ArrayList

There are two general-purpose List implementations in the Collection Framework, ArrayList and LinkedList, which of the two List implementations you use depends on your specific needs. If you need to support random access, without inserting or removing elements from any place to other than the end, then ArrayList offers you the optimal collection, the LinkedList class provides uniformly named methods to get, remove and insert an element at the beginning and end of the list.

Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.

An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.

Note that these implementation is not synchronized. If multiple threads access a set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be "wrapped" using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:

List list = Collections.synchronizedList(new ArrayList(...));