What is the difference between list and vector in Java?

Sonoo Michael James Andy

umesh irfan kumar

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ArrayList and Vectors both implement the List interface, and both use [dynamically resizable] arrays for their internal data structure, much like using an ordinary array.

Syntax:

ArrayList: ArrayList al = new ArrayList[]; Vector: Vector v = new Vector[];

Vector vs. ArrayList in Java

S. No. ArrayList Vector

1.	ArrayList is not synchronized.	Vector is synchronized.
2.	ArrayList increments 50% of the current array size if the number of elements exceeds ts capacity.	Vector increments 100% means doubles the array size if the total number of elements exceeds its capacity.
3.	ArrayList is not a legacy class. It is introduced in JDK 1.2.	Vector is a legacy class.
4.	ArrayList is fast because it is non-synchronized.	Vector is slow because it is synchronized, i.e., in a multithreading environment, it holds the other threads in a runnable or non-runnable state until the current thread releases the lock of the object.
5.	ArrayList uses the Iterator interface to traverse the elements.	A Vector can use the Iterator interface or Enumeration interface to traverse the elements.

Significant Differences between ArrayList and Vector:

• Synchronization: Vector is synchronized, which means only one thread at a time can access the code, while ArrayList is not synchronized, which means multiple threads can work on ArrayList at the same time. For example, if one thread is performing an add operation, then there can be another thread performing a remove operation in a multithreading environment.If multiple threads access ArrayList concurrently, then we must synchronize the block of the code which modifies the list structurally or allow simple element modifications. Structural modification means the addition or deletion of element[s] from the list. Setting the value of an existing element is not a structural modification.

• Performance: ArrayList is faster. Since it is non-synchronized, while vector operations give slower performance since they are synchronized [thread-safe], if one thread works on a vector, it has acquired a lock on it, which forces any other thread wanting to work on it to have to wait until the lock is released.
• Data Growth: ArrayList and Vector both grow and shrink dynamically to maintain optimal use of storage – but the way they resize is different. ArrayList increments 50% of the current array size if the number of elements exceeds its capacity, while vector increments 100% – essentially doubling the current array size.
• Traversal: Vector can use both Enumeration and Iterator for traversing over vector elements, while ArrayList can only use Iterator for traversing.
• Applications: Most of the time, programmers prefer ArrayList over Vector because ArrayList can be synchronized explicitly using Collections.synchronizedList.

Note: ArrayList is preferable when there is no specific requirement to use vector.

How to choose between ArrayList and Vector?

• ArrayList is unsynchronized and not thread-safe, whereas Vectors are. Only one thread can call methods on a Vector, which is slightly overhead but helpful when safety is a concern. Therefore, in a single-threaded case, ArrayList is the obvious choice, but where multithreading is concerned, vectors are often preferable.
• If we don’t know how much data we will have but know the rate at which it grows, Vector has an advantage since we can set the increment value in vectors.
• ArrayList is newer and faster. If we don’t have any explicit requirements for using either of them, we use ArrayList over vector.

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Vector: Vector is a type of dynamic array which has the ability to resize automatically after insertion or deletion of elements. The elements in vector are placed in contiguous storage so that they can be accessed and traversed using iterators. Element is inserted at the end of the vector.
Example:

vector v; v.insert[5]; v.delete[];

List: List is a double linked sequence that supports both forward and backward traversal. The time taken in the insertion and deletion in the beginning, end and middle is constant. It has the non-contiguous memory and there is no pre-allocated memory.
Example:

list l; l.insert_begin[5]; l.delete_end[];

Below is a table of differences between Vector and List:

Vector List

It has contiguous memory.	While it has non-contiguous memory.
It is synchronized.	While it is not synchronized.
Vector may have a default size.	List does not have default size.
In vector, each element only requires the space for itself only.	In list, each element requires extra space for the node which holds the element, including pointers to the next and previous elements in the list.
Insertion at the end requires constant time but insertion elsewhere is costly.	Insertion is cheap no matter where in the list it occurs.
Vector is thread safe.	List is not thread safe.
Deletion at the end of the vector needs constant time but for the rest it is O[n].	Deletion is cheap no matter where in the list it occurs.
Random access of elements is possible.	Random access of elements is not possible.
Iterators become invalid if elements are added to or removed from the vector.	Iterators are valid if elements are added to or removed from the list.

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