JFrame

package main;

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;

public class main {

    public static void main(String[] args){

        // Frames

        JFrame mainFrame = new JFrame("It's the frame");

        // Panels

        JPanel panel = new JPanel();

        // Buttons

        JButton button1 = new JButton();
        JButton button2 = new JButton();
        JButton button3 = new JButton();
        JButton button4 = new JButton();

        // Frame
        mainFrame.setVisible(true);
        mainFrame.add(panel);
        mainFrame.setResizable(false);
        mainFrame.setSize(500, 150);

        // Panel

        panel.add(button1);
        panel.add(button2);
        panel.add(button3);
        panel.add(button4);

        // Button 1

        button1.setBackground(Color.RED);
        button1.setForeground(Color.RED);
        button1.setPreferredSize(new Dimension(100, 100));

        // Button 2

        button2.setBackground(Color.GREEN);
        button2.setForeground(Color.GREEN);
        button2.setPreferredSize(new Dimension(100, 100));

        // Button 3

        button3.setBackground(Color.BLUE);
        button3.setForeground(Color.BLUE);
        button3.setPreferredSize(new Dimension(100, 100));

        // Button 4

        button4.setBackground(Color.YELLOW);
        button4.setForeground(Color.YELLOW);
        button4.setPreferredSize(new Dimension(100, 100));

        // Pack it up

        mainFrame.pack();

        }
    }