Exemple

public class Outer {

private final class Inner1 {

private final class Inner2 {

private final class Inner3 {

}
}
}

}

Solution
Déplacez toutes les classes internes imbriquées vers le niveau de classe interne le plus externe.

public class Outer {

private final class Inner1 {}

private final class Inner2 {}

private final class Inner3 {}

}