使用Applet绘制线条现在搞砸了

Question

我们最近开始使用Applet并用它做事，所以最后一堂课是绘图线。因此，在写完一个成功的代码之后，我出于某种原因改变了一行代码，而我现在并不知道这一行，而是绘制一条奇怪的光线。这是代码所以任何帮助将不胜感激。

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

public class SimplePaint extends Applet {
    private int lastx;
    private int lasty;
    @Override
    public void init() {
        this.setForeground(Color.BLUE);
        this.addMouseListener(new PositionRecorder());
        this.addMouseMotionListener(new LineDrawer());
    }
    public int getLastx() {
        return lastx;
    }
    public void setLastx(int lastx) {
        this.lastx = lastx;
    }
    public int getLasty() {
        return lasty;
    }
    public void setLasty(int lasty) {
        this.lasty = lasty;
    }
    private class PositionRecorder extends MouseAdapter{
        public void mousePressed(MouseEvent e) {
            setLastx(e.getX());
            setLasty(e.getY());

        }
    }
    private class LineDrawer extends MouseMotionAdapter{
        public void mouseDragged(MouseEvent e) {
            int x=e.getX();
            int y=e.getY();
            Graphics g;
            g=getGraphics();
            g.drawLine(lastx, lasty, x, y);

        }

    }
}

我特别记得如果有帮助的话，搞乱这部分代码

private class PositionRecorder extends MouseAdapter{
   public void mousePressed(MouseEvent e) {
     setLastx(e.getX());
     setLasty(e.getY());

Answer 1

AWT中的绘画是一个记录良好的过程。你应该从阅读开始：

• 特别是Painting in AWT and Swing，我建议你阅读有关The Paint Method的部分
• A Closer Look at the Paint Mechanism

“近距离观察油漆机制”的行情

paint方法（由java.awt.Component定义。）只要需要渲染组件，此方法将由绘制子系统执行。

在AWT中，绘画具有破坏性，预计随时调用paint，您将完全重绘组件状态。这引发了一些您需要解决的问题。

对于你需要绘制的任何东西，你需要一种可靠的方法来存储这些状态，并在调用paint时重新应用它们。在你的情况下，像ArrayList这样简单的东西是合适的，但你也可以使用BufferedImage作为后备缓冲区并将提交的绘图元素绘制到它。

第一种方法的好处是提供撤消更改的能力，但就绘画而言，并不像简单地绘制图像那么快。

更聪明的方法是使用两者。维持绘制状态在ArrayList中，渲染每个像BufferedImage一样，当你想要执行撤消时，从一个新的BufferedImage开始并将所有状态绘制到你不需要的点......但那是超出了问题的范围

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

public class SimplePaint extends Applet {

    private Point clickPoint;
    private Point dragPoint;

    @Override
    public void init() {
        this.setForeground(Color.BLUE);
        this.addMouseListener(new PositionRecorder());
        this.addMouseMotionListener(new LineDrawer());
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);
        if (clickPoint != null && dragPoint != null) {
            g.drawLine(clickPoint.x, clickPoint.y, dragPoint.x, dragPoint.y);
        }
    }

    private class PositionRecorder extends MouseAdapter {

        public void mousePressed(MouseEvent e) {
            clickPoint = e.getPoint();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            // This is where I'd reset the clickPoint and dragPoint
            // to null and apply the line to some structure which can
            // redraw it in the `paint` method, maybe something like
            // a ArrayList
            // Alternativly, you could render the result to a BufferedImage
            // and paint that inside the paint method
        }
    }

    private class LineDrawer extends MouseMotionAdapter {

        public void mouseDragged(MouseEvent e) {
            dragPoint = e.getPoint();
            repaint();
        }

    }
}