9. Стек неограниченного размера

import java.util.ArrayList;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        ArrayList<Integer> stack = new ArrayList<>(10000);
        int size = 0;
        String command;

        while (true) {
            command = sc.nextLine();
            if (command.contains("push")) {
                stack.add(Integer.parseInt(command.substring(5)));
                size++;
                System.out.println("ok");
            }
            if (command.contains("pop")) {
                if (size <= 0) {
                    System.out.println("error");
                } else {
                    System.out.println(stack.remove(size - 1));
                    size--;
                }
            }
            if (command.contains("back")) {
                if (size <= 0) {
                    System.out.println("error");
                } else {
                    System.out.println(stack.get(size - 1));
                }
            }
            if (command.contains("size")) {
                System.out.println(size);
            }
            if (command.contains("clear")) {
                if (size == 0) {
                    System.out.println("ok");
                } else {
                    stack.clear();
                    size = 0;
                    System.out.println("ok");
                }
            }
            if (command.contains("exit")) {
                System.out.println("bye");
                break;
            }
        }
    }
}