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Java Scientific calculator GUI

I am working on a calculator with a GUI. I'm not sure how I should gather all the numbers from the user and do the math for them. I tried storing them in variables, but that doesn't really work since I need to be able to have multiple numbers and operators at one time. Any input would be greatly appreciated. Thanks!
import java.util.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class GUI extends JFrame implements ActionListener {
private JFrame frame;
private JPanel buttons;
//
private JPanel text;
private JTextField field;
//
private ImageIcon WCU;
private JLabel labelWCU;
///
JButton[] nums = new JButton[10];
JButton[] functions = new JButton[15];
ArrayList<Double> Addition = new ArrayList<>();
double A;
double S;
double M;
double D;
double Answer;
private JButton num1, num2, num3, num4, num5, num6, num7, num8, num9, num0;
private JButton decimal, add, subtract, multiply, divide, equals, squareRoot;
private JButton sin, cos, tan, exclamation, oneOverX, xSquared, log;
private JButton clear;
double input1 = 0, input2 = 0, answer = 0;
char op;
public GUI() {
frame = new JFrame();
buttons = new JPanel();
//
text = new JPanel();
field = new JTextField(30);
//
WCU = new ImageIcon("WCU.png");
labelWCU = new JLabel(WCU);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setTitle("Calculator"); // make sure to center
frame.setSize(525,750);
frame.setLocationRelativeTo(null);
frame.setResizable(false);
frame.setLayout(new BorderLayout());
frame.add(text, BorderLayout.NORTH);
frame.add(buttons, BorderLayout.CENTER);
text.add(field);
field.setEditable(false);
text.setBackground(Color.RED);
buttons.setBackground(Color.red);
buttons.setBorder(BorderFactory.createEmptyBorder(20,50,300,50));
buttons.setLayout(new GridLayout(6,4,20,20));
//
buttons.add(decimal = new JButton ("."));
functions[0] = decimal;
buttons.add(add = new JButton ("+"));
functions[1] = add;
buttons.add(subtract = new JButton ("-"));
functions[2] = subtract;
buttons.add(multiply = new JButton ("*"));
functions[3] = multiply;
buttons.add(divide = new JButton ("/"));
functions[4] = divide;
buttons.add(equals = new JButton ("="));
functions[5] = equals;
buttons.add(squareRoot = new JButton ("√"));
functions[6] = squareRoot;
buttons.add(num1 = new JButton ("1"));
nums[1] = num1;
buttons.add(num2 = new JButton ("2"));
nums[2] = num2;
buttons.add(num3 = new JButton ("3"));
nums[3] = num3;
buttons.add(num4 = new JButton ("4"));
nums[4] = num4;
buttons.add(num5 = new JButton ("5"));
nums[5] = num5;
buttons.add(num6 = new JButton ("6"));
nums[6] = num6;
buttons.add(num7 = new JButton ("7"));
nums[7] = num7;
buttons.add(num8 = new JButton ("8"));
nums[8] = num8;
buttons.add(num9 = new JButton ("9"));
nums[9] = num9;
buttons.add(num0 = new JButton ("0"));
nums[0] = num0;
buttons.add(sin = new JButton ("sin"));
functions[7] = sin;
buttons.add(cos = new JButton ("cos"));
functions[8] = cos;
buttons.add(tan = new JButton ("tan"));
functions[9] = tan;
buttons.add(exclamation = new JButton ("!"));
functions[10] = exclamation;
buttons.add(oneOverX = new JButton ("1/x"));
functions[11] = oneOverX;
buttons.add(xSquared = new JButton ("x^2"));
functions[12] = xSquared;
buttons.add(log = new JButton ("log"));
functions[13] = log;
text.add(clear = new JButton("Clear"));
functions[14] = clear;
labelWCU.setIcon(WCU);
frame.setVisible(true);
//
for (int i = 0; i < 15; i++){
functions[i].addActionListener(this);
functions[i].setFocusable(false);
}
for (int i = 0; i < 10; i++){
nums[i].addActionListener(this);
nums[i].setFocusable(false);
}
}
public void actionPerformed(ActionEvent e){
for (int i = 0; i < 10; i++){
if(e.getSource() == nums[i]){
field.setText(field.getText().concat(String.valueOf(i)));
}
}
if(e.getSource() == decimal){
field.setText(field.getText().concat("."));
}
if(e.getSource() == add){
A = Double.parseDouble(field.getText());
op = '+';
field.setText("");
}
if(e.getSource() == subtract){
input1 = Double.parseDouble(field.getText());
op = '-';
field.setText("");
}
if(e.getSource() == multiply){
input1 = Double.parseDouble(field.getText());
op = '*';
field.setText("");
}
if(e.getSource() == divide){
input1 = Double.parseDouble(field.getText());
op = '/';
field.setText("");
}
if(e.getSource() == equals){
field.setText(String.valueOf(Addition));
}
if(e.getSource() == clear){
field.setText("");
A = 0;
}
}
}

