我的项目有问题，我的项目是绘制线条（例如在Windows中绘制）。我想用mouseDragged，mousePressed和mouseReleased画多条线。但是当我运行测试时，它显示了很多错误，这是我的代码
package image;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Point;
import java.awt.Graphics;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class paint extends JFrame{
private Point points[] = new Point[10000];
private Point pointends[] = new Point[10000];
private int pointCount = 0;
public paint()
{
panel paint2 = new panel();
add(paint2,BorderLayout.CENTER);
}
private class panel extends JPanel
{
public panel()
{
setBackground(Color.BLUE);
MouseHandler handler = new MouseHandler();
this.addMouseMotionListener(handler);
this.addMouseListener(handler);
}
@Override
protected void paintComponent(Graphics g)
{
// TODO Auto-generated method stub
super.paintComponent(g);
for(int i = 0;i < pointCount;i++)
{
g.setColor(Color.RED);
g.drawLine(points[pointCount].x, points[pointCount].y, pointends[pointCount].x, pointends[pointCount].y);
}
}
}
private class MouseHandler extends MouseAdapter
{
@Override
public void mouseDragged(MouseEvent e)
{
// TODO Auto-generated method stub
pointends[ pointCount ] = e.getPoint();
repaint();
}
@Override
public void mousePressed(MouseEvent e) {
// TODO Auto-generated method stub
super.mousePressed(e);
if(pointCount < points.length)
{
points[ pointCount ] = e.getPoint();
}
}
@Override
public void mouseReleased(MouseEvent e) {
// TODO Auto-generated method stub
super.mouseReleased(e);
pointends[pointCount]=e.getPoint();
repaint();
pointCount++;
}
}
}
这是我的虚空主干
package image;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.BorderLayout;
import java.awt.BorderLayout;
public class test
{
public static void main(String[] args) {
paint paint1 = new paint();
/*paintP.add(paint1, BorderLayout.CENTER);
paintP.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
paintP.setSize(400,400);
paintP.setVisible(true);*/
paint1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
paint1.setSize(400,400);
paint1.setVisible(true);
}
}
在您的paintComponent方法中，更改行
g.drawLine(points[pointCount].x, points[pointCount].y, pointends[pointCount].x, pointends[pointCount].y);
对此：
g.drawLine(points[i].x, points[i].y, pointends[i].x, pointends[i].y);
NullPointerException一旦您松开鼠标按钮，这将消除，线条将正确绘制。（之前，您不仅试图在循环的每次迭代中绘制同一行，而且还尝试绘制一条尚不存在的行，从而绘制NullPointerException。）
还有另一个问题：在releaseMouse和mouseDragged方法中，您要在index处设置直线的终点pointCount，但您最多只能绘制pointCount - 1。pointCount开始绘制线条时，您必须增加计数器，否则仅当释放鼠标时才会绘制新的线条。解决此问题的一种方法是将鼠标侦听器更改为此：
private class MouseHandler extends MouseAdapter {
public void mouseDragged(MouseEvent e) {
pointends[ pointCount - 1 ] = e.getPoint(); // note the "- 1"
repaint();
}
public void mousePressed(MouseEvent e) {
if(pointCount < points.length) {
points[ pointCount ] = e.getPoint();
pointends[ pointCount ] = e.getPoint(); // add end point
pointCount++;
repaint();
}
}
public void mouseReleased(MouseEvent e) { // do nothing
}
}
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