Elementary data structures - Stack http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_3_A

Main.java

import java.util.Arrays;
import java.util.Scanner;

public class Main {
    public Main() {
        Scanner scan = new Scanner(System.in);

        Stack stack = new Stack(200);

        while(scan.hasNext()){
            String str = scan.next();
            stack.push(str);
        }
        System.out.println(stack.pop());
    }

    public static void main(String[] args) {
        new Main();
    }

    private class Stack{
        private int top = 0;
        private int stack[];

        public Stack(int size){
            stack = new int[size];
        }

        public void push(int n){
            stack[top++] = n;
        }

        public void push(String str){
            if(isInteger(str)){
                int n = Integer.parseInt(str);
                push(n);
            }else{
                int o1 = pop();
                int o2 = pop();

                if(str.equals("+")){
                    push(o2+o1);
                }
                else if(str.equals("-")){
                    push(o2-o1);
                }else{
                    push(o2*o1);
                }
            }
        }

        public int pop(){
            return stack[--top];
        }

        public boolean isInteger(String str) {
            try {
                Integer.parseInt(str);
            } catch(NumberFormatException e) {
                return false;
            }
            return true;
        }

    }

}

Main.scala

import scala.collection.mutable.ArrayBuffer
import scala.io.StdIn

object Main extends App {
  val inputList = StdIn.readLine().split(" ").map {
    str =>
      try {
        str.toInt
      } catch {
        case e: NumberFormatException =>
          str
      }
  }.toList

  val stack = new Stack

  inputList.foreach { elm =>
    stack.push(elm)
  }
  println(stack.pop())
}

class Stack {
  val array = ArrayBuffer.empty[Int]

  def push(elm: Any): Unit = {
    elm match {
      case n: Int =>
        array += n
      case str: String =>
        val (o1, o2) = pop2()
        str match {
          case "+" =>
            push(o2 + o1)
          case "-" =>
            push(o2 - o1)
          case "*" =>
            push(o2 * o1)
        }
    }
  }

  def pop(): Int = {
    array.remove(array.length - 1)
  }

  def pop2(): (Int, Int) = {
    (pop(), pop())
  }
}