Question

我有2个班级Drawing.java和Front.java。我希望使用java.awt.graphics类中的drawing库创建形状，只有在front类中调用该类的函数时才会创建形状。这是洞察力的代码。

Drawing.java类：

import java.awt.Graphics;
import javax.swing.JPanel;

public class Drawing extends JPanel {

Graphics g;
int x, y;
String name;

public Drawing(int x, int y, String name) {
    this.name = name;
    this.x = x;
    this.y = y;
}

char[] n = name.toCharArray();

public void drawProcess(){
    g.drawOval(x, y, 100, 100);
    g.drawChars(n, 0, 40, x+5, y);
}

Front.java类：

import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import static java.lang.Math.sqrt;
import javax.swing.JOptionPane;

public class Front extends javax.swing.JFrame { 
.
.
.
canvas.addMouseListener(new MouseAdapter(){
    @Override
    public void mouseClicked(MouseEvent evt){
        System.out.print(" " + evt.getX() + " " + evt.getY() + " " + name);

        Drawing dr = new Drawing(evt.getX(), evt.getY(), name);
        dr.drawProcess();
    }
});

运行Front类时，出现以下错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at dfd_analyzer.Drawing.<init>(Drawing.java:14)
at dfd_analyzer.Front$5.mouseClicked(Front.java:294)
at java.awt.AWTEventMulticaster.mouseClicked(AWTEventMulticaster.java:270)
at java.awt.Component.processMouseEvent(Component.java:6519)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6281)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4872)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4698)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4501)
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:4698)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:747)
at java.awt.EventQueue.access$300(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:706)
at java.awt.EventQueue$3.run(EventQueue.java:704)
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:720)
at java.awt.EventQueue$4.run(EventQueue.java:718)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:717)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

现在，根据我的理解，我想我在引用图形对象或该方向上的某些东西时遇到问题，或者我对正在发生的事情缺乏清楚的了解，将不胜感激。提前致谢。

更新：我能够创建drawing类的实例，但是调用dr.drawProcess()会导致nullpointer异常。我知道这是因为Graphics抽象类未正确引用。我是Swing的新手，真的可以在这里使用一些帮助。

如何通过另一个类中的函数调用创建图形对象？

0 个答案: