1 = y1;
this.x2 = x2;
this.y2 = y2;
}
public static int getX2()
{
return x2;
}
public static int getY2()
{
return y2;
}
public static void paintLine(Graphics g, int x1, int y1, int x2, int y2)
{
g.drawLine(x1,y1,x2,y2);
}
}
制作椭圆形：
import java.awt.*;
public class Oval extends Dot
{
protected static int w,h;
public Oval()
{
super(0,0);
}
public Oval(int x1, int y1)
{
super(x1,y1);
}
public void setPoint(int x1, int y1)
{
this.x1 = x1;
this.y1 = y1;
this.w = w;
this.h = h;
}
public static int getWidth()
{
return w;
}
public static int getHeight()
{
return h;
}
public static void paintOval(Graphics g, int x1,int y1, int w, int h)
{
g.drawOval(x1,y1,w,h);
}
}
并制作一个三角形：
import java.awt.*;
public class Traingle extends Line
{
protected static int x3,y3;
public Traingle()
{
setPoint(0,0,0,0,0,0);
}
public Traingle(int x1, int y1, int x2, int y2)
{
setPoint(x1,y1,x2,y2,x3,y3);
}
public void setPoint(int x1, int y1, int x2, int y2,int x3, int y3)
{
this.x1 = x1;
this.y1 = y1;
this.x2 = x2;
this.y2 = y2;
this.x3 = x3;
this.y3 = y3;
}
public static int getX3()
{
return x3;
}
public static int getY3()
{
return y3;
}
public static void paintTriangle(Graphics g, int x1, int y1, int x2, int y2, int x3, int y3)
{
g.drawLine(x1,y1,x2,y2);
g.drawLine(x2,y2,x3,y3);
g.drawLine(x1,y1,x3,y3);
}
}
错误是这样的：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Dot.paintPoint(Dot.java:36)
at ShapeDriver.actionPerformed(ShapeDriver.java:451)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:682)
at java.awt.EventQueue$3.run(EventQueue.java:680)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:696)
at java.awt.EventQueue$4.run(EventQueue.java:694)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:693)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)
任何帮助，将不胜感激。提前致谢。