AWT-EventQueue-0执行
[英]AWT-EventQueue-0 exepction
问题描述
I'm studying a little example about thread and the program for the example is follow, the compile well, but when i press the button to start to draw the circle Netbeans throw me an exception, someone could help me. 
我正在研究一个有关线程的小例子，该例子的程序如下，编译良好，但是当我按下按钮开始绘制圆圈时，Netbeans抛出了异常，有人可以帮助我。
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at feedingschedule.FillDemo.fill(FillDemo.java:63) at feedingschedule.FillDemo.actionPerformed(FillDemo.java:56) at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022) at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348) 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:6535) at javax.swing.JComponent.processMouseEvent(JComponent.java:3324) at java.awt.Component.processEvent(Component.java:6300) at java.awt.Container.processEvent(Container.java:2236) at java.awt.Component.dispatchEventImpl(Component.java:4891) at java.awt.Container.dispatchEventImpl(Container.java:2294) at java.awt.Component.dispatchEvent(Component.java:47
Thanks. 谢谢。
package feedingschedule;
import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
/**
*
* @author ManagerCompufi
*/
public class FillDemo extends JFrame implements ActionListener {
public static final int WIDTH_S = 300;
public static final int HEIGHT_S = 200;
public static final int FILL_WITH = 300;
public static final int FILL_HEIGHT = 100;
public static final int CIRCLE_SIZE = 10;
public static final int PAUSE = 100;
private final JPanel box;
public static void main(String[] args) {
FillDemo gui = new FillDemo();
gui.setVisible(true);
}
public FillDemo(){
setSize(WIDTH_S, HEIGHT_S);
setTitle("FillDemo");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLayout(new BorderLayout());
box = new JPanel();
JPanel buttonPanel = new JPanel();
buttonPanel.setLayout(new FlowLayout());
JButton startButton = new JButton();
startButton.addActionListener(this);
buttonPanel.add(startButton);
add(buttonPanel, "South");
}
@Override
public void actionPerformed(ActionEvent e) {
fill();
}
public void fill(){
Graphics g = box.getGraphics();
for (int y = 0; y < FILL_HEIGHT; y = y + CIRCLE_SIZE)
for (int x = 0; x < FILL_WITH; x = x + CIRCLE_SIZE){
g.fillOval(x, y, CIRCLE_SIZE, CIRCLE_SIZE);
doNothing(PAUSE);
}
}
public void doNothing(int milliseconds){
try {
Thread.sleep(milliseconds);
}
catch (InterruptedException e) {
System.out.println("Unexpected interrup");
System.exit(0);
}
}
}
1 个解决方案
解决方案1
0 2016-04-07 12:51:21
Into your program, 进入您的程序，
Graphics g = box.getGraphics(); // which returns null
and afterward , you are try to perform some operation on it, 然后，您尝试对其执行一些操作，
g.fillOval(x, y, CIRCLE_SIZE, CIRCLE_SIZE); // which leads to NullPointerException
So, better to put checks over there likewise, 所以，最好也把支票放在那边，
Graphics g = box.getGraphics();
if(g != null){
for (int y = 0; y < FILL_HEIGHT; y = y + CIRCLE_SIZE)
for (int x = 0; x < FILL_WITH; x = x + CIRCLE_SIZE){
......
}
}
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