String key = Greenfoot.getKey();
if (key != null) System.out.println(key);

   public static String delete(String aString, int index){
        String placeholderString1 = "";
        String placeholderString2 = "";
        if (index != 0){
            placeholderString1 = aString.substring(0, index-1);
        }
        if (index != aString.length()){
            placeholderString2 = aString.substring(index, aString.length());
        }
        aString = placeholderString1 + placeholderString2;
        return aString;
    }
    
    public static String insert(String aString, int index, String character){
        String placeholderString1 = "";
        String placeholderString2 = "";
        if (index != 0){
            placeholderString1 = aString.substring(0, index);
        }
        if (index != aString.length()){
            placeholderString2 = aString.substring(index, aString.length());
        }
        aString = placeholderString1 + character + placeholderString2;
        return aString;
    }

//Button class
public void buttonPress(){ /**override this in subclasses*/
        if(getWorld() instanceof Calculator){//if world is 'Calculator'
            /**possible source of issue*/
            Calculate calculate = new Calculate();
            calculate.add(button);//call add method in calculate
            if (!button.equals("=")){
                /**possible source of issue*/
                Calculator world = new Calculator();
                world.updateDisplay();
            }
            else{
                Calculator world = new Calculator();
                /**world.showAnswer*/
            }
            /**call method from world that updates the display.
             * the method must call on getcurrentcalculation method of calculate
             * and assign that string to a string called displayCalculation
             * the method must call on getcurrentindex method of calculate
             * and then insert ":" at index+1 of displayCalculation
             * (possibly using calculate's insert method)
             * and showText(displaycalculation) etc
             */
        }
        /**delete*/
        System.out.println(button);
    }

//Calculate class
public void add(String chars){
        /**check if chars == mode alpha store. if so, then 'return;'*/
        if (processingCalculation == false){
            holder = chars;
            //check to see if input is a number
            if (checkIfNumber(holder)){
                currentCalculation = insert(currentCalculation, currentIndex, holder);
            }
            else if (holder.equals("Clear")){
                currentCalculation = "";
                currentIndex = -1; //+1 is added at end of method
                System.out.println("CLEARED");
                System.out.println("_______");
            }
            else if (holder.equals("Delete")){
                if (! currentCalculation.equals("")){
                    currentCalculation = delete(currentCalculation, currentIndex);
                }
                if (currentIndex > 0){
                    currentIndex = currentIndex - 2;
                    //currentIndex - 2 + 1 = intended currentIndex
                    //will not matter if currentIndex results as -1...
                    //...because +1 is added at the end of the method
                }
            }
        }
        /**make sure subclasses of button arent registered!*/
        /**starts at zero each time a button is pressed then increases by 1, to equal 1?*/
        currentIndex = currentIndex + 1;
        System.out.println("a" + currentIndex); //error checking
    }

    public void buttonPress(){ /**override this in subclasses*/
        if(getWorld() instanceof Calculator){//if world is 'Calculator'
            Calculator world = (Calculator) getWorld();
            calculate.add(button);//call add method in calculate
            if (!button.equals("=")){
                world.updateDisplay();
            }

public Calculator()
    {    
        super(420, 600, 1);
        makeNumButtons();
        makeOpButtons();
        makeAdvOpButtons();
        makeActionButtons();
        makeArrowButtons();
        prepare();
        updateDisplay();
    }

public void makeNumButtons(){
        for(int i = 0; i < 15; i++) { /**change 17 to length of array*/
            if(!numButton[i].equals("")){
                Button myButton = new Button(numButton[i]);
                addObject(myButton, xCoord1, yCoord1);
            }
            xCoord1 += buttonSpacingXVal;
            if((i+1)%4 == 0){
                xCoord1 = xCoordStart;
                yCoord1 = (yCoordStart - buttonSpacingYVal) - buttonSpacingYVal * ((i-3)/4);
            }
        }
    }

    public void makeOpButtons(){
        for(int i = 0; i < 16; i++) { /**change 17 to length of array*/
            if(!operationButton[i].equals("")){
                Button myButton = new Button(operationButton[i]);
                addObject(myButton, xCoord2, yCoord2);
            }
            xCoord2 += buttonSpacingXVal;
            if((i+1)%4 == 0){
                xCoord2 = xCoordStart;
                yCoord2 = (yCoordStart - buttonSpacingYVal) - buttonSpacingYVal * ((i-3)/4);
            }
        }
    }
    
    public void makeAdvOpButtons(){ /**change 17 to length of array*/
        for(int i = 0; i < 28; i++) {
            if(!advOperationButton[i].equals("")){
                Button myButton = new Button(advOperationButton[i], advOperationButtonName[i]);
                addObject(myButton, xCoord3, yCoord3);
            }
            xCoord3 += buttonSpacingXVal;
            if((i+1)%4 == 0){
                xCoord3 = xCoordStart;
                yCoord3 = (yCoordStart - buttonSpacingYVal) - buttonSpacingYVal * ((i-3)/4);
            }
        }
        
        //Clear and Delete buttons
        Button clear = new Button("Clear", " AC");
        addObject(clear, xCoordStart + (buttonSpacingXVal * 3), yCoordStart - (buttonSpacingYVal * 8));
        Button delete = new Button("Delete", "Del");
        addObject(delete, xCoordStart + (buttonSpacingXVal * 5 / 2), yCoordStart - (buttonSpacingYVal * 7));
    }
    
    public void makeActionButtons(){
        StoreButton store = new StoreButton();
        ModeButton mode = new ModeButton();
        AlphaButton alpha = new AlphaButton();
        addObject(store, xCoordStart, yCoordStart - (buttonSpacingYVal * 5));
        addObject(mode, xCoordStart, yCoordStart - (buttonSpacingYVal * 8));
        addObject(alpha, xCoordStart + (buttonSpacingXVal/2), yCoordStart - (buttonSpacingYVal * 7));
    }
    
    public void makeArrowButtons(){ /**change 17 to length of array*/
        //make the area that the buttons will occupy
        Arrows arrowCircle = new Arrows();
        addObject(arrowCircle, (getWidth() / 2 + 5), (getHeight() / 2 - 60));
        
        DownArrow down = new DownArrow();
        addObject(down, (getWidth() / 2 + 5), (getHeight() / 2 - 27));
        
        UpArrow up = new UpArrow();
        addObject(up, (getWidth() / 2 + 5), (getHeight() / 2 - 97));
        
        LeftArrow left = new LeftArrow();
        addObject(left, (getWidth() / 2 - 30), (getHeight() / 2 - 62));
        
        RightArrow right = new RightArrow();
        addObject(right, (getWidth() / 2 + 39), (getHeight() / 2 - 62));
    }
    
    public void prepare(){
        CheckKeyPress keypress = new CheckKeyPress();
        addObject(keypress, 0, 0);
    }

Calculate calculate = ((Calculator)getWorld()).myCalculator;

Calculate calculate = new Calculate();