How to use paint in Java

Usage of the paint method: First, the first class in the Java file must inherit Jframe or JPanel; then write a constructor method to initialize the window, and call the constructor method through the main method; finally, use the paint method. Draw pictures.

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The correct way to paint Usage:

(1) The first class in the newly created .java file inherits (extends) Jframe (container) or JPanel (lightweight container)

(2) Write a constructor to initialize the window (size, position, visible, closed, etc.).

(3) Write a main method to call the constructor.

(4) Write a paint method to draw, add @Override in front of the method.

paint principle

JFrame and JPanel classes have their own methods, and paint is their method, and the inherited paint method will be automatically called when the program is running. .

If you do not inherit, the paint method is a newly written method belonging to the current class, so the paint method will not be automatically called when the program is running. So in general, if you don't inherit, you can't draw anything.

Code example

Code that can draw something:

package Test;
import java.awt.Graphics;
import javax.swing.JFrame;
public class Test extends JFrame{
@Override
public void paint(Graphics g) {
super.paint(g);
g.drawRect(50, 50, 100, 100);
}
private void win() {
this.setVisible(true);
this.setSize(500, 500);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public static void main(String[] args) {
Test f = new Test();
f.win();
}
}

Code that cannot draw something:

package Test;
import java.awt.Graphics;
import javax.swing.JFrame;
public class Test2{
JFrame jf = new JFrame();
public Test2() {
jf.setBounds(500, 250, 500, 500);
jf.setVisible(true);
jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public void paint(Graphics g) {
g.drawRect(50, 50, 100, 100);
}
public static void main(String[] args) {
new Test2();
}
}

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