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import java.awt.Color;
import java.awt.Graphics;

import javax.swing.JFrame;

public class GUI extends JFrame
{
	public static void main(String[] args) {
		GUI g = new GUI();
	}

	public GUI()
	{
		this.setTitle("Fenster");
		this.setSize(1000, 1000);
		this.setLocation(20, 20);
		this.setVisible(true);
	}

	public void paint(Graphics g) {
		g.setColor(Color.black);
		g.drawLine(0, 0, 100, 100);
	}
}

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import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;

public class GUI extends JFrame
{
	private boolean shallPaint = false;

	public static void main(String[] args) {
		GUI g = new GUI();
	}

	public GUI()
	{
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		this.setTitle("Fenster");
		this.setSize(1000, 1000);
		this.setLocation(20, 20);
		this.setLayout(new FlowLayout());

		JButton button = new JButton("zeichnen");
		button.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e)
			{
				shallPaint = true;
				Graphics g = GUI.this.getGraphics();
				g.setColor(Color.red);
				//diese Linie wird wieder verschwinden
				g.drawLine(0, 100, 100, 0);
				GUI.this.paint(g);
			}
		});
		this.add(button);
		this.setVisible(true);
	}

	public void paint(Graphics g) {
		if (shallPaint) {
			g.setColor(Color.green);
			//die Linie bleibt auch beim Neuzeichnen erhalten
			g.drawLine(0, 0, 100, 100);
		}
	}
}

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

import javax.swing.Timer;

public class PENDEL
{
	private double theta;
	private double thetaPunkt;
	private double thetaPunktPunkt;
	private double m;
	private double l;
	private double g;
	private double omega;
	private double I;
	private double tau;
	private double x;
	private double y;
	private int xAlt;
	private int yAlt;
	private GUI h;

	public static void main(String[] args) {
		new PENDEL();
	}

	public PENDEL()
	{
		// initialize instance variables
		theta = 0.785;
		thetaPunkt = 0;
		thetaPunktPunkt = 0;
		m = 0.05;
		l = 0.10;
		g = 9.81;
		I = m * l * l;
		omega = Math.sqrt(m * g * l / I);
		tau = 0.0001;
		x = Math.sin(theta) * l;
		int X = (int) x;
		xAlt = X;
		y = Math.cos(theta) * l;
		int Y = (int) y;
		yAlt = Y;
		h = new GUI();

		final Timer t = new Timer(1, new ActionListener() {
			@Override
			public void actionPerformed(ActionEvent arg0) {
				PENDEL.this.pendelSchwingt();
			}
		});
		t.start();
	}

	public double getX() {
		return x;
	}

	public double getY() {
		return y;
	}

	void pendelSchwingt()
	{
		h.zeichnen(this);
		x = (int) xAlt;
		y = (int) yAlt;
		theta = thetaPunkt * tau + theta;
		thetaPunkt = thetaPunktPunkt * tau + thetaPunkt;
		thetaPunktPunkt = omega * omega * Math.sin(theta);
		x = Math.sin(theta) * l;
		System.out.println("x=" + x);
		y = Math.cos(theta) * l;
		System.out.println("y=" + y);
	}
}

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import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;

import javax.swing.JFrame;

public class GUI extends JFrame
{
	public GUI()
	{
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		this.setTitle("Fenster");
		this.setSize(1000, 1000);
		this.setLocation(20, 20);
		this.setLayout(new FlowLayout());
		this.setVisible(true);
	}

	private PENDEL pendel;

	public void zeichnen(PENDEL pendel)
	{
		this.pendel = pendel;
		Graphics g = GUI.this.getGraphics();
		GUI.this.paint(g);
	}

	public void paint(Graphics g)
	{
		int x0 = 200;
		int y0 = 200;
		int x1 = (int) (pendel.getX() * 500 + x0);
		int y1 = (int) (-pendel.getY() * 500 + y0);
		g.setColor(Color.white);
		g.fillRect(-1, -1, 1000, 1000);
		g.setColor(Color.black);
		g.drawLine(x0, y0, x1, y1);
		g.fillOval(x0 - 5, y0 - 5, 10, 10);
		g.setColor(Color.blue);
		g.fillOval(x1 - 5, y1 - 5, 10, 10);
	}
}