This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12个答案)
4年前关闭。
我正在测试另一个简单的程序来画一条线。第一个问题是错误。
这是第57行的代码
如果我可以问,如何保存绘制的线条?谢谢
(12个答案)
4年前关闭。
我正在测试另一个简单的程序来画一条线。第一个问题是错误。
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at GUI$2.paint(GUI.java:57)
at javax.swing.JComponent.paintChildren(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
at javax.swing.JComponent.paintChildren(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
这是第57行的代码
g.drawLine(pointStart.x, pointStart.y, pointEnd.x, pointEnd.y);
panel = new JPanel(){
Point pointStart = null;
Point pointEnd = null;
int x = 1;
{
addMouseListener(new MouseAdapter(){
public void mousePressed(MouseEvent e){
pointStart = e.getPoint();
}
public void mouseReleased(MouseEvent e){
pointEnd = e.getPoint();
}
});
addMouseMotionListener(new MouseAdapter(){
public void mouseMoved(MouseEvent e){
pointEnd = e.getPoint();
}
public void mouseDragged(MouseEvent e){
pointEnd = e.getPoint();
repaint();
}
});
}
public void paint(Graphics g){
super.paint(g);
g.setColor(a);
g.drawLine(pointStart.x, pointStart.y, pointEnd.x, pointEnd.y);
}
};
如果我可以问,如何保存绘制的线条?谢谢
最佳答案
尝试以下操作(请注意,这是paintComponent()
方法,而不是paint()
):
public void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(a);
if(pointStart!=null && pointEnd!=null){
g.drawLine(pointStart.x, pointStart.y, pointEnd.x, pointEnd.y);
}
}
09-04 05:43