What is the difference between ArrayList and Vector ?
Vector is synchronized while ArrayList is not . Vector is slow while ArrayList is fast . Every time when needed, Vector increases the capacity twice of its initial size while ArrayList increases its ArraySize by 50%.
What is the difference between HashMap and Hashtable ?
a. HashMap allows one null key and any number of null values while Hashtable does not allow null keys and null values.
b. HashMap is not synchronized or thread-safe while Hashtable is synchronized or thread-safe .
What is the difference between peek(),poll() and remove() method of the Queue interface ?
Both poll() and remove() method is used to remove head object of the Queue. The main difference lies when the Queue is empty().
If Queue is empty then poll() method will return null . While in similar case , remove() method will throw NoSuchElementException .
peek() method retrieves but does not remove the head of the Queue. If queue is empty then peek() method also returns null.
What is the difference between Iterator and ListIterator.
Using Iterator we can traverse the list of objects in forward direction . But ListIterator can traverse the collection in both directions that is forward as well as backward.
What is the difference between Array and ArrayList in Java ?
a. Array is static in size while ArrayList is dynamic in size.
b. Array can contain primitive data types while ArrayList can not contain primitive data types.When you add any primitive data type to ArrayList, it will automatically convert it to its corresponding Wrapper class(auto Boxing).
What is the difference between HashSet and TreeSet ?
a. HashSet maintains the inserted elements in random order while TreeSet maintains elements in the sorted order
b. HashSet can store null object while TreeSet can not store null object.
What is the difference between HashMap and ConcurrentHashMap ?
Main differences between HashMap and ConcurrentHashMap are :
a. HashMap is not synchronized while ConcurrentHashMap is synchronized.
b. HashMap can have one null key and any number of null values while ConcurrentHashMap does not allow null keys and null values .
What is the difference between LinkedList and ArrayList in Java ?
Main differences between LinkedList and ArrayList are :
a. LinkedList is the doubly linked list implementation of list interface , while , ArrayList is the resizable array implementation of list interface.
Why Map interface does not extend the Collection interface in Java Collections Framework ?
Map interface is not compatible with the Collection interface.
Explanation : Since Map requires key as well as value , for example , if we want to add key-value pair then we will use put(Object key , Object value) . So there are two parameters required to add element to the HashMap object . In Collection interface add(Object o) has only one parameter.
The other reasons are Map supports valueSet , keySet as well as other appropriate methods which have just different views from the Collection interface.
When to use ArrayList and when to use LinkedList in application?
ArrayList has constant time get operation O(1).Hence, ArrayList is preferred when the list is collection required more querying than modifying.
Insertion , Deletion operations take constant time O(1) for LinkedList. Hence, LinkedList is preferred when there are more insertions or deletions involved in the application.