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Vector

The Vector class implements a growable array of objects. Like an array, it contains components that can be accessed using an integer index. However, the size of a Vector can grow or shrink as needed to accommodate adding and removing items after the Vector has been created. Vectors basically fall in legacy classes but now it is fully compatible with collections.

Vector defines three protected data member:

• int capacityIncreament: Contains the increment value.
• int elementCount: Number of elements currently in vector stored in it.
• Object elementData[]: Array that holds the vector is stored in it.

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.

If a thread-safe implementation is not needed, it is recommended to use ArrayList in place of Vector. Vector proves to be very useful if you don’t know the size of the array in advance or you just need one that can change sizes over the lifetime of a program.

Synchronization

Unlike the new collection implementations, Vector is synchronized.

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.

Example of usage

import java.util.*;

public class VectorDemo {

   public static void main(String args[]) {
        // Initial capacity(size) of 3
        Vector<String> vector = new Vector<>(3);

        vector.addElement("First");
        vector.addElement("Second");
        vector.addElement("Third");
        vector.addElement("Fourth");

        System.out.println("Size is: " + vector.size());
        System.out.println("Capacity increment is: " + vector.capacity());

        vector.addElement("Fifth");
        vector.addElement("Sixth");
        vector.addElement("Seventh");

        System.out.println("Size is: " + vector.size());
        System.out.println("Capacity increment is: " + vector.capacity());

        // Display Vector elements
        System.out.println("Elements are:");
        for (String str : vector) {
            System.out.println(str);
        }
   }
}

Output:

Size is: 4
Capacity increment is: 6
Size is: 7
Capacity increment is: 12
Elements are:
First
Second
Third
Fourth
Fifth
Sixth
Seventh

Conclusion

Vector are:

• Inherits the AbstractList class;
• Implements List interface;
• Implements Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess interfaces;
• Synchronized;
• Perform get and set operation for constant time - O(1);
• Perform add and remove operation for amortized constant time - O(n).

Links

https://docs.oracle.com/javase/8/docs/api/java/util/Vector.html
https://www.geeksforgeeks.org/java-util-vector-class-java/
https://www.tutorialspoint.com/java/java_vector_class.htm

Next questions

What you know about ArrayList?
What you know about LinkedList?

credit goes to @swayangjit