Formular.java

package Rechner;
import java.awt.BorderLayout;
import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JTextField;
import javax.swing.JTextArea;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;


public class Formular extends JFrame {

    private JPanel contentPane;
    private JTextField textFieldSumme;
    private JTextField textFieldZinsen;
    private JTextField textFieldAnzahl;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Formular frame = new Formular();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the frame.
     */
    public Formular() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 450, 300);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        contentPane.setLayout(null);
        setContentPane(contentPane);

        textFieldSumme = new JTextField();
        textFieldSumme.setBounds(42, 24, 86, 20);
        contentPane.add(textFieldSumme);
        textFieldSumme.setColumns(10);

        textFieldZinsen = new JTextField();
        textFieldZinsen.setBounds(42, 55, 86, 20);
        contentPane.add(textFieldZinsen);
        textFieldZinsen.setColumns(10);

        textFieldAnzahl = new JTextField();
        textFieldAnzahl.setBounds(42, 86, 86, 20);
        contentPane.add(textFieldAnzahl);
        textFieldAnzahl.setColumns(10);

        final JTextArea textAreaAusgabe = new JTextArea();
        textAreaAusgabe.setBounds(42, 123, 344, 111);
        contentPane.add(textAreaAusgabe);

        JButton btnOk = new JButton("Ok");
        btnOk.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                String kreditsumme = textFieldSumme.getText();
                String zinsen = textFieldZinsen.getText();
                String anzahl = textFieldAnzahl.getText();
                Kreditrechner rech = new Kreditrechner(kreditsumme,zinsen,anzahl);
                textAreaAusgabe.setText(Double.toString(rech.getKreditsumme()));
            }
        });
        btnOk.setBounds(166, 67, 89, 23);
        contentPane.add(btnOk);
    }
}