вопрос по стилю кода

не могу понять что надо сделать? Ошибка в файле com/javarush/task/task17/task1722/Solution.java в строке 15 Distance between variable 'counter' declaration and its first usage is 4, but allowed 3. Consider to make that variable as final if you still need to store its value in advance (before method calls that might do side effect on original value).

public class Solution {
    //-------------------------------------------------------------------------
    public static void main(String[] args) throws InterruptedException {
        Counter counter1 = new Counter();
        Counter counter2 = new Counter();
        Counter counter3 = new Counter();
        Counter counter4 = new Counter();

        counter1.start();
        counter2.start();
        counter3.start();
        counter4.start();

        counter1.join();
        counter2.join();
        counter3.join();
        counter4.join();