I don't fully understand the purpose of all fields in your class, so I'll propose my own set. If we (ab)use the text field as storage for the current value, two state fields are enough for the basic functionality. It's not necessary to store any more data:
private String firstBinaryOperand = null;
private char binaryOp = ' ';
These fields are used if there is a pending binary operation such as + or -. We don't need them for unary operations. For example, if the user types "700 + 200 =", then
When the "+" button is pressed for the first time, the currently displayed value (700) is stored in firstBinaryOperand and '+' in binaryOp.
When the "=" button is pressed, the current value (200) and the value cached in firstBinaryOperand are added and the result (900) written into the text field.
We therefore need a method that evaluates a pending binary operation if there is one and writes the result into the text field:
// Evaluate pending binary operation, if there is one.
private void evaluateBinaryOp() {
if (binaryOp != ' ') {
double firstOperand = Double.parseDouble(firstBinaryOperand);
double secondOperand = Double.parseDouble(field.getText());
double result;
if (binaryOp == '+') {
field.setText(Double.toString(firstOperand + secondOperand));
} else if (binaryOp == '-') {
field.setText(Double.toString(firstOperand - secondOperand));
}
// Implementation of further binary operands ...
// Update state, there is no longer a pending binary operation
firstBinaryOperand = null;
binaryOp = ' ';
}
}
Addition as an example of a binary operation:
if (e.getSource() == add){
evaluateBinaryOp(); // evaluate pending binary op, if any
firstBinaryOperand = field.getText();
binaryOp = '+';
field.setText("0");
}
Unary operations are simpler, but we still have to check whether there is a pending binary operation and evaluate it first if this is the case:
if (e.getSource() == oneOverX) {
evaluateBinaryOp(); // evaluate pending binary op, if any
double currentValue = Double.parseDouble(field.getText());
field.setText(Double.toString(1.0 / currentValue));
}
All other unary and binary operations can be implemented using the same pattern.
This is going to give you a very basic calculator whose behavior differs a bit from the usual. For example, if the user presses +, they will not see the intermediate result but always 0. The intermediate result isn't lost though as it is stored in the firrstBinaryOperand field in the background. The correct result will be shown once = or another unary operator is used. I hope this answers your question and is enough to get you started, you can then refine the behavior in a second step.

