Java Graphics Not Executing Paint Function Without extends

问题

I've been battling a bit (or a lot) with Java's Graphics, I've been going through the documentation and tutorials for drawing Java Graphics.

Every example I've found seems to have a main class that extends a JPanel and then calls itself which executes the paint function somehow.

Is it possible to draw graphics without using extends at all? I have a basic program

import javax.swing.*;
import awt.Graphics;

public class basicWindow {

    public static void main(String[] args) {

        JFrame frame = new JFrame();
        frame.setTitle("Basic Window");
        frame.setSize(300, 300);
        frame.setDefaultCloseOperation(JInternalFrame.DISPOSE_ON_CLOSE);

        JPanel panel = new JPanel();

        frame.add(panel);

        frame.setVisible(true);

    }

    public void paint(Graphics g) {

        g.fillRect(5, 15, 50, 75);

    }

}

This opens a window but doesn't draw anything which I was I expect because I don't know how to execute the paint function.

If I try

Graphics graphic = new Graphics();

paint(graphic);

It doesn't work because it's complaining about the class being abstract.

回答1:

Is it possible to draw graphics without using extends at all?

Yes, it sure is possible. As mentioned by @HovercraftFullOfEels in a comment:

You can draw on a BufferedImage..

If you display that image in a JLabel (lets call it label) then to get the screen to update after the image has changed, simply call:

label.repaint();

回答2:

You will want to read the tutorials and follow them. Java Swing uses passive graphics, and your code needs to respect this.

As per the tutorials, have your drawing class extend JPanel.
Draw within the paintComponent method override
Don't forget the @Override annotation.

Regarding,

Is it possible to draw graphics without using extends at all?

You can draw on a BufferedImage and display that within the paintComponent method. This still requires that you extend the component.
You can draw with a Graphics object obtained by calling getGraphics() on a component, however if you do this, the image will not be stable, and you risk throwing a NPE, and so this is not recommended.

来源：https://stackoverflow.com/questions/35484267/java-graphics-not-executing-paint-function-without-extends

标签

java

swing

graphics

paint