[java] What are the differences between ArrayList and Vector?

What are the differences between the two data structures ArrayList and Vector, and where should you use each of them?

Vector is a broken class that is not threadsafe, despite it being "synchronized" and is only used by students and other inexperienced programmers.

ArrayList is the go-to List implementation used by professionals and experienced programmers.

Professionals wanting a threadsafe List implementation use a CopyOnWriteArrayList.

ArrayList and Vector both implements List interface and maintains insertion order.But there are many differences between ArrayList and Vector classes...

ArrayList -

1. ArrayList is not synchronized.
2. ArrayList increments 50% of current array size if number of element exceeds from its capacity.
3. ArrayList is not a legacy class, it is introduced in JDK 1.2.
4. ArrayList is fast because it is non-synchronized.
5. ArrayList uses Iterator interface to traverse the elements.

Vector -

6. Vector is synchronized.
7. Vector increments 100% means doubles the array size if total number of element exceeds than its capacity.

8. Vector is a legacy class.

9. Vector is slow because it is synchronized i.e. in multithreading environment, it will hold the other threads in runnable or non-runnable state until current thread releases the lock of object.

10. Vector uses Enumeration interface to traverse the elements. But it can use Iterator also.

See Also : https://www.javatpoint.com/difference-between-arraylist-and-vector

There are 2 major differentiation's between Vector and ArrayList.

1. Vector is synchronized by default, and ArrayList is not. Note : you can make ArrayList also synchronized by passing arraylist object to Collections.synchronizedList() method. Synchronized means : it can be used with multiple threads with out any side effect.

2. ArrayLists grow by 50% of the previous size when space is not sufficient for new element, where as Vector will grow by 100% of the previous size when there is no space for new incoming element.

Other than this, there are some practical differences between them, in terms of programming effort:

1. To get the element at a particular location from Vector we use elementAt(int index) function. This function name is very lengthy. In place of this in ArrayList we have get(int index) which is very easy to remember and to use.
2. Similarly to replace an existing element with a new element in Vector we use setElementAt() method, which is again very lengthy and may irritate the programmer to use repeatedly. In place of this ArrayList has add(int index, object) method which is easy to use and remember. Like this they have more programmer friendly and easy to use function names in ArrayList.

When to use which one?

1. Try to avoid using Vectors completely. ArrayLists can do everything what a Vector can do. More over ArrayLists are by default not synchronized. If you want, you can synchronize it when ever you need by using Collections util class.
2. ArrayList has easy to remember and use function names.

Note : even though arraylist grows by 100%, you can avoid this by ensurecapacity() method to make sure that you are allocating sufficient memory at the initial stages itself.

Hope it helps.

Basically both ArrayList and Vector both uses internal Object Array.

ArrayList: The ArrayList class extends AbstractList and implements the List interface and RandomAccess (marker interface). ArrayList supports dynamic arrays that can grow as needed. It gives us first iteration over elements. ArrayList uses internal Object Array; they are created with an default initial size of 10. When this size is exceeded, the collection is automatically increases to half of the default size that is 15.

Vector: Vector is similar to ArrayList but the differences are, it is synchronized and its default initial size is 10 and when the size exceeds its size increases to double of the original size that means the new size will be 20. Vector is the only class other than ArrayList to implement RandomAccess. Vector is having four constructors out of that one takes two parameters Vector(int initialCapacity, int capacityIncrement) capacityIncrement is the amount by which the capacity is increased when the vector overflows, so it have more control over the load factor.

Some other differences are:

ArrayList is newer and 20-30% faster.

If you don't need something explitly apparent in Vector, use ArrayList

As the documentation says, a Vector and an ArrayList are almost equivalent. The difference is that access to a Vector is synchronized, whereas access to an ArrayList is not. What this means is that only one thread can call methods on a Vector at a time, and there's a slight overhead in acquiring the lock; if you use an ArrayList, this isn't the case. Generally, you'll want to use an ArrayList; in the single-threaded case it's a better choice, and in the multi-threaded case, you get better control over locking. Want to allow concurrent reads? Fine. Want to perform one synchronization for a batch of ten writes? Also fine. It does require a little more care on your end, but it's likely what you want. Also note that if you have an ArrayList, you can use the Collections.synchronizedList function to create a synchronized list, thus getting you the equivalent of a Vector.