How does java.util.Arrays.asList(T...) work?

Question

It seems to be a silly question at the first sight, but I found that the mechanism isn't trivial. The implementation from JDK 8 (copied from here) is just a few lines:

@SafeVarargs
@SuppressWarnings("varargs")
public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}

The point is that there are only 3 constructors in ArrayList:

• one doesn't take any parameter
• one takes an int (the initial capacity)
• one takes a Collection (the elements) as shown below (copied from here):

public class ArrayList<E> extends AbstractList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{

    // ...

    transient Object[] elementData; // non-private to simplify nested class access

    private int size;

    // ...

    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        size = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);

    }

    // ...

}

Since the only possibility is that the 3rd constructor is called, it seems that the parameter a (the generic vararg) is somehow "casted" to a Collection. But as far as I know, varargs are nothing more than arrays with a shorter syntax, and are not convertible to Collections (so that's why this made me puzzled)...

Does anybody know how the magic works?

Thank you!

Answer 1

You're looking at the wrong ArrayList class. The one used in Arrays.asList(..) is java.util.Arrays.ArrayList and has a constructor that accepts an array.

Answer 2

The ArrayList type that's described here is not java.util.ArrayList; it's a private type defined inside that file (see line 3799). That one does have an array constructor, and it's the one referenced by Arrays.asList.

Hope this helps!