Design calculator

abstract class Operation:
float arg1, arg2;
abstract float getValue()

class Literal extends Operation:
float getValue() {
    retur arg1;
}

class Add extends Operation:
float getValue() {
    return arg1 + arg2
}

class Negate extends Operation:
float getValue() {
    return arg2 - arg1
}

class Calculator
string expression
float evaluate() {
    result = 0;
    operation = '+';
    for (var i = 0; i < lengt; i++) {
        number = compute(expression[i]);
    if (number != null) {
           result = getResult(result, number, operation)
        } else {
           operation = expression[i];
        }
    }
    return new Literal(result, null).getValue();
}

float getResult(result, number, operation) {
           if (operation == '+') {
            return new Add(result, number).getValue()
           } else if (operation == '-') {
            return new Add(result, new Negate(number, null)).getValue()
           }
   return new Literal(result, null).getValue();
}

float compute(number) {
   var number = parseInt(expression[i]);
   if (number!== NaN) {
      return number;
   }
   return null;
}