Data commit

Task/Animate-a-pendulum/Java/animate-a-pendulum.java

@@ -0,0 +1,52 @@
+import java.awt.*;
+import javax.swing.*;
+
+public class Pendulum extends JPanel implements Runnable {
+
+    private double angle = Math.PI / 2;
+    private int length;
+
+    public Pendulum(int length) {
+        this.length = length;
+        setDoubleBuffered(true);
+    }
+
+    @Override
+    public void paint(Graphics g) {
+        g.setColor(Color.WHITE);
+        g.fillRect(0, 0, getWidth(), getHeight());
+        g.setColor(Color.BLACK);
+        int anchorX = getWidth() / 2, anchorY = getHeight() / 4;
+        int ballX = anchorX + (int) (Math.sin(angle) * length);
+        int ballY = anchorY + (int) (Math.cos(angle) * length);
+        g.drawLine(anchorX, anchorY, ballX, ballY);
+        g.fillOval(anchorX - 3, anchorY - 4, 7, 7);
+        g.fillOval(ballX - 7, ballY - 7, 14, 14);
+    }
+
+    public void run() {
+        double angleAccel, angleVelocity = 0, dt = 0.1;
+        while (true) {
+            angleAccel = -9.81 / length * Math.sin(angle);
+            angleVelocity += angleAccel * dt;
+            angle += angleVelocity * dt;
+            repaint();
+            try { Thread.sleep(15); } catch (InterruptedException ex) {}
+        }
+    }
+
+    @Override
+    public Dimension getPreferredSize() {
+        return new Dimension(2 * length + 50, length / 2 * 3);
+    }
+
+    public static void main(String[] args) {
+        JFrame f = new JFrame("Pendulum");
+        Pendulum p = new Pendulum(200);
+        f.add(p);
+        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
+        f.pack();
+        f.setVisible(true);
+        new Thread(p).start();
+    }
+}