АПС Лаб 4 Постфикс нотација

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.NoSuchElementException;


interface Stack<E> {

    // Elementi na stekot se objekti od proizvolen tip.

    // Metodi za pristap:

    public boolean isEmpty ();
        // Vrakja true ako i samo ako stekot e prazen.

    public E peek ();
        // Go vrakja elementot na vrvot od stekot.

    // Metodi za transformacija:

    public void clear ();
        // Go prazni stekot.

    public void push (E x);
        // Go dodava x na vrvot na stekot.

    public E pop ();
        // Go otstranuva i vrakja elementot shto e na vrvot na stekot.
}


class SLLNode<E> {
    protected E element;
    protected SLLNode<E> succ;

    public SLLNode(E elem, SLLNode<E> succ) {
        this.element = elem;
        this.succ = succ;
    }

    @Override
    public String toString() {
        return element.toString();
    }
}


class ArrayStack<E> implements Stack<E> {

    // Stekot e pretstaven na sledniot nacin:
    //depth e dlabochinata na stekot, a
    // elems[0...depth-1] se negovite elementi.
    private E[] elems;
    private int depth;

    @SuppressWarnings("unchecked")
    public ArrayStack (int maxDepth) {
        // Konstrukcija na nov, prazen stek.
        elems = (E[]) new Object[maxDepth];
        depth = 0;
    }


    public boolean isEmpty () {
        // Vrakja true ako i samo ako stekot e prazen.
        return (depth == 0);
    }


    public E peek () {
        // Go vrakja elementot na vrvot od stekot.
        if (depth == 0)
            throw new NoSuchElementException();
        return elems[depth-1];
    }


    public void clear () {
        // Go prazni stekot.
        for (int i = 0; i < depth; i++)  elems[i] = null;
        depth = 0;
    }


    public void push (E x) {
        // Go dodava x na vrvot na stekot.
        elems[depth++] = x;
    }


    public E pop () {
        // Go otstranuva i vrakja elementot shto e na vrvot na stekot.
        if (depth == 0)
            throw new NoSuchElementException();
        E topmost = elems[--depth];
        elems[depth] = null;
        return topmost;
    }
}










public class PostFixEvaluation {


    public static int postFix(String exp[]){

        ArrayStack <Integer> stack = new ArrayStack<Integer> (exp.length);

        for (int i =0 ;i<exp.length; i++){

            if (exp[i].equals("+") || exp[i].equals("-") || exp[i].equals("*") || exp[i].equals("/")){



                if(exp[i].equals("*")){
                    int desno = stack.pop();
                    int levo = stack.pop();
                    int mnozi = levo*desno;
                    stack.push(mnozi);

                }
                if(exp[i].equals("+")){

                    int soberi = 1000;
                    int desno = stack.pop();
                    int levo = stack.pop();
                     soberi = levo+desno;
                    stack.push(soberi);

                }
                if(exp[i].equals("/")){
                    int desno= stack.pop();
                    int levo = stack.pop();
                    int deli = levo/desno;
                    stack.push(deli);

                }
                if(exp[i].equals("-")){

                    int levo = stack.pop();
                    int desno = stack.pop();
                    int odzemi = desno-levo;
                    stack.push(odzemi);

                }


            }
            else{

                int a = Integer.parseInt(exp[i]);

                stack.push(a);

            }

        }

        int kraj = stack.peek();

        return kraj;
    }





    public static void main(String[] args) throws Exception{

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        String [] expression = br.readLine().split(" ");;
        //char exp[] = expression.toCharArray();


        System.out.println(postFix(expression));

        br.close();

    }

}