swing Java 2D API:空指针异常

Question

请看一下下面的代码

package java2D;

import java.awt.BasicStroke;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.*;
import java.awt.*;

public class Joins extends JFrame
{
    private JPanel centerPanel;

    public Joins()
    {   
        this.setLayout(new BorderLayout());
        this.add(createCenterPanel(),"Center");

        this.setSize(200,250);
        this.setVisible(true);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    private JPanel createCenterPanel()
    {
        centerPanel = new JPanel();
        //centerPanel.setLayout(new FlowLayout());

        Graphics g = centerPanel.getGraphics();

        Graphics2D g2d =  (Graphics2D)g;

        BasicStroke bs1 = new BasicStroke(8, BasicStroke.CAP_ROUND, BasicStroke.JOIN_BEVEL);
        g2d.setStroke(bs1);
        g2d.drawRect(15, 15, 80, 50);

        BasicStroke bs2 = new BasicStroke(8, BasicStroke.CAP_ROUND, BasicStroke.JOIN_MITER);
        g2d.setStroke(bs2);
        g2d.drawRect(125, 15, 80, 50);

        BasicStroke bs3 = new BasicStroke(8, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND);
        g2d.setStroke(bs3);
        g2d.drawRect(235, 15, 80, 50);


        return centerPanel;
    }

    public static void main(String[]args)
    {
        SwingUtilities.invokeLater(new Runnable(){

            @Override
            public void run() {
                new Joins();
            }
        });
    }
}

此代码生成NullPointerException。我需要在centerPanel中绘制图形。当我开发Swing应用程序时，我通常将JFrame区域划分为若干个JPanel，我在这里也这样做了，但最终导致了NullPointerException

下面是错误

run:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at java2D.Joins.createCenterPanel(Joins.java:34)
    at java2D.Joins.<init>(Joins.java:17)
    at java2D.Joins$1.run(Joins.java:55)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:705)
    at java.awt.EventQueue.access$000(EventQueue.java:101)
    at java.awt.EventQueue$3.run(EventQueue.java:666)
    at java.awt.EventQueue$3.run(EventQueue.java:664)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:675)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
BUILD SUCCESSFUL (total time: 6 seconds)

请帮忙

Answer 1

错误在于：

Graphics g = centerPanel.getGraphics();

根据javadocs：

Creates a graphics context for this component. This method will return null if this component is currently not displayable.

也许你需要先把JPanel添加到Joins框架中，这样你就可以使用getGraphics()了

Answer 2

没有可见内容，因此getGraphics（）返回null。试试看：

    centerPanel = new JPanel();
    JFrame frame = new JFrame();
    frame.add(centerPanel);
    frame.setVisible(true);
    centerPanel.setVisible(true);

Answer 3

正如已经指出的那样，Graphics g = centerPanel.getGraphics();是你问题的根源

永远不要使用getGraphics来执行自定义绘制。充其量getGraphics是上一个绘制周期的快照，充其量，好吧，你已经看到了最糟糕的结果，它是null

要执行自定义绘制，您应该自己创建一个自定义组件（例如从JPanel这样的组件），并重写其paintComponent方法（不要忘记调用super.paintComponet）

更多细节请看Performing Custom Painting

Answer 4

不要使用getGraphics。创建一个新的JPanel并将绘制功能放入paintComponent方法中，记住调用super.paintComponent(g)