Java:程序不保存旧图形

Question

所以，我已经学习 Java 几个月了，现在我正在学习图形。所以，我正在尝试制作一种油漆之类的东西，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，程序都会绘制一个小点。它有点有效，除了它不能保存我的旧点这一事实。它只是创建一个新的!如果有人能帮助我解决这个问题，那就太好了:

主类:

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”类:

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

}

“南瓜派”类:

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自定义绘画的两种常见方法:

1. 将要绘制的对象添加到 ArrayList 并迭代列表以绘制所有对象
2. 绘制 BufferedImage 并重新绘制 BufferedImage

对于您的情况，我建议采用方法 2。

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

不要重写 update()。没有理由这样做。这是一种旧的 AWT 方法，在 Swing 中不需要。