Java JPanel Graphics - 了解如何绘制简单形状

Question

Disclaimer: I'm new to Java. I'm new to Swing. And I'm sure it shows.

I've viewed quite a number of examples/tutorials of how to draw on a jpanel "canvas". But they mostly have the same basic format and put all of their drawLine/drawRect/drawArc inside the paintComponent() method. It seems assumed that people want to draw static things to the jpanel one time. But what if I want to change the jpanel object over the course of the program's runtime, like a paint program, or a game?

I suppose I need to be able to access the jpanel object, and internal methods to paint. I'm betting what I'm doing isn't best practices, but here is what I have:

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class PaintPanel extends JPanel {
    public static JFrame frame;
    private Graphics g = getGraphics();

    public static void main(String[] args) {
        frame = new JFrame();
        frame.getContentPane().setBackground(new Color(32, 32, 32));
        frame.setResizable(false);
        frame.setBounds(1, 1, 800, 600);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);
        frame.setVisible(true);
        frame.setLocationRelativeTo(null); // center frame on screen
        
        PaintPanel paintPanel = new PaintPanel();
        paintPanel.setBounds(10, 10, 100, 100);
        paintPanel.setBackground(Color.red);
        frame.add(paintPanel);
    }
    
    // constructor
    public PaintPanel() {

    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
    }
    
    public void DrawRect(Integer x, Integer y, Integer w, Integer h, Color color) {
        g.setColor(color);
        g.fillRect(x, y, w, h);
        this.repaint(); // doesn't seem to do anything
    }
}

This code results in a red panel box, but my user method DrawRect() doesn't draw anything.

1. I've read in some places that it's necessary to override the paintComponent() method. If there's nothing in it, what's the purpose?
2. How can I get my DrawRect() method to work?

Answer 1

The piece of the puzzle you're missing is the model object. There should be an external object that describes what should be drawn. In a game for example, it would be something that describes the current state of the game.

Your custom component looks at this model and takes the necessary steps to paint it. This is implemented in paintComponent and in helper methods you see fit to add.

To make an animation, you make a loop that modifies the model over time, and asks the custom component to redraw itself with repaint().