JPanel size unknown at startup

Put the calculations in your paintComponent method. In general, you want to avoid code in paintComponent that will slow it down, but these calls shouldn't cost too much of a penalty. Also, if your program is resizable, you will need to resize things in the path like this:

public void paintComponent(Graphics g){
    super.paintComponent(g);
    int x = getWidth() - 50;
    int y = getHeight() - 50;
    g.fillRect(x, y, 100, 100);
}

      

        
        
        
      

    

but of course, in your real program, you will want to avoid the magic numbers

Another problem: don't name setSize()

on your JFrame. Instead, if you want to specify a hard size, do so in the JPanel by overriding its method getPreferredSize()

. It will also give you suggested parameters that can be used for your calculations.

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For example:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

@SuppressWarnings("serial")
public class DrawRect extends JPanel {
   private static final int PREF_W = 800;
   private static final int PREF_H = PREF_W;
   private static final int DELTA = 50;
   private static final Color RECT_COLOR = Color.red;
   private static final int RECT_WIDTH = 100;
   private static final int TIMER_DELAY = 15;
   private int rectX = PREF_W - DELTA;
   private int rectY = PREF_H - DELTA;

   public DrawRect() {
      new Timer(TIMER_DELAY, new TimerListener()).start();
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(RECT_COLOR);
      g.fillRect(rectX, rectY, RECT_WIDTH, RECT_WIDTH);
   }

   @Override
   public Dimension getPreferredSize() {
      if (isPreferredSizeSet()) {
         return super.getPreferredSize();
      }
      return new Dimension(PREF_W, PREF_H);
   }

   private class TimerListener implements ActionListener {
      @Override
      public void actionPerformed(ActionEvent e) {
         rectX--;
         rectY--;
         repaint();
      }
   }

   private static void createAndShowGui() {
      JFrame frame = new JFrame("DrawRect");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(new DrawRect());
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

      

        
        
        
      

    

---

Also, check the key bind animation code from this answer .

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.EnumMap;
import java.util.HashMap;
import java.util.Map;

public class GamePanel extends JPanel {
   private static final int ANIMATION_DELAY = 15;
   private final int HEIGHT = 400;
   private final int WIDTH = 600;
   private Square square;
   private EnumMap<Direction, Boolean> dirMap = new EnumMap<>(Direction.class);
   private Map<Integer, Direction> keyToDir = new HashMap<>();
   // !! private Circle circle;
   private Timer animationTimer;

   public GamePanel() {
      for (Direction dir : Direction.values()) {
         dirMap.put(dir, Boolean.FALSE);
      }
      keyToDir.put(KeyEvent.VK_UP, Direction.UP);
      keyToDir.put(KeyEvent.VK_DOWN, Direction.DOWN);
      keyToDir.put(KeyEvent.VK_LEFT, Direction.LEFT);
      keyToDir.put(KeyEvent.VK_RIGHT, Direction.RIGHT);
      // !! addKeyListener(new DirectionListener());
      setKeyBindings();
      setBackground(Color.white);
      setPreferredSize(new Dimension(WIDTH, HEIGHT));
      setFocusable(true);
      square = new Square();
      animationTimer = new Timer(ANIMATION_DELAY, new AnimationListener());
      animationTimer.start();
   }

   private void setKeyBindings() {
      int condition = WHEN_IN_FOCUSED_WINDOW;
      final InputMap inputMap = getInputMap(condition);
      final ActionMap actionMap = getActionMap();
      boolean[] keyPressed = { true, false };
      for (Integer keyCode : keyToDir.keySet()) {
         Direction dir = keyToDir.get(keyCode);
         for (boolean onKeyPress : keyPressed) {
            boolean onKeyRelease = !onKeyPress; // to make it clear how bindings work
            KeyStroke keyStroke = KeyStroke.getKeyStroke(keyCode, 0,
                  onKeyRelease);
            Object key = keyStroke.toString();
            inputMap.put(keyStroke, key);
            actionMap.put(key, new KeyBindingsAction(dir, onKeyPress));
         }
      }
   }

   public void paintComponent(Graphics g) {
      super.paintComponent(g);
      square.display(g);
   }

   private class AnimationListener implements ActionListener {
      @Override
      public void actionPerformed(ActionEvent evt) {
         boolean repaint = false;
         for (Direction dir : Direction.values()) {
            if (dirMap.get(dir)) {
               square.move(dir);
               repaint = true;
            }
         }
         if (repaint) {
            repaint();
         }
      }
   }

   private class KeyBindingsAction extends AbstractAction {
      private Direction dir;
      boolean pressed;

      public KeyBindingsAction(Direction dir, boolean pressed) {
         this.dir = dir;
         this.pressed = pressed;
      }

      @Override
      public void actionPerformed(ActionEvent evt) {
         dirMap.put(dir, pressed);
      }
   }

   private static void createAndShowGUI() {
      GamePanel gamePanel = new GamePanel();
      JFrame frame = new JFrame("GamePanel");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(gamePanel);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
      gamePanel.requestFocusInWindow();
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGUI();
         }
      });
   }
}

enum Direction {
   UP(0, -1), DOWN(0, 1), LEFT(-1, 0), RIGHT(1, 0);
   private int incrX;
   private int incrY;

   private Direction(int incrX, int incrY) {
      this.incrX = incrX;
      this.incrY = incrY;
   }

   public int getIncrX() {
      return incrX;
   }

   public int getIncrY() {
      return incrY;
   }
}

class Square {
   private int x = 0;
   private int y = 0;
   private int w = 20;
   private int h = w;
   private int step = 1;
   private Color color = Color.red;
   private Color fillColor = new Color(255, 150, 150);
   private Stroke stroke = new BasicStroke(3f, BasicStroke.CAP_ROUND,
         BasicStroke.JOIN_ROUND);

   public void display(Graphics g) {
      Graphics2D g2d = (Graphics2D) g.create();
      g2d.setColor(fillColor);
      g2d.fillRect(x, y, w, h);
      g2d.setStroke(stroke);
      g2d.setColor(color);
      g2d.drawRect(x, y, w, h);
      g2d.dispose();
   }

   public void setStep(int step) {
      this.step = step;
   }

   public void move(Direction dir) {
      x += step * dir.getIncrX();
      y += step * dir.getIncrY();
   }

}