Can't figure out how to code my calculator

I am trying to code a simple calculator like the one you would see in windows (non-scientific). I can not for the life of me figure out how to properly change the text after pressing an operator but still see the original entry that wont change until the next digit is pressed, have the equals key do the same operation if used multiple times (ie: 5+5 = 10 +5 = 15), and when I input a digit that is the same (5+5) it won't accept the second digit. I know I am very close and am probably missing a certain boolean that could allow my code to work properly, but Im not sure where to look. Thanks for any help.
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class JCalculator extends JFrame implements ActionListener {
/** intialize the jlabel, jbutton, string that makes the buttons and jpanel that will hold the buttons
*
*/
private JTextField display = new JTextField("0", SwingConstants.RIGHT);
private JPanel buttonPanel = new JPanel();
private JPanel displayPanel = new JPanel();
private JButton newButton, divButton, multiButton, equalsButton, clearButton, plusButton, minusButton;
private String[] arrayButtons = {"7","8", "9", "/", "4", "5", "6", "x", "1", "2", "3", "-", "0","C", "=", "+"};
private int operand = 0;
private int firstInput = 0, secondInput = 0, answer = 0;
JCalculator() {
/** creates the jframe */
JFrame jfrm = new JFrame("Calculator");
/** sets the size of the jfrm and then positions it in the center of screen */
jfrm.setSize(480, 300);
jfrm.setLocationRelativeTo(null);
/** sets the program to close on exit */
jfrm.setDefaultCloseOperation(jfrm.EXIT_ON_CLOSE);
/** ImageIcon img = new ImageIcon(JCalculator.png);
*
*/
/**sets initial layout to border */
jfrm.setLayout(new BorderLayout());
/** makes a panel named display that holds the Jtext display
* enables it and justifies it right
*/
displayPanel.setLayout(new BorderLayout());
displayPanel.add(display, BorderLayout.CENTER);
display.setHorizontalAlignment(JTextField.RIGHT);
display.setEnabled(false);
/** makes a panel for buttons that is a grid layout
*
*/
buttonPanel.setLayout(new GridLayout(4,4));
/** makes a loop to make all of the 16 buttons for the calculator
* the operands all have their own specific buttons
*/
for (int i = 0; i < arrayButtons.length; i++) {
if (arrayButtons[i].equals("/")) {
divButton = new JButton(arrayButtons[i]);
divButton.addActionListener(this);
buttonPanel.add(divButton);
} else if (arrayButtons[i].equals("x")) {
multiButton = new JButton(arrayButtons[i]);
multiButton.addActionListener(this);
buttonPanel.add(multiButton);
} else if (arrayButtons[i].equals("=")) {
equalsButton = new JButton(arrayButtons[i]);
equalsButton.addActionListener(this);
buttonPanel.add(equalsButton);
} else if (arrayButtons[i].equals("C")) {
clearButton = new JButton(arrayButtons[i]);
clearButton.addActionListener(this);
buttonPanel.add(clearButton);
} else if (arrayButtons[i].equals("+")) {
plusButton = new JButton(arrayButtons[i]);
plusButton.addActionListener(this);
buttonPanel.add(plusButton);
} else if (arrayButtons[i].equals("-")) {
minusButton = new JButton(arrayButtons[i]);
minusButton.addActionListener(this);
buttonPanel.add(minusButton);
} else {
newButton = new JButton(arrayButtons[i]);
newButton.addActionListener(this);
buttonPanel.add(newButton);
}
}
/** adds the two panels to the jfrm
* sets jfrm visibility to true
* sets equalsButton to the default button
*/
jfrm.add(displayPanel, BorderLayout.NORTH);
jfrm.add(buttonPanel, BorderLayout.CENTER);
jfrm.getRootPane().setDefaultButton(equalsButton);
jfrm.setVisible(true);
}
public void actionPerformed(ActionEvent ae) {
String s = ae.getActionCommand();
char stringCheck = s.charAt(0);
if (answer == Integer.parseInt(display.getText())) {
display.setText("0");
}
if (Character.isDigit(stringCheck) && display.getText().length() < 8) {
if (display.getText().equals("0")) {
display.setText(s);
} else {
display.setText(display.getText().concat(s));
}
} if (s.equals("+")){
answer = Integer.parseInt(display.getText());
operand = 1;
} if (s.equals( "-")){
answer = Integer.parseInt(display.getText());
operand = 2;
} if (s.equals("x")){
answer = Integer.parseInt(display.getText());
operand = 3;
} if (s.equals("/")){
answer = Integer.parseInt(display.getText());
operand = 4;
} if (s.equals("C")){
firstInput = 0;
secondInput = 0;
answer = 0;
operand = 0;
display.setText("0");
} if (s.equals("=")){
switch(operand){
case 1:
answer += Integer.parseInt(display.getText());
display.setText(Integer.toString(answer));
break;
case 2:
answer -= Integer.parseInt(display.getText());
display.setText(Integer.toString(answer));
break;
case 3:
answer *= Integer.parseInt(display.getText());
display.setText(Integer.toString(answer));
break;
case 4:
if(Integer.parseInt(display.getText()) > 0 || Integer.parseInt(display.getText()) < 0) {
answer /= Integer.parseInt(display.getText());
display.setText(Integer.toString(answer));
break;
} else {
display.setText("Div by 0");
}
}
if(answer > 100000000 || answer < -100000000){
display.setText("Overlow");
}
}
}
public static void main (String [] args){
/**
* run program
*/
SwingUtilities.invokeLater(new Runnable() {
public void run() {
new JCalculator();
}
});
}
}

