NPR with Stack Java

import java.util.Scanner;

public class Stack<T> {

    private int stackPos;
    private T[] stack;

    public Stack(int size) {
        this.stackPos = -1;
        this.stack = (T[]) new Object[size];
    }

    public void push(T value) {
        if (this.stackPos < this.stack.length - 1) {
            this.stack[++stackPos] = value;
        }
    }

    public T pop() {
        if (this.isEmpty()) {
            return null;
        }
        return this.stack[stackPos--];
    }

    public int length() {
        if (this.isEmpty()) {
            return 0;
        }
        return this.stackPos + 1;
    }

    public T head() {
        if (this.isEmpty()) {
            return null;
        }
        return this.stack[stackPos];
    }

    public boolean isEmpty() {
        return this.stackPos == -1;
    }

    public void print() {
        if(this.isEmpty()) {
            System.out.println("Pilha vazia..");
        }else{
            for(int i = stackPos; i >= 0; i--){
                System.out.println(stack[i]);
            }
        }
    }

}

public class NPR {

    public static int operate(String operation, int left, int right) {
        switch (operation) {
            case "+": return left + right;
            case "-": return left - right;
            case "*": return left * right;
            case "/":
                if (right == 0) {
                    System.out.println("ERR: Não é possível dividir por zero");
                    return left;
                }
                return left / right;
            default: return left;
        }
    }

    public static int parseOperation(String operation, Stack<Integer> stack) {
        int result = (stack.isEmpty()) ? 0 : stack.pop();
        if (!stack.isEmpty()) {
            result = operate(operation, stack.pop(), result);
        }
        return result;
    }

    public static boolean isInteger(String input) {
        if (input == null) return false;

        try {
            Integer.parseInt(input);
        } catch (NumberFormatException ex) {
            return false;
        }
        return true;
    }

    public static boolean isOperation(String input) {
        if (input == null) return false;

        return input.equals("+") || input.equals("-") || input.equals("*") || input.equals("/");
    }

    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);
        Stack<Integer> stack = new Stack<Integer>(30);

        System.out.println("Digite a operação (s para sair)");
        String input = "";

        while (!input.equals("s")) {
            input = reader.nextLine();
            if (isInteger(input)) {
                stack.push(Integer.parseInt(input));
            } else if (isOperation(input)) {
                int current = parseOperation(input, stack);
                if (stack.length() == 0) {
                    System.out.println(">> " + current);
                } else {
                    System.out.println("> " + current);
                }
                stack.push(current);
            }
        }

        reader.close();
    }

}