This section describes several mini-implementations that can be more convenient and more efficient than general-purpose implementations when you don't need their full power. All the implementations in this section are made available via static factory methods rather than public classes.

List View of an Array

The Arrays.asList method returns a List view of its array argument. Changes to the List write through to the array and vice versa. The size of the collection is that of the array and cannot be changed. If the add or the remove method is called on the List, an UnsupportedOperationException will result.

The normal use of this implementation is as a bridge between array-based and collection-based APIs. It allows you to pass an array to a method expecting a Collection or a List. However, this implementation also has another use. If you need a fixed-size List, it's more efficient than any general-purpose List implementation. This is the idiom.

List<String> list = Arrays.asList(new String[size]);

Note that a reference to the backing array is not retained.

Immutable Multiple-Copy List

Occasionally you'll need an immutable List consisting of multiple copies of the same element. The Collections.nCopies method returns such a list. This implementation has two main uses. The first is to initialize a newly created List; for example, suppose you want an ArrayList initially consisting of 1,000 null elements. The following incantation does the trick.

List<Type> list =
    new ArrayList<Type>(Collections.nCopies(1000, (Type)null);

Of course, the initial value of each element need not be null. The second main use is to grow an existing List. For example, suppose you want to add 69 copies of the string "fruit bat" to the end of a List<String>. It's not clear why you'd want to do such a thing, but let's just suppose you did. The following is how you'd do it.

lovablePets.addAll(Collections.nCopies(69, "fruit bat"));

By using the form of addAll that takes both an index and a Collection, you can add the new elements to the middle of a List instead of to the end of it.

Immutable Singleton Set

Sometimes you'll need an immutable singleton Set, which consists of a single, specified element. The Collections.singleton method returns such a Set. One use of this implementation is to remove all occurrences of a specified element from a Collection.

c.removeAll(Collections.singleton(e));

A related idiom removes all elements that map to a specified value from a Map. For example, suppose you have a Map — job — that maps people to their line of work and suppose you want to eliminate all the lawyers. The following one-liner will do the deed.

job.values().removeAll(Collections.singleton(LAWYER));

One more use of this implementation is to provide a single input value to a method that is written to accept a collection of values.

Empty Set, List, and Map Constants

The Collections class provides methods to return the empty Set, List, and Map — emptySet, emptyList, and emptyMap. The main use of these constants is as input to methods that take a Collection of values when you don't want to provide any values at all, as in this example.

tourist.declarePurchases(Collections.emptySet());