public Evaluator(String expression){
		Stack<Expression> expressionStack = new Stack<Expression>();
		for (String token : expression.split(" ")){
			if (token.equals("+")) {
				Expression subExpression = new Plus(expressionStack.pop(), expressionStack.pop());
				expressionStack.push(subExpression);
			} else if (token.equals("-")){
				Expression left = expressionStack.pop();
				Expression right = expressionStack.pop();
				Expression subExpression = new Minus(left, right);
				expressionStack.push(subExpression);
			}
			else expressionStack.push(new Variable(token));
		}
		syntaxTree = expressionStack.pop();
	}