Postfix Notacija

package kolokviumska;
import java.util.NoSuchElementException;
import java.util.Scanner;

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> {

    // 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 Postfix {

    public static int resultPostfix(char[] izraz)
    {
        ArrayStack<Integer> stekce=new ArrayStack<>(110);
        int number=0;

        for(int i=0;i<izraz.length;i++)
        {
            if(Character.isDigit(izraz[i]) && Character.isDigit(izraz[i]))
            {
                number=number*10 + Character.getNumericValue(izraz[i]);
            }
            if(Character.isDigit(izraz[i]) && !Character.isDigit(izraz[i]))
            {
                number=number*10 + Character.getNumericValue(izraz[i]);
                stekce.push(number);
                number=0;
            }
            if(izraz[i]=='+' || izraz[i]=='-' || izraz[i]=='*' || izraz[i]=='/')
            {
                int operandA=stekce.pop();
                int operandB=stekce.pop();

                if(izraz[i]=='+')
                {
                    stekce.push(operandA+operandB);
                }
                else if(izraz[i]=='-')
                {
                    stekce.push(operandB-operandA);
                }
                else if(izraz[i]=='*')
                {
                    stekce.push(operandA*operandB);
                }
                else
                    stekce.push(operandB/operandA);
            }
        }
        return stekce.pop();
    }

    public static void main(String[] args) {
        Scanner s=new Scanner(System.in);
        String s1=s.nextLine();
        char[]c=s1.toCharArray();
        int result=Postfix.resultPostfix(c);
        System.out.println(result);
    }
}