List array Java

[old thread, but just 2 cents as none mention Guava or other libs and some other details]

It's worth pointing out the Guava way, which greatly simplifies these shenanigans:

Usage

For an Immutable List

Use the ImmutableList class and its of[] and copyOf[] factory methods [elements can't be null]:

List il = ImmutableList.of["string", "elements"]; // from varargs List il = ImmutableList.copyOf[aStringArray]; // from array

For A Mutable List

Use the Lists class and its newArrayList[] factory methods:

List l1 = Lists.newArrayList[anotherListOrCollection]; // from collection List l2 = Lists.newArrayList[aStringArray]; // from array List l3 = Lists.newArrayList["or", "string", "elements"]; // from varargs

Please also note the similar methods for other data structures in other classes, for instance in Sets.

The main attraction could be to reduce the clutter due to generics for type-safety, as the use of the Guava factory methods allow the types to be inferred most of the time. However, this argument holds less water since Java 7 arrived with the new diamond operator.

But it's not the only reason [and Java 7 isn't everywhere yet]: the shorthand syntax is also very handy, and the methods initializers, as seen above, allow to write more expressive code. You do in one Guava call what takes 2 with the current Java Collections.

For an Immutable List

Use the JDK's Arrays class and its asList[] factory method, wrapped with a Collections.unmodifiableList[]:

List l1 = Collections.unmodifiableList[Arrays.asList[anArrayOfElements]]; List l2 = Collections.unmodifiableList[Arrays.asList["element1", "element2"]];

Note that the returned type for asList[] is a List using a concrete ArrayList implementation, but it is NOT java.util.ArrayList. It's an inner type, which emulates an ArrayList but actually directly references the passed array and makes it "write through" [modifications are reflected in the array].

It forbids modifications through some of the List API's methods by way of simply extending an AbstractList [so, adding or removing elements is unsupported], however it allows calls to set[] to override elements. Thus this list isn't truly immutable and a call to asList[] should be wrapped with Collections.unmodifiableList[].

See the next step if you need a mutable list.

For a Mutable List

Same as above, but wrapped with an actual java.util.ArrayList:

List l1 = new ArrayList[Arrays.asList[array]]; // Java 1.5 to 1.6 List l1b = new ArrayList[Arrays.asList[array]]; // Java 1.7+ List l2 = new ArrayList[Arrays.asList["a", "b"]]; // Java 1.5 to 1.6 List l2b = new ArrayList[Arrays.asList["a", "b"]]; // Java 1.7+ // for Java 1.5+ static List arrayToList[final T[] array] { final List l = new ArrayList[array.length]; for [final T s : array] { l.add[s]; } return [l]; } // for Java < 1.5 [no generics, no compile-time type-safety, boo!] static List arrayToList[final Object[] array] { final List l = new ArrayList[array.length]; for [int i = 0; i < array.length; i++] { l.add[array[i]]; } return [l]; }