GetChangeGui manual JFrame

I'm trying to write a code for my CS 1 class.
The point of the code is to write a code where you are making change out of a hundred dollar bill out for what ever amount, I need to give back the appropriate bills and coins.
I have to write the JFrame manually
It would be helpful if someone could show me where I'm going wrong in the computation.
/**
*
* #author esamayoa
*/
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class GetChange extends JFrame {
//Declare variables
JButton bCompute, bReset;
JTextField tAmount, tQuarters, tDimes, tNickels, tPennies, tTwenty, tTen, tFive, tOne;
JLabel lAmount, lQuarters, lDimes, lNickels, lPennies, lTwenty, lTen, lFive, lOne;
double amount, diff, totalPaid, quarter, dime, nickel, penny, twenty, ten, five, one;
public GetChange (){
//Set the attributes of the Jframe
setTitle("Eric");
setLocation(500,10);
setSize(450,1000);
setDefaultCloseOperation(EXIT_ON_CLOSE);
setResizable(false);
setVisible(true);
//Create your Gui components
lAmount = new JLabel("Amount");
lQuarters = new JLabel("Quarters:");
lDimes = new JLabel("Dimes:");
lNickels = new JLabel("Nickels:");
lPennies = new JLabel("Pennies:");
lTwenty = new JLabel("Twenties");
lTen = new JLabel("Tens");
lFive = new JLabel("Fives");
lOne = new JLabel("Ones");
tAmount = new JTextField();
tQuarters = new JTextField();
tDimes = new JTextField();
tNickels = new JTextField();
tPennies = new JTextField();
tTwenty = new JTextField();
tTen = new JTextField();
tFive = new JTextField();
tOne = new JTextField();
bReset = new JButton("Reset");
bCompute = new JButton("Compute");
//Add you Gui components to the Jframe
setLayout(new GridLayout(10,2));
add(lAmount);
add(tAmount);
add(lQuarters);
add(tQuarters);
add(lDimes);
add(tDimes);
add(lNickels);
add(tNickels);
add(lPennies);
add(tPennies);
add(lTwenty);
add(tTwenty);
add(lTen);
add(tTen);
add(lFive);
add(tFive);
add(lOne);
add(tOne);
add(bCompute);
add(bReset);
//Updates frame
this.validate();
//Add Action Listeners to buttons
bReset.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent ae) {
tAmount.setText("");
tQuarters.setText("");
tDimes.setText("");
tNickels.setText("");
tPennies.setText("");
tTwenty.setText("");
tTen.setText("");
tFive.setText("");
tOne.setText("");
tAmount.setText("");
}
});
//Create computation for compute button
bCompute.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent ae) {
totalPaid = 100;
amount = Double.parseDouble(tAmount.getText());
diff = totalPaid-amount;
twenty = diff/20;
diff = diff%20;
tTwenty.setText(""+ twenty);
ten = diff/10;
diff = diff%10;
tTen.setText(""+ ten);
five = diff/5;
diff = diff%5;
tFive.setText(""+ five);
one = diff/1;
diff = diff%1;
tOne.setText(""+ one);
quarter = diff/.25;
diff = diff%.25;
tQuarters.setText(""+ quarter);
dime = diff/.1;
diff = diff%.1;
tDimes.setText(""+ dime);
nickel = diff/.05;
diff = diff%.05;
tNickels.setText(""+ nickel);
penny = diff/.01;
diff = diff%.01;
tPennies.setText(""+ penny);
}
});
}
//Main Method
public static void main(String[] args) {
// TODO code application logic here
GetChange myApp = new GetChange();
}
}

To Solve the First problem(Frame not showing) insert the following after all components are added,
this.validate(); // updates frame
To add implementation to the reset button, just use .setText("");
bReset.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent ae) {
tAmount.setText("");
tQuarters.setText("");
tDimes.setText("");
tNickels.setText("");
tPennies.setText("");
tTwenty.setText("");
tTen.setText("");
tFive.setText("");
tOne.setText("");
tResult.setText("");
}
});
To add implementation to the Compute Button...
bCompute.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent ae) {
double Amount = Double.parseDouble(tAmount.getText());
double q = (double) Integer.parseInt(tQuarters.getText()) *.25;
double d = (double) Integer.parseInt(tDimes.getText()) *.10;
double n = (double) Integer.parseInt(tNickels.getText()) *.05;
double p = (double) Integer.parseInt(tPennies.getText()) *.01;
double T = (double) Integer.parseInt(tTwenty.getText()) *20;
double Ten = (double) Integer.parseInt(tTen.getText()) *10;
double Five = (double) Integer.parseInt(tFive.getText()) *5;
double one = (double) Integer.parseInt(tOne.getText()) *1;
double TotalPaid = q+d+n+p+T+Ten+Five+1;
double diff = TotalPaid-Amount;
//Heres an example to create how many twenties you need
int totalTwenties = (int)diff /20;
diff = diff%20;
tTwenty.setText("" + totalTwenties);
}
});
Basically, I got the difference between Amount paid and Amount Cost.
A) Then, the number of times a 20 can fit into the difference is found by /20. Next, I modulus or found the remainder of the difference and twenty.
B) To Find the number of other coins/bills that you need to give as change, repeat the process described in A.

