Arrays.asList() vs Collections.singletonList()

Question

Is there an advantage (or much of a difference) to using Arrays.asList(something) over Collections.singletonList(something) to make a list containing one item? The latter makes the returned list immutable as well.

Answer 1

Collections.singletonList(something) is immutable whereas Arrays.asList(something) is a fixed size List representation of an Array where the List and Array gets joined in the heap.

Arrays.asList(something) allows non-structural changes made to it, which gets reflected to both the List and the conjoined array. It throws UnsupportedOperationException for adding, removing elements although you can set an element for a particular index.

Any changes made to the List returned by Collections.singletonList(something) will result in UnsupportedOperationException.

Also, the capacity of the List returned by Collections.singletonList(something) will always be 1 unlike Arrays.asList(something) whose capacity will be the size of the backed array.