Why do I get a NullPointerException?

[nessa203]

I'm having a go at writing a calculator. The Calculator is the controlling class, which calls all the other ones. The Evaluator is the one that does the calculations.

package calculator;

public class Calculator {
	/*
	 * The main class that controls the program.
	 */
	
	public String currentCalculation;
	Reader r;
	Evaluator e;
	Splitter s;
	
	public Calculator() {
		r = new Reader();
		s = new Splitter();
		e = new Evaluator();
	}

	private void calculate() {
		/*
		 * Keeps the calculator calculating:
		 * - reads in the current calculation
		 * - splits it into a string[] to be analysed
		 * - analyses the string[] for brackets
		 * - calculates and prints out the answer
		 * - starts again
		 */
		currentCalculation = r.prompt();
		System.out.println("Read in.");
		String[] currentArray = s.split(currentCalculation);
		System.out.println("Array split.");
		e.evaluate(currentArray);
		System.out.println("E evaluated.");
		
		calculate();
	}
	
	public static void main(String[] args) {
		System.out.println("Type quit at any point to exit");
		Calculator c = new Calculator();
		c.calculate();
	}
}

package calculator;

public class Evaluator {
	private String add = "+";
	private String subtract = "-";
	private String multiply = "*";
	private String divide = "/";
	
	public Evaluator() {
		
	}
	
	public String evaluate(String[] s) {
		String[] newS = null;
		String result = null;
	
		System.out.println(s.length);
		
		if (s.length != 1) {
			for (int i = 0; i < s.length; i++) {
				if (s[i].equals(divide)) {
					System.out.println("divide");
					s[i] = divide(s[i-1], s[i+1]);
					System.out.println("newS");
					newS = new String[s.length-2];
					for (int j = 0, k = 0; j < s.length; j++) {
						if (s[j].equals(s[i-1]) || s[j].equals(s[i+1])) {
							// ignore them
						} else {
							newS[k++] = s[j];
						}
					}
				}
				if (newS != null) {
					evaluate(newS);	
				}
			}
		} else if (s.length == 1) {
			result = s[0].toString();
		} else {
			System.out.println("No result found.");
			result = "0";
		}
		return result;
	}
	
	private String divide(String s1, String s2) {
		Double d1 = Double.parseDouble(s1);
		Double d2 = Double.parseDouble(s2);
		Double result = d1 / d2;
		return result.toString();
	}

}

My reader class works just fine, but everytime I try to run the program and input a calculation I receive the following:

java calculator/Calculator
Type quit at any point to exit
>>> 8 / 5
8 / 5
Read in.
8
/
5
Array split.
Exception in thread "main" java.lang.NullPointerException
at calculator.Evaluator.evaluate(Evaluator.java:17)
at calculator.Calculator.calculate(Calculator.java:35 )
at calculator.Calculator.main(Calculator.java:45)

Have I missed initialising something? Where am I pointing to nothing? :confused:
Any help would be much appreciated!

[nessa203]

Don't worry! Found my problem!

[quad64bit]

Good luck :D