NumberFormatException: empty string when clicking the JButton?

I made a program that has a JFrame that contains a JTextField, a button, and two JLabels. When a number is typed into the JTextField, either pressing the enter key or clicking on the JButton should display the number in scientific notation on the second JLabel. When I hit the enter key, it works, however, when I click on the JButton, it does not. It gives me a NumberFormatException: empty string.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class MyMath extends JPanel implements ActionListener
{
private JTextField textField;
private static JLabel textArea;
private static JLabel comment;
private JButton button;
private static JFrame frame;
public MyMath()
{
comment = new JLabel("Enter a number");
textField = new JTextField();
textField.setSize(new Dimension(10 , 10));
textField.addActionListener(this);
button = new JButton("Go");
button.addActionListener(this);
}
public static void addComponentsToPane(Container pane)
{
textArea = new JLabel(" ");
pane.setLayout(new BoxLayout(pane, BoxLayout.PAGE_AXIS));
pane.add(new MyMath().textField);
pane.add(new MyMath().button);
pane.add(new MyMath().comment);
pane.add(textArea);
}
public void actionPerformed(ActionEvent evt)
{
String text = textField.getText();
textArea.setText(SciNotation.convertToSciNotation(text));
textArea.setHorizontalAlignment(SwingConstants.CENTER);
comment.setText(text + " in Scientific Notation:");
textField.selectAll();
}
private static void createAndShowGUI()
{
frame = new JFrame("Scientific Notation");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
addComponentsToPane(frame.getContentPane());
Container bg = frame.getContentPane();
Dimension d = new Dimension(300, 150);
bg.setPreferredSize(d);
frame.setResizable(false);
frame.getContentPane().setBackground(Color.GREEN);
Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
Point screenCenter = new Point (screen.width/2 , screen.height/2);
Point center = new Point(screenCenter.x - (150), screenCenter.y - (75));
frame.setLocation(center);
frame.pack();
frame.setVisible(true);
}
public static void main(String[] args)
{
SwingUtilities.invokeLater(new Runnable()
{
public void run()
{
createAndShowGUI();
}
});
}
}
Here is SciNotation.java
import java.awt.Component;
import java.awt.font.TextAttribute;
import java.text.AttributedString;
public class SciNotation
{
public static String convertToSciNotation(String num)
{
num = num.replaceAll("," , "");
if (num.contains("E")) //not working
{
String[] partsE = num.split("E");
String beforeE = partsE[0];
String afterE = partsE[1];
char first = num.charAt(0);
num = first + beforeE;
}
double number = Double.parseDouble(num);
double resultNumber = 0;
int power = 0;
String numString = Double.toString(number);
String[] parts = numString.split("\\.");
String decimals = parts[1];
int decimalInt = Integer.parseInt(decimals);
int numberInt = (int) number;
if(number > -1 && number < 1)
{
String[] low = numString.split("\\.");
String afterLow = low[1];
for(int i = 1; i < 10; i++)
{
String decNums = Integer.toString(i);
afterLow = afterLow.replaceAll(decNums, "");
int zeros = afterLow.length();
power = -1 * (zeros + 1);
resultNumber = number * Math.pow(10, zeros + 1);
decimals = "";
}
}
if(decimalInt == 0)
{
decimals = "";
}
if( number >= 10)
{
power = Integer.toString(numberInt).length() - 1;
resultNumber = numberInt/(Math.pow(10,(power)));
}
if((number >= 1 && number < 10) || (number <= -1 && number > -10))
{
resultNumber = number;
decimals = "";
}
if(number <= -10)
{
power = Integer.toString(numberInt).length() - 2;
resultNumber = numberInt/(Math.pow(10,(power)));
}
return resultNumber + decimals + " x 10^" + power;
}
}

