RPN Calculator (March 19th)

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.border.Border;

public class RPNCalc {

    //Frame constructs
    public static JFrame frame;
    public static JPanel mainPanel;
    public static JPanel numbersPanel;
    public static JPanel buttonPanel;
    public static JTextArea output;
    public static JTextArea input;

    //numerical buttons
    public static JButton one;
    public static JButton two;
    public static JButton three;
    public static JButton four;
    public static JButton five;
    public static JButton six;
    public static JButton seven;
    public static JButton eight;
    public static JButton nine;
    public static JButton zero;

    //operator buttons
    public static JButton add;
    public static JButton subtract;
    public static JButton multiply;
    public static JButton divide;
    public static JButton equals;

    //other functions
    public static JButton clear;
    public static JButton clearAll;
    public static JButton decimal;
    public static JButton back;

    //Action listener
    public static ActionListener buttonListener;

    //Doubles for storing math
    public static double numOne = 0;
    public static double numTwo = 0;

    //String for displaying the operator
    public static String displayOperator = "";

    //Boolean for when to overwrite the current input with totally new input
    public static boolean overwrite = false;

    //JButton array
    public static JButton[] buttons;

    public static void main(String[] args){

        //initialize all the components
        Initialize();
        //set up the GUI
        GUI();
        //update the button presses and do the math
        Update();

    }

    public static void Initialize(){

        //simple inits for frame, panels and text fields
        frame = new JFrame("Calculator");

        mainPanel = new JPanel();
        numbersPanel = new JPanel();
        buttonPanel = new JPanel();

        output = new JTextArea();
        input = new JTextArea();

        frame.setSize(300,380);
        frame.setResizable(false);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        mainPanel.setSize(300,380);

        //create a border for the numbersPanel
        Border inputOutputBorder = BorderFactory.createLineBorder(Color.black);

        numbersPanel.setPreferredSize(new Dimension(280,60));
        numbersPanel.setBackground(Color.white);
        numbersPanel.setBorder(inputOutputBorder);

        buttonPanel.setPreferredSize(new Dimension(290,380));

        output.setPreferredSize(new Dimension(268,19));
        output.setEditable(false);

        input.setPreferredSize(new Dimension(268,29));
        input.setEditable(false);

        //universal dimension to make it easier to change later
        Dimension buttonDimension = new Dimension(65,50);

        //create buttons
        one = new JButton("1");
        two = new JButton("2");
        three = new JButton("3");
        four = new JButton("4");
        five = new JButton("5");
        six = new JButton("6");
        seven = new JButton("7");
        eight = new JButton("8");
        nine = new JButton("9");
        zero = new JButton("0");

        //operator buttons
        add = new JButton("+");
        subtract = new JButton("-");
        multiply = new JButton("x");
        divide = new JButton("\\");
        equals = new JButton("=");

        //other buttons
        clear = new JButton("C");
        clearAll = new JButton("Clear All");
        decimal = new JButton(".");
        back = new JButton("<==");

        //add buttons to buttons array
        buttons = new JButton[]{one,two,three,add,
                                four,five,six,subtract,
                                seven,eight,nine,multiply
                                ,zero,decimal,clear,divide,
                                clearAll, equals, back};

        //set dimensions
        for(int i=0; i < buttons.length; i++){
            buttons[i].setPreferredSize(buttonDimension);
        }

        clearAll.setPreferredSize(new Dimension(100,50));
    }

    public static void GUI(){

        //add everything to the appropriate frame/panel
        frame.add(mainPanel);

        mainPanel.add(numbersPanel);
        mainPanel.add(buttonPanel);

        numbersPanel.add(output);
        numbersPanel.add(input);

        //add buttons to button panel
        for(int i = 0; i < buttons.length; i++){
            buttonPanel.add(buttons[i]);
        }

        //make sure it's all visible
        frame.setVisible(true);
    }

    public static void Update(){

        //the action listener to control all the buttons
        buttonListener = new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                //input for numerical buttons
                if(e.getSource().equals(one))
                    overwriteCheck(1);
                else if(e.getSource().equals(two))
                    overwriteCheck(2);
                else if(e.getSource().equals(three))
                    overwriteCheck(3);
                else if(e.getSource().equals(four))
                    overwriteCheck(4);
                else if(e.getSource().equals(five))
                    overwriteCheck(5);
                else if(e.getSource().equals(six))
                    overwriteCheck(6);
                else if(e.getSource().equals(seven))
                    overwriteCheck(7);
                else if(e.getSource().equals(eight))
                    overwriteCheck(8);
                else if(e.getSource().equals(nine))
                    overwriteCheck(9);
                else if(e.getSource().equals(zero))
                    overwriteCheck(0);

                //for adding decimals
                else if(e.getSource().equals(decimal))
                    input.append(".");

                //for inputting math operators
                else if(e.getSource().equals(add) || e.getSource().equals(subtract)
                        || e.getSource().equals(multiply) || e.getSource().equals(divide)){
                    //don't deal with empty strings
                    if(input.getText().equals(""))
                        numOne = 0;
                    else
                        numOne = Double.parseDouble(input.getText());

                    //double check the button and do the appropriate math
                    if(e.getSource().equals(add)){
                        numTwo += numOne;
                        displayOperator = "+";
                        overwrite = true;
                    }
                    else if(e.getSource().equals(subtract)){
                        numTwo -= numOne;
                        displayOperator = "-";
                        overwrite = true;
                    }
                    else if(e.getSource().equals(multiply)){
                        numTwo *= numOne;
                        displayOperator = "*";
                        overwrite = true;
                    }
                    else if(e.getSource().equals(divide)){
                        numTwo /= numOne;
                        displayOperator = "/";
                        overwrite = true;
                    }

                    //display the math
                    output.append(numOne + " " + displayOperator + " ");
                }

                //clear the input
                else if(e.getSource().equals(clear)){
                    //clear text field
                    input.setText("");

                    //clear number in memory
                    numOne = 0;
                }

                //clear the whole calculator
                else if(e.getSource().equals(clearAll)){
                    //clear the text fields
                    input.setText("");
                    output.setText("");

                    //clear the numbers in memory
                    numOne = 0;
                    numTwo = 0;
                }

                //Actually display the answer to the users
                else if(e.getSource().equals(equals)){
                    //clear math
                    output.setText("");

                    //display answer in input
                    input.setText("" + numTwo);

                    //allow past input to be totally overwritten
                    overwrite = true;
                }

                //delete last entered number
                else if(e.getSource().equals(back)){
                    //copy the input for modification
                    String changedInput = input.getText();

                    //delete the last inputed character in the string
                    if(!input.getText().equals(""))
                        changedInput = changedInput.substring(0, changedInput.length()-1);

                    //set the input to the reduced string
                    input.setText(changedInput);
                }
            }
        };

        //add all the button listeners
        for(int i = 0; i < buttons.length; i++){
            buttons[i].addActionListener(buttonListener);
        }
    }

    //if overwrite is true, overwrite current input and then switch to false
    public static void overwriteCheck(int x){
        if(overwrite == true){
            input.setText("" + x);
            overwrite = false;
        }
        else
            input.append("" + x);
    }
}