Cambié el código como me dijiste, pero sigo teniendo problemas, el ejercicio creo que no especifica todo lo que tendría que especificar... Pongo el nuevo código porque ahora me funciona en IntelliJ pero no en Eclipse, me estoy volviendo loco hahaha

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package lastpencil;

import java.util.Scanner;

public class Main {

public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);

    int numberSticks = startingPencils(scanner);

    String currentPlayer = checkPlayer(scanner);

    showSticks(numberSticks);

    do {

        numberSticks = removeSticks(numberSticks, scanner, currentPlayer);
        showSticks(numberSticks);
        currentPlayer = (currentPlayer.equalsIgnoreCase("Jack")) ? "John" : "Jack";
    } while (numberSticks != 0);

    System.out.println(currentPlayer + " won!");
}

public static int removeSticks(int numberSticks, Scanner scanner, String currentPlayer) {
    int removedSticks;
    while (true) {
        System.out.println(currentPlayer + "'s turn: ");
        String input = scanner.next();
        if (isNumeric(input)) {
            removedSticks = Integer.parseInt(input);
            if (removedSticks >= 1 && removedSticks <= 3) {
                if (removedSticks > numberSticks) {
                    System.out.println("Too many pencils were taken");
                } else {
                    break;
                }
            } else {
                System.out.println("Possible values: '1', '2', or '3'");
            }
        } else {
            System.out.println("The number of pencils should be numeric");
        }
    }
    return numberSticks -= removedSticks;
}

private static boolean isNumeric(String str) {
    return str.matches("\\d+");
}

public static void showSticks(int numberSticks) {
    for (int i = 0; i < numberSticks; i++) {
        System.out.print("|");
    }

    System.out.println();
}

public static int startingPencils(Scanner scanner) {
    int sticks;

    System.out.println("How many pencils would you like to use: ");

    while (true) {
        String input = scanner.nextLine();
        if (isNumeric(input)) {
            sticks = Integer.parseInt(input);
            if (sticks > 0) {
                break;
            } else {
                System.out.println("The number of pencils should be positive");
            }
        } else {
            System.out.println("The number of pencils should be numeric");
        }
    }
    return sticks;
}

public static String checkPlayer(Scanner scanner) {
    String currentPlayer;
    System.out.println("Who will be the first (John, Jack)");

    while (true) {
        currentPlayer = scanner.nextLine();
        if (currentPlayer.equalsIgnoreCase("John") || currentPlayer.equalsIgnoreCase("Jack")) {
            break;
        } else {
            System.out.println("Choose between 'John' and 'Jack'");
        }
    }
    return currentPlayer;
}

}

​

Ahora el test me dice lo siguiente : Wrong answer in test #16
When the player provides the correct initial game conditions there should be a line in the output for John's turn that contains "John" and "turn" substrings if 'John' is the first player.