Create ArrayList of arrays using Arrays.asList

Question

I am trying to create ArrayList of arrays with method Arrays.asList but I am struggling to achieve this goal when I have only one array to pass into the list.

List<String[]> notWithArrays = Arrays.asList(new String[] {"Slot"}); // compiler does not allow this
List<String[]> withArrays = Arrays.asList(new String[] {"Slot"},new String[] {"ts"}); // this is ok

The problem is that sometimes I have only one array to pass as argument and since it is only one iterable method asList creates List of strings out of it instead of required List<String[]>. Is there a way or method to make the list of arrays notWithArrays without having to create it manually?

Example of creating it manually:

List<String[]> withArraysManual = new ArrayList<>();
withArraysManual.add(new String[] {"Slot"});

Answer 1

I think you want to create a List<String[]> using Arrays.asList, containing the string array {"Slot"}.

You can do that like this:

List<String[]> notWithArrays = Arrays.asList(new String[][] {{"Slot"}});

or you can explicitly specify the generic type parameter to asList, like this:

List<String[]> notWithArrays = Arrays.<String[]>asList(new String[] {"Slot"});

Answer 2

Arrays.asList has a varargs parameter, T.... The issue that you’re facing is that Java has two ways of calling a method with a vararg parameter:

1. With multiple arguments of type T
2. With a single argument of type T[]

In addition, Arrays.asList is generic and infers the type parameter T from the type of its arguments. If only a single argument of array type is given, interpretation (2) takes precedence.

This means that when you write Arrays.asList(new String[] {"x"}), Java interprets this as a call of form (2) with T = String.

With multiple arguments there’s no confusion: Java always interprets it as a call of form (1), and infers T to be of type String[].

So the solution, as khelwood has shown, is to disambiguate calls with a single argument, either by packing the argument into an additional array layer, or by explicitly specifying the generic type parameter T.