Simple Java 2D graphics: draw a rectangle?

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To draw a rectangle in Swing you should:

  • First of all, never draw directly in the JFrame or other top-level window.
  • Instead draw in a JPanel, JComponent or other class that eventually extends from JComponent.
  • You should override the paintComponent(Graphics g) method.
  • You should be sure to call the super method
  • You should draw your rectangle with the Graphics object provided to the method by the JVM.
  • You should read the painting in Swing tutorial.

Clear?

e.g.,

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

public class DrawRect extends JPanel {
   private static final int RECT_X = 20;
   private static final int RECT_Y = RECT_X;
   private static final int RECT_WIDTH = 100;
   private static final int RECT_HEIGHT = RECT_WIDTH;

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      // draw the rectangle here
      g.drawRect(RECT_X, RECT_Y, RECT_WIDTH, RECT_HEIGHT);
   }

   @Override
   public Dimension getPreferredSize() {
      // so that our GUI is big enough
      return new Dimension(RECT_WIDTH + 2 * RECT_X, RECT_HEIGHT + 2 * RECT_Y);
   }

   // create the GUI explicitly on the Swing event thread
   private static void createAndShowGui() {
      DrawRect mainPanel = new DrawRect();

      JFrame frame = new JFrame("DrawRect");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

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