Refactoring some Java

Exceptions.java

package com.davelnewton.exceptions;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.DecimalFormat;

import static java.lang.Math.abs;

public class Exceptions extends JFrame implements ActionListener {
  JLabel mort         = new JLabel("Enter the mortgage amount: ");
  JTextField mortText = new JTextField(8);

  double[] term = { 7.0, 15.0, 30.0 };
  double[] rate = { .0535, .055, .0575 };

  JTextArea edit = new JTextArea(20, 60);
  JScrollPane scroll = new JScrollPane(edit, ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS, ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

  JLabel payment = new JLabel("Your monthly payment is: ");

  JLabel out = new JLabel("           ");

  JMenuBar jmb = new JMenuBar();

  JMenu termRate      = new JMenu("Term & Rate");
  JMenuItem termRate1 = new JMenuItem("7 Year at 5.35%");
  JMenuItem termRate2 = new JMenuItem("15 Year at 5.50%");
  JMenuItem termRate3 = new JMenuItem("30 Year at 5.75%");

  JMenu exit      = new JMenu("Exit");
  JMenuItem reset = new JMenuItem("Reset");
  JMenuItem close = new JMenuItem("Close");

  DecimalFormat decimalFormatter = new DecimalFormat("###,##0.00");

  public Exceptions() {
    Container contentPane = getContentPane();
    FlowLayout layout = new FlowLayout(FlowLayout.LEFT, 35, 20);
    contentPane.setLayout(layout);

    jmb.add(termRate);
    termRate.add(termRate1);
    termRate.add(termRate2);
    termRate.add(termRate3);

    termRate1.addActionListener(this);
    termRate2.addActionListener(this);
    termRate3.addActionListener(this);

    jmb.add(exit);
    exit.add(reset);
    exit.add(close);

    reset.addActionListener(this);
    close.addActionListener(this);

    setJMenuBar(jmb);

    contentPane.add(scroll);
    contentPane.add(mort);
    contentPane.add(mortText);
    contentPane.add(payment);
    contentPane.add(out);

    setContentPane(contentPane);
  }

  public static void main(String[] arguments) throws Exception {
    Exceptions calc = new Exceptions();
    calc.setTitle("Mortgage Calculator");
    calc.setDefaultCloseOperation(EXIT_ON_CLOSE);
    calc.setDefaultCloseOperation(DISPOSE_ON_CLOSE);
    calc.setSize(800, 500);
    calc.setVisible(true);
  }

  public void actionPerformed(ActionEvent event) {
    Object source = event.getSource();

    if (source == close) {
      System.exit(0);
    }

    if (source == reset) {
      clearFields();
      return;
    }

    double mortgage = Double.parseDouble(mortText.getText());

    String result = "";

    if (source == termRate1) {
      result = doCalc(0, mortgage);
    }

    if (source == termRate2) {
      result = doCalc(1, mortgage);
    }

    if (source == termRate3) {
      result = doCalc(2, mortgage);
    }

    edit.append("\n\nPeriod \t Payment \t Interest  \t Balance \t Balance \n");
    edit.append(result);
  }

  public String doCalc(int idx, double n) {
    double tr = n * (rate[idx] / 12) / (1.0 - Math.pow((rate[idx] / 12 + 1), -term[idx] * 12));

    String output = " $" + decimalFormatter.format(tr);
    out.setText(output);

    StringBuilder s = new StringBuilder();
    for (int i = 0; i < term[idx] * 12; i++) {
      double interestPaid  = n * (rate[idx] / 12);
      double principalPaid = tr - interestPaid;

      n -= principalPaid;

      s.append("\n  ")
       .append(i + 1)
       .append(" \t ")
       .append(decimalFormatter.format(interestPaid + principalPaid))
       .append(" \t ")
       .append(decimalFormatter.format(interestPaid))
       .append(" \t ")
       .append(decimalFormatter.format(principalPaid))
       .append(" \t ")
       .append(decimalFormatter.format(abs(n)));
    }

    return s.toString();
  }

  public void clearFields() {
    edit.setText("");
    mortText.setText("");
    out.setText("");
  }
}

Guts.java

  public void actionPerformed(ActionEvent event) {
    Object source = event.getSource();

    if (source == close) {
      System.exit(0);
    }

    if (source == reset) {
      clearFields();
      return;
    }

    double mortgage = Double.parseDouble(mortText.getText());

    String result = "";

    if (source == termRate1) {
      result = doCalc(0, mortgage);
    }

    if (source == termRate2) {
      result = doCalc(1, mortgage);
    }

    if (source == termRate3) {
      result = doCalc(2, mortgage);
    }

    edit.append("\n\nPeriod \t Payment \t Interest  \t Balance \t Balance \n");
    edit.append(result);
  }

  public String doCalc(int idx, double n) {
    double tr = n * (rate[idx] / 12) / (1.0 - Math.pow((rate[idx] / 12 + 1), -term[idx] * 12));

    String output = " $" + decimalFormatter.format(tr);
    out.setText(output);

    StringBuilder s = new StringBuilder();
    for (int i = 0; i < term[idx] * 12; i++) {
      double interestPaid  = n * (rate[idx] / 12);
      double principalPaid = tr - interestPaid;

      n -= principalPaid;

      s.append("\n  ")
       .append(i + 1)
       .append(" \t ")
       .append(decimalFormatter.format(interestPaid + principalPaid))
       .append(" \t ")
       .append(decimalFormatter.format(interestPaid))
       .append(" \t ")
       .append(decimalFormatter.format(principalPaid))
       .append(" \t ")
       .append(decimalFormatter.format(abs(n)));
    }

    return s.toString();
  }

MoreRefactors.java

public double getMortgage() {
  double mortgage = getMortgageAmount(sMortgage);
  validateMortgageRange(mortgage);
  return mortgage;
}

// This method stands on its own: it's easy to test, operates
// at a single level of abstraction (i.e., "it does only one
// thing, and does it completely"), and is easy to think about.
public void validateMortgage(double mortgage) throws Exception { 
  if ((mortgage < 5000) || (mortgage > 1000000)) {
    throw new IllegalMortgageAmountException("Mortgage must be between $5000 and $1000000");
  }
}

// Ditto.
public double parseMortgage(sMortgageAmount) {
  try {
    return Double.parseDouble(sMortgageAmount);
  } catch (Exception e) {
    throw new IllegalMortgageAmountException("Unable to parse mortgage amount '" + sMortgageAmount + "'");
  }
}