Java unchecked: unchecked generic array creation for varargs parameter

Because java compiler uses an implicit array creation for varargs, and java doesn't allow a generic array creation (because type argument is not reifiable).

The code below is correct (these operations are allowed with arrays), so unchecked warning is needed:

public static <T> List<List<T>> createCombinations(List<T> ... lists) {
    ((Object[]) lists)[0] = new ArrayList<Integer>();
    // place your code here
}

See a comprehensive explanation here

As janoh.janoh mentioned above, varargs in Java is just a syntactic sugar for arrays plus the implicit creation of an array at the calling site. So

List<List<String>> combinations =
    Utils.createCombinations(cocNumbers, vatNumbers, ibans);

is actually

List<List<String>> combinations =
    Utils.createCombinations(new List<String>[]{cocNumbers, vatNumbers, ibans});

But as you may know, new List<String>[] is not allowed in Java, for reasons that have been covered in many other questions, but mainly have to do with the fact that arrays know their component type at runtime, and check at runtime whether elements added match its component type, but this check is not possible for parameterized types.

Anyway, rather than failing, the compiler still creates the array. It does something similar to this:

List<List<String>> combinations =
    Utils.createCombinations((List<String>[])new List<?>[]{cocNumbers, vatNumbers, ibans});

This is potentially unsafe, but not necessarily unsafe. Most varargs methods simply iterate over the varargs elements and read them. In this case, it doesn't care about the runtime type of the array. This is the case with your method. Since you are on Java 7, you should add the @SafeVarargs annotation to your method, and you won't get this warning anymore. This annotation basically says, this method only cares about the types of the elements, not the type of the array.

However, there are some varargs methods that do use the runtime type of the array. In this case, it is potentially unsafe. That's why the warning is there.