Each vector tries to optimize storage management by maintaining a capacity and a capacityIncrement. The capacity is always at least as large as the vector size, it is usually larger because as components are added to the vector, the vector’s storage increases in chunks the size of capacityIncrement. An application can increase the capacity of a vector before inserting a large number of components, this reduces the amount of incremental reallocation. The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the vector is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by the elements method are not fail-fast. Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs. As of the Java 2 platform v1.2, this class was retrofitted to implement the List interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, Vector is synchronized. If a thread-safe implementation is not needed, it is recommended to use ArrayList in place of Vector. If the current capacity of this vector is less than minCapacity, then its capacity is increased by replacing its internal data array, kept in the field elementData, with a larger one. The size of the new data array will be the old size plus capacityIncrement, unless the value of capacityIncrement is less than or equal to zero, in which case the new capacity will be twice the old capacity, but if this new size is still smaller than minCapacity, then the new capacity will be minCapacity. This method is identical in functionality to the get(int) method (which is part of the List interface). This method is identical in functionality to the set(int, E) method (which is part of the List interface). Note that the set method reverses the order of the parameters, to more closely match array usage. Note also that the set method returns the old value that was stored at the specified position. This method is identical in functionality to the remove(int) method (which is part of the List interface). Note that the remove method returns the old value that was stored at the specified position. The index must be a value greater than or equal to 0 and less than or equal to the current size of the vector. (If the index is equal to the current size of the vector, the new element is appended to the Vector.) This method is identical in functionality to the add(int, E) method (which is part of the List interface). Note that the add method reverses the order of the parameters, to more closely match array usage. This method is identical in functionality to the add(E) method (which is part of the List interface). This method is identical in functionality to the remove(Object) method (which is part of the List interface). This method is identical in functionality to the clear() method (which is part of the List interface). If the Vector fits in the specified array with room to spare (i.e., the array has more elements than the Vector), the element in the array immediately following the end of the Vector is set to null. (This is useful in determining the length of the Vector only if the caller knows that the Vector does not contain any null elements.) This method eliminates the need for explicit range operations (of the sort that commonly exist for arrays). Any operation that expects a List can be used as a range operation by operating on a subList view instead of a whole List. For example, the following idiom removes a range of elements from a List: The semantics of the List returned by this method become undefined if the backing list (i.e., this List) is structurally modified in any way other than via the returned List. (Structural modifications are those that change the size of the List, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.) Source.