This bug is pretty tricky. You constructed three MyMath instances. So your code was holding a wrong reference to your JTextFiled instance. That's why you couldn't get the right input.
public static void addComponentsToPane(Container pane) {
textArea = new JLabel(" ");
pane.setLayout(new BoxLayout(pane, BoxLayout.PAGE_AXIS));
pane.add(new MyMath().textField); //MyMath instance 1
pane.add(new MyMath().button); //MyMath instance 2
pane.add(new MyMath().comment); //MyMath instance 3
pane.add(textArea);
}
Here is the right code:
public static void addComponentsToPane(Container pane) {
MyMath myMath = new MyMath();
textArea = new JLabel(" ");
pane.setLayout(new BoxLayout(pane, BoxLayout.PAGE_AXIS));
pane.add(myMath.textField);
pane.add(myMath.button);
pane.add(myMath.comment);
pane.add(textArea);
}
:)

java Clear Table on button click

I am writing a Mortgage Calculator for class and I have it working the way I need it to, except everytime I click the "Calculate" button it will just continue to add to the table instead of the table clearing and showing new values. I know my code might look a little sloppy and I have some things commented out that don't need to be there because I'm still working it, but do you have any suggestions?
FYI I am still a beginner learning Java and it has taken me over 20hrs to get this far (and i"m pretty proud of myself!) Thank you!!
//Import all required Packages
import javax.swing.*;
import javax.swing.table.*;
import java.awt.*;
import java.text.*;
import java.awt.event.*;
public class MortgageCalculator extends JFrame implements ActionListener {
// Loan Values
double intPrincipal, interestRate, calcPayment, monthlyInterest, currentInterest, principalPaid, newBalance;
int totalMonths;
double[] loanInterest = {5.35, 5.5, 5.75}; // Yearly interest in decimal form
int[] loanTerm = {7, 15, 30}; // Total months of term
String principal;
String comboArray[] = {"7 Years at 5.35%", "15 Years at 5.5%", "30 Years at 5.75%"};
int termYears, termMonths, done, i=0, m=0, p=0;
//Set up panels
JPanel contentPanel;
//Set up labels
JLabel mortgageLabel, paymentLabel, termLabel;
//Set up buttons
JButton calculateButton, clearButton, exitButton;
//TextFields
JTextField txtMortgage = new JTextField(10);
JTextField txtPayment = new JTextField(10);
//New Text Area
JTextArea textarea = new JTextArea();
DecimalFormat df = new DecimalFormat("$###,###.00"); //Formatting the results to decimal form
//Combo Box
JComboBox loansList = new JComboBox();
DefaultTableModel model = new DefaultTableModel();
JTable table = new JTable(model);
//Build GUI
public MortgageCalculator()
{
super();
initializeContent();
}
public void initializeContent()
{
this.setSize(700, 500);
this.setLocation(0, 0);
this.setContentPane(contentPanel());
this.setTitle("Mortgage Calculator");
}
public JPanel contentPanel()
{
contentPanel = new JPanel();
contentPanel.setLayout(null);
//Add labels to the panel
mortgageLabel = new JLabel("Mortgage:");
mortgageLabel.setLocation(200, 30);
mortgageLabel.setSize(100, 25);
contentPanel.add(mortgageLabel);
termLabel = new JLabel("Term & Rate:");
termLabel.setLocation(183, 55);
termLabel.setSize(100, 30);
contentPanel.add(termLabel);
paymentLabel = new JLabel("Monthly Payment:");
paymentLabel.setLocation(158, 85);
paymentLabel.setSize(100, 30);
contentPanel.add(paymentLabel);
//Text Fields
txtMortgage = new JTextField(10);
txtMortgage.setLocation(280, 30);
txtMortgage.setSize(150, 25);
contentPanel.add(txtMortgage);
txtPayment = new JTextField(10);
txtPayment.setLocation(280, 85);
txtPayment.setSize(150, 25);
contentPanel.add(txtPayment);
//Combo Box
loansList.addItem(comboArray[0]);
loansList.addItem(comboArray[1]);
loansList.addItem(comboArray[2]);
loansList.setLocation(280, 55);
loansList.setSize(150, 25);
loansList.addActionListener(this);
contentPanel.add(loansList);
//textarea.setPreferredSize(new Dimension(650, 300));
