Java JFrame size and centre
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JComponent;
import javax.swing.JFrame;

public class DiceSimulator {

    public static void main(String[] args) {
        JFrame frame = new JFrame("DiceSimulator");
        frame.setVisible(true);
        frame.setSize(400, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        draw object = new draw();
        frame.add(object);
        frame.setLocationRelativeTo(null);
        object.drawing();
    }
}

import javax.swing.*;
import java.awt.*;
//import java.util.Random;
public class draw extends JComponent {

    public void drawing() {
        repaint();
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        //Random1 random = new Random1();

        Graphics2D g2 = (Graphics2D) g;
        g.setColor(Color.BLACK);
        Rectangle box = new Rectangle(115, 60, 150, 150);
        g2.fill(box);

        g.setColor(Color.WHITE);
        g.fillOval(145, 75, 30, 30);

        g.setColor(Color.WHITE);
        g.fillOval(205, 75, 30, 30);

        g.setColor(Color.WHITE);
        g.fillOval(145, 115, 30, 30);

        g.setColor(Color.WHITE);
        g.fillOval(205, 115, 30, 30);

        g.setColor(Color.WHITE);
        g.fillOval(145, 155, 30, 30);

        g.setColor(Color.WHITE);
        g.fillOval(205, 155, 30, 30);
    }
}

int width = getSize().width;
int height = getSize().height;

g.translate(5, 5);