Aips lab4 Постфикс нотација

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

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();
    }
}
interface Stack<E> {

    public boolean isEmpty ();
    public E peek ();

    public void clear ();

    public void push (E x);

    public E pop ();
}
class ArrayStack<E> implements Stack<E> {

    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 boolean isNumber(String a) {
        for(int i=0; i<a.length(); i++) {
            if(Character.isDigit(a.charAt(i))) {
                continue;
            } else {
                return false;
            }
        }
        return true;
    }
    public static int calculate(char[]exp) {
        ArrayStack<Integer> stack  = new ArrayStack<>(exp.length);
        String izraz=new String();
        for(int i=0; i<exp.length; i++) {
            izraz+=exp[i];
        }
        String []parts=izraz.split("\\s");
        int rezultat=0;

        for(int i=0; i<parts.length; i++) {
            if (isNumber(parts[i])) {
                stack.push(Integer.parseInt(parts[i]));
            } else {
                int pop1= stack.pop();
                int pop2=stack.pop();
                char operator = parts[i].charAt(0);
                if(operator == '+') {
                    rezultat = pop2 + pop1;
                } else if(operator=='-') {
                    rezultat=pop2-pop1;
                } else if(operator=='*') {
                    rezultat=pop2*pop1;
                } else {
                    rezultat=pop2/pop1;
                }
                if(i== parts.length-1) {
                    if (stack.isEmpty()) {
                        return rezultat;
                    } else {
                        return 0;
                    }
                } else {
                    stack.push(rezultat);
                }
            }
        }
        return -1;
    }


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

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

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

        System.out.print(calculate(exp));



        br.close();

    }

}