BiFunc.java

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<Float>();
        for (String s : digitString) {
            switch (s) {
                case "+": {
                    stack.push(operate((a, b) -> a + b, stack));
                    break;
                }
                case "-": {
                    stack.push(operate((a, b) -> a - b, stack));
                    break;
                }
                case "*": {
                    stack.push(operate((a, b) -> a * b, stack));
                    break;
                }
                case "/": {
                    stack.push(operate((a, b) -> a / b, stack));
                    break;
                }
                default: {
                    stack.push(Float.parseFloat(s));
                    break;
                }
            }
        }
        return stack.pop();
    }

    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);
    }
}