Arrays.asList() vs Collections.singletonList()

问题:

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.

回答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.

回答2:

I would just add that the singletonlist is not backed by an array and just has a reference to that one item. Presumably, it would take less memory and can be significant depending on the number of lists you want to create.

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