import java.awt.BorderLayout;
import java.awt.EventQueue;

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

public class Rechner extends JFrame {

	/* constants for operations */
	static final int PLUS =0;
	static final int MINUS =1;
	static final int MULTIPLICATION =2;
	static final int DIVISION =3;
	
	private JPanel contentPane;
	private JTextField operand;
	private JTextField ergebnis;
	private JTextArea verlauf;
	
	/**
	 * Launch the application.
	 */
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {
					Rechner frame = new Rechner();
					frame.setVisible(true);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
	}

	/**
	 * Create the frame.
	 */
	public Rechner() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 651, 431);
		contentPane = new JPanel();
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		setContentPane(contentPane);
		contentPane.setLayout(null);
		
		JButton clear = new JButton("clear");
		clear.setBounds(488, 268, 89, 23);
		contentPane.add(clear);
		
		JButton plus = new JButton("+");
		plus.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				doOperation(Rechner.PLUS);	
			}
		});
		
		plus.setBounds(371, 268, 89, 23);
		contentPane.add(plus);
		
		JButton minus = new JButton("-");
		minus.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				doOperation(Rechner.MINUS);	
			}
		});
		minus.setBounds(371, 294, 89, 23);
		contentPane.add(minus);
		
		JButton mul = new JButton("*");
		mul.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				doOperation(Rechner.MULTIPLICATION);
				
			}
			
		});
		mul.setBounds(371, 320, 89, 23);
		contentPane.add(mul);
		
		
		JButton div = new JButton("/");
		div.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				doOperation(Rechner.DIVISION);	
			}
		});
		div.setBounds(371, 345, 89, 23);
		contentPane.add(div);
		
		
		
		operand = new JTextField();
		operand.setBounds(202, 269, 127, 20);
		contentPane.add(operand);
		operand.setColumns(10);
		
		JLabel lblOperand = new JLabel("Operand");
		lblOperand.setFont(new Font("Tahoma", Font.BOLD, 14));
		lblOperand.setBounds(47, 268, 113, 32);
		contentPane.add(lblOperand);
		
		JLabel lblVerlauf = new JLabel("Verlauf:");
		lblVerlauf.setFont(new Font("Tahoma", Font.BOLD, 14));
		lblVerlauf.setBounds(47, 38, 113, 77);
		contentPane.add(lblVerlauf);
		
		JLabel lblErgebnis = new JLabel("Ergebnis:");
		lblErgebnis.setFont(new Font("Tahoma", Font.BOLD, 14));
		lblErgebnis.setBounds(47, 217, 113, 58);
		contentPane.add(lblErgebnis);
		
		ergebnis = new JTextField();
		ergebnis.setColumns(10);
		ergebnis.setBounds(202, 238, 127, 20);
		contentPane.add(ergebnis);
		
		verlauf = new JTextArea();
		verlauf.setBounds(202, 38, 375, 189);
		contentPane.add(verlauf);
	}
	
	private void doOperation(int type) {
		try {
			if ("".equals(operand.getText())) {
				verlauf.setText(verlauf.getText()+"\nKeine Zahl eingeben!\n");
				operand.setBackground(Color.RED);
				return;
			} 
			if ("".equals(ergebnis.getText())) {
				ergebnis.setText("0");
			}
			operand.setBackground(Color.WHITE);
			double z= Double.parseDouble(operand.getText());
			double e= Double.parseDouble(ergebnis.getText());
			switch (type) {
			 	case Rechner.PLUS : {
			 		verlauf.setText(verlauf.getText()+"+"+operand.getText()+"\n");
			 		ergebnis.setText(String.valueOf(e+z));
			 		break;
			 	}
			 	case Rechner.MINUS : {
			 		verlauf.setText(verlauf.getText()+"-"+operand.getText()+"\n");
			 		ergebnis.setText(new Double(e-z).toString());
			 		break;
			 	}
			 	case Rechner.DIVISION : {
			 		verlauf.setText(verlauf.getText()+"/"+operand.getText()+"\n");
			 		ergebnis.setText(String.valueOf(e/z));
			 		break;
			 	}
			 	case Rechner.MULTIPLICATION : {
			 		verlauf.setText(verlauf.getText()+"*"+operand.getText()+"\n");
			 		ergebnis.setText(new Double(e*z).toString());
					break;
			 	}
			}
			operand.setText("");
		} catch (Exception e)  {
			verlauf.setText("Fehler:"+e.toString()+"\n");
			operand.setText("0");
		}
	}

}