This is the calculator class:

public class Calculator { public enum Key { DIGIT_0, DIGIT_1, DIGIT_2, DIGIT_3, DIGIT_4, DIGIT_5, DIGIT_6, DIGIT_7, DIGIT_8, DIGIT_9, NEGATE, DECIMAL_POINT, CLEAR, CLEAR_ENTRY, ADD, SUBTRACT, MULTIPLY, DIVIDE, EQUALS } private enum Operation { NOP, ADD, SUBTRACT, MULTIPLY, DIVIDE } public String getDisplay() { return display.toString(); } public Calculator() { display = new StringBuffer(); clear(); } public void keyPressed( Key key ) { switch( key ) { case CLEAR: clear(); break; case DIGIT_0: case DIGIT_1: case DIGIT_2: case DIGIT_3: case DIGIT_4: case DIGIT_5: case DIGIT_6: case DIGIT_7: case DIGIT_8: case DIGIT_9: case DECIMAL_POINT: if( ! errorMode ) digitKeyPressed( key ); break; case NEGATE: if( ! errorMode ) negateKeyPressed(); break; case ADD: if( ! errorMode ) addKeyPressed(); break; case SUBTRACT: if( ! errorMode ) subtractKeyPressed(); break; case MULTIPLY: if( ! errorMode ) multiplyKeyPressed(); break; case DIVIDE: if( ! errorMode ) divideKeyPressed(); break; case EQUALS: if( ! errorMode ) equalsKeyPressed(); break; } } private void clear() { display.setLength( 0 ); ac = 0.0; operand = 0.0; operation = Operation.NOP; validAC = false; validOperand = false; errorMode = false; clearDisplayUponNextDigit = true; } private void digitKeyPressed( Key key ) { if( clearDisplayUponNextDigit ) { display.setLength(0); clearDisplayUponNextDigit = false; } validOperand = false; char keyChar ='\0'; switch( key ) { case DIGIT_0: keyChar = '0'; break; case DIGIT_1: keyChar = '1'; break; case DIGIT_2: keyChar = '2'; break; case DIGIT_3: keyChar = '3'; break; case DIGIT_4: keyChar = '4'; break; case DIGIT_5: keyChar = '5'; break; case DIGIT_6: keyChar = '6'; break; case DIGIT_7: keyChar = '7'; break; case DIGIT_8: keyChar = '8'; break; case DIGIT_9: keyChar = '9'; break; case DECIMAL_POINT: keyChar = '.'; break; } display.append(keyChar); } public void addKeyPressed() { if( !validAC ) { loadAC(); if( validAC ) { operation = Operation.ADD; } } else { loadOperand(); if( validOperand ) { performSavedOperation(); operation = Operation.ADD; } } } public void subtractKeyPressed() { if( !validAC ) { loadAC(); if( validAC ) { operation = Operation.SUBTRACT; } } else { loadOperand(); if( validOperand ) { performSavedOperation(); operation = Operation.SUBTRACT; } } } public void multiplyKeyPressed() { if( !validAC ) { loadAC(); if( validAC ) { operation = Operation.MULTIPLY; } } else { loadOperand(); if( validOperand ) { performSavedOperation(); operation = Operation.MULTIPLY; } } } public void divideKeyPressed() { if( !validAC ) { loadAC(); if( validAC ) { operation = Operation.DIVIDE; } } else { loadOperand(); if( validOperand ) { performSavedOperation(); operation = Operation.DIVIDE; } } } private void equalsKeyPressed() { if( validAC ) { if( ! validOperand ) loadOperand(); if( validOperand ) performSavedOperation(); } } private void negateKeyPressed() { if( display.length() > 0 ) { if( display.charAt(0) == '-' ) { display.deleteCharAt(0); } else { display.insert( 0, '-'); } } } private void loadAC() { try { ac = Double.parseDouble(display.toString()); // display.setLength(0); // Keep the current digits in the display until new digit received. clearDisplayUponNextDigit = true; validAC = true; } catch( NumberFormatException e ) { display.setLength(0); display.append( "ERROR" ); errorMode = true; validAC = false; } } private void loadOperand() { try { operand = Double.parseDouble(display.toString()); display.setLength(0); validOperand = true; } catch( NumberFormatException e ) { display.setLength(0); display.append( "ERROR" ); errorMode = true; validOperand = false; } } private void performSavedOperation() { double result = 0.0; if( operation == Operation.ADD ) { result = ac + operand; } else if(operation == Operation.SUBTRACT ) { result = ac - operand; } else if( operation == Operation.MULTIPLY ) { result = ac * operand; } else if( operation == Operation.DIVIDE ) { result = ac / operand; } if( operation != Operation.NOP ) { display.setLength(0); display.append( result ); clearDisplayUponNextDigit = true; ac = result; } } private StringBuffer display; private double ac; private double operand; private boolean validAC; private boolean validOperand; private Operation operation; private boolean errorMode; private boolean clearDisplayUponNextDigit; }