Java: 程序不保存旧图形

Question

所以，我已经 Java 几个月了，我现在正在学习图形。所以，我正在尝试做一种绘画之类的东西，我可以用鼠标画画。没什么特别的，只是让我开始的东西。每当我拖动鼠标时，程序只会画一个小点。它有点管用，除了它不能保存我的旧点。它只是创建一个新的！如果有人能帮我解决这个问题，那就太好了：

主要class：

import javax.swing.JFrame;

public class Test{
    public static void main(String args[]){

        Ploofer ploof = new Ploofer();
        PumpkinPie f = new PumpkinPie(ploof);

        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(1000,1000);
        f.setResizable(false);
        ploof.setSize(1000,1000);
        f.add(ploof);
        f.setVisible(true);
    }
}

"Ploofer" class:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JPanel;

public class Ploofer extends JPanel{

    PumpkinPie pObj = new PumpkinPie(this);

    @Override
    public void paintComponent(Graphics g){

        super.paintComponent(g);

        Graphics2D g2d = (Graphics2D) g;

        this.setBackground(Color.WHITE);

        g2d.setColor(new Color(190, 50, 0));

        if(pObj.draw==true){
            g2d.fillRect(pObj.x, pObj.y, 2, 2);
            pObj.draw = false;
        }
    }

    @Override
    public void update(Graphics g){
        paint(g);
    }

    //I tried to override update, but it didn't really help

}

"PumpkinPie" class:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JPanel;

public class Ploofer extends JPanel{

    PumpkinPie pObj = new PumpkinPie(this);

    @Override
    public void paintComponent(Graphics g){

        super.paintComponent(g);

        Graphics2D g2d = (Graphics2D) g;

        this.setBackground(Color.WHITE);

        g2d.setColor(new Color(190, 50, 0));
    /*  g2d.fillRect(475, 475, 50, 50);
        g2d.drawString("^Red^", 484, 540); */

        if(pObj.draw==true){
            g2d.fillRect(pObj.x, pObj.y, 2, 2);
            pObj.draw = false;
        }
    }

    @Override
    public void update(Graphics g){
        paint(g);
    }

    //I tried to override update, but it didn't really help

}

Answer 1

It sort of works, except for the fact that it doesn't save my old dots. It just creates a new one!

查看 Custom Painting Approaches 进行自定义绘画的两种常用方法：

将要绘制的对象添加到 ArrayList 并遍历列表以绘制所有对象
绘制到 BufferedImage 并重新绘制 BufferedImage

对于你的情况，我推荐方法 2。

//I tried to override update, but it didn't really help

不要覆盖 update()。没有理由这样做。这是 Swing 中不需要的旧 AWT 方法。

Java: 程序不保存旧图形

Java: Program doesn't save old graphics

java

swing

jpanel

paintcomponent