why did my color of JPanel fill up all the window gui? I didn't know if there's something wrong in the code or the compiler?

the window appeared like this: this is my code:

import java.awt.Color;

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class gui2 {

    public static void main(String[] args) {
        JPanel bluepanel = new JPanel();
        bluepanel.setBackground(Color.blue);
        bluepanel.setBounds(0, 0, 250, 250);
        
        JPanel greenpanel = new JPanel();
        greenpanel.setBackground(Color.green);
        greenpanel.setBounds(250, 0, 250, 250);
        
        JPanel redpanel = new JPanel();
        redpanel.setBackground(Color.red);
        redpanel.setBounds(0, 250, 250, 250);
        
        JFrame frame = new JFrame();
        frame.setTitle("what?");
        frame.setSize(500,500);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.add(bluepanel);
        frame.add(greenpanel);
        frame.add(redpanel);
    }

}

I had tried to add new JPanel or even setBackground Color, but it cannot solve the problem.

You should set JFrame layout to null.

Here is updated code.

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

public class gui2 {

    public static void main(String[] args) {
        // Create panels
        JPanel bluepanel = new JPanel();
        bluepanel.setBackground(Color.blue);
        bluepanel.setBounds(0, 0, 250, 250);
        
        JPanel greenpanel = new JPanel();
        greenpanel.setBackground(Color.green);
        greenpanel.setBounds(250, 0, 250, 250);
        
        JPanel redpanel = new JPanel();
        redpanel.setBackground(Color.red);
        redpanel.setBounds(0, 250, 250, 250);
        
        // Create frame
        JFrame frame = new JFrame();
        frame.setTitle("what?");
        frame.setSize(500, 500);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(null); // Set layout manager to null
        frame.add(bluepanel);
        frame.add(greenpanel);
        frame.add(redpanel);
        frame.setVisible(true);
    }
}