Solution.java

class Solution {
   public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<>();
        for(String s: tokens)
        {
            if(s.matches("-?\\d+"))
            {
                stack.push(Integer.valueOf(s));
            }
            else
            {
                Integer el1 = stack.pop();
                Integer el2 = stack.pop();
                Integer res=0;

                Operation operation = new OperationBuilder().build(el1,el2, s);
                res = operation.apply();

                stack.push(res);
            }

        }
        return stack.pop();
    }

    enum Operand {
        ADDITION("+"),
        SUBTRACTION("-"),
        DIVISION("/"),
        MULIPLICATION("*");

        private final String operationSymbol;

        Operand(String operationSymbol)
        {
            this.operationSymbol = operationSymbol;
        }
    }

    class OperationBuilder {
        Operation build(int el1, int el2, String s)
        {
            if (Operand.ADDITION.operationSymbol.equals(s))
            {
                return new Addition(el1, el2);
            }
            else if (Operand.SUBTRACTION.operationSymbol.equals(s))
            {
                return new Subtraction(el1, el2);
            }
            else if (Operand.MULIPLICATION.operationSymbol.equals(s))
            {
                return new Multiplication(el2, el1);
            }
            else if (Operand.DIVISION.operationSymbol.equals(s))
            {
                return new Division(el1, el2);
            }
            return null;
        }
    }

    abstract class Operation
    {
        private Integer op1;

        public Integer getOp1()
        {
            return op1;
        }

        public Integer getOp2()
        {
            return op2;
        }

        private Integer op2;

        public Operation(Integer op1, Integer op2)
        {
            this.op1 = op1;
            this.op2 = op2;
        }

        abstract Integer apply();
    }

    class Addition extends Operation
    {

        public Addition(Integer op1, Integer op2)
        {
            super(op1, op2);
        }

        @Override
        public Integer apply()
        {
            return getOp1() + getOp2();
        }
    }

    class Subtraction extends  Operation
    {

        public Subtraction(Integer op1, Integer op2)
        {
            super(op1, op2);
        }

        @Override
        public Integer apply()
        {
            return getOp2() - getOp1();
        }
    }
    class Multiplication extends Operation
    {

        public Multiplication(Integer op1, Integer op2)
        {
            super(op1, op2);
        }

        @Override
        public Integer apply()
        {
            return getOp1() * getOp2();
        }
    }

    class Division extends Operation
    {

        public Division(Integer op1, Integer op2)
        {
            super(op1, op2);
        }

        @Override
        public Integer apply()
        {
            return getOp2() / getOp1();
        }
    }
}