HashMapImplementation.java

import java.util.HashMap;
import java.util.Stack;
import java.util.function.BiFunction;

class ReversePolishNotation {

    public static void main(String[] args) {
        System.out.println(evaluate("4 2 /"));
    }

    public static double evaluate(String expr) {
        String[] digitString = expr.split(" ");

        Stack<Float> stack = new Stack<>();
        HashMap<String, BiFunction<Float, Float, Float>> map = constructMapForOperator();

        for (String s : digitString) {
            if (map.containsKey(s)) stack.push(operate(map.get(s), stack));
            else stack.push(Float.parseFloat(s));
        }
        return stack.pop();
    }

    private static HashMap<String, BiFunction<Float, Float, Float>> constructMapForOperator() {
        HashMap<String, BiFunction<Float, Float, Float>> map = new HashMap();
        map.put("+", (a, b) -> a + b));
        map.put("-", (a, b) -> a - b);
        map.put("*", (a, b) -> a * b);
        map.put("/", (a, b) -> a / b);
        return map;
    }

    public static Float operate(BiFunction<Float, Float, Float> function, Stack<Float> stack) {
        float numberOne = stack.pop();
        float numberTwo = stack.pop();
        return function.apply(numberTwo, numberOne);
    }
}