//JScrollPane scroller = new JScrollPane(textarea);
JScrollPane scroller = new JScrollPane(table);
contentPanel.add(scroller);
scroller.setSize(650,300);
scroller.setLocation(20, 150);
textarea.setLineWrap(true);
model.addColumn("Payment Number");
model.addColumn("Current Interest");
model.addColumn("Principal Paid");
model.addColumn("New Balance");
//Buttons
exitButton = new JButton("Exit");
exitButton.setLocation(450, 30);
exitButton.setSize(100, 25);
contentPanel.add(exitButton);
clearButton = new JButton("Clear");
clearButton.setLocation(450, 55);
clearButton.setSize(100, 25);
contentPanel.add(clearButton);
calculateButton = new JButton("Calculate");
calculateButton.setLocation(450, 85);
calculateButton.setSize(100, 25);
contentPanel.add(calculateButton);
//setup up buttons
calculateButton.addActionListener(this);
clearButton.addActionListener(this);
exitButton.addActionListener(this);
return contentPanel;
}
//Define actions performed for buttons
public void actionPerformed(ActionEvent e)
{
String arg = e.getActionCommand();
if (e.getSource() == loansList) {
switch (loansList.getSelectedIndex()) {
case 0:
i = 0;
break;
case 1:
i = 1;
break;
case 2:
i = 2;
break;
}
}
if (arg == "Calculate")
{
txtPayment.setText("");
principal = txtMortgage.getText();
try {
intPrincipal = Double.parseDouble(principal);
if (intPrincipal <= 0) throw new NumberFormatException();
}
catch(NumberFormatException n){
txtPayment.setText("Please Enter a Postive Numeric Number");
done = 1;
}
if (done == 1)
done = 0;
else {
interestRate = loanInterest[i];
termYears = loanTerm[i];
monthlyInterest = interestRate/(12*100); //calculates monthly interest
termMonths = termYears*12; //calculates term length in months
calcPayment = monthlyInterest*intPrincipal/(1-Math.pow((1+monthlyInterest), -termMonths)); //calculates monthly payment
txtPayment.setText(" " + df.format(calcPayment));
for (m=0; m<=totalMonths; m++) {
totalMonths = loanTerm[i]*12;
currentInterest = intPrincipal * monthlyInterest;
principalPaid = calcPayment - currentInterest;
newBalance = intPrincipal - principalPaid;
intPrincipal = newBalance;
/* printAndAppend(
(m+1) + " " +
df.format(currentInterest) + " " +
df.format(principalPaid) + " " +
df.format(newBalance) + "\n");
//textarea.setText(df.format(currentInterest));
if(intPrincipal <= 1){ break;}*/
// Create a couple of columns
model.addRow(new Object[]{m+1, df.format(currentInterest), df.format(principalPaid), df.format(newBalance)});
if(intPrincipal <= 1){ break;}
}
}
}
else if (e.getSource() == clearButton)
{
txtMortgage.setText(""); //clear Mortgage textfield
txtPayment.setText(""); //clear Payment textfield
txtMortgage.requestFocusInWindow(); //move cursor back to Mortgage textfield
loansList.setSelectedIndex(0);
}
else if (e.getSource() == exitButton)
System.exit(0);
}
public void printAndAppend(String text) {
textarea.append(text);
}
public static void main(String[] args)
{
new MortgageCalculator().setVisible(true);
}
}

To clear all you need to do is set the row count of the model to 0 -- that's it:
else if (e.getSource() == clearButton) {
txtMortgage.setText("");
txtPayment.setText("");
txtMortgage.requestFocusInWindow();
loansList.setSelectedIndex(0);
model.setRowCount(0); //!! added
}
Also, this is not good:
if (arg == "Calculate") {
As you shouldn't use == to compare Strings. If you want to compare Strings, use the equals method:
if (arg.equals("Calculate")) {
or the equalsIgnoreCase method:
if (arg.equalsIgnoreCase("Calculate")) {
The reason this is important is because == checks to see if one String object is the same as another String object, and you really don't care about this. Instead you want to know if one String holds the same chars as another, and that's what equals tests for.
Also, I'd set the model's row count to 0 at the beginning of your calculate method, and this way you can recalculate things without having to clear.