java collections interview questions

What is an Iterator interface? Is Iterator a Class or Interface? What is its use?
The Iterator is an interface, used to traverse through the elements of a Collection. It is not advisable to modify the collection itself while traversing an Iterator.
What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.
What is the List interface?
The List interface provides support for ordered collections of objects.
How can we access elements of a collection?
We can access the elements of a collection using the following ways:
1.Every collection object has get(index) method to get the element of the object. This method will return Object.
2.Collection provide Enumeration or Iterator object so that we can get the objects of a collection one by one.
What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
What’s the difference between a queue and a stack?
Stack is a data structure that is based on last-in-first-out rule (LIFO), while queues are based on First-in-first-out (FIFO) rule.
What is the Map interface?
The Map interface is used associate keys with values.
What is the Properties class?
The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.
Which implementation of the List interface provides for the fastest insertion of a new element into the middle of the list?
a. Vector
b. ArrayList
c. LinkedList
d. None of the above
ArrayList and Vector both use an array to store the elements of the list. When an element is inserted into the middle of the list the elements that follow the insertion point must be shifted to make room for the new element. The LinkedList is implemented using a doubly linked list; an insertion requires only the updating of the links at the point of insertion. Therefore, the LinkedList allows for fast insertions and deletions.
How can we use hashset in collection interface?
This class implements the set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the Null element.
This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets.
What are differences between Enumeration, ArrayList, Hashtable and Collections and Collection?
Enumeration: It is series of elements. It can be use to enumerate through the elements of a vector, keys or values of a hashtable. You can not remove elements from Enumeration.
ArrayList: It is re-sizable array implementation. Belongs to 'List' group in collection. It permits all elements, including null. It is not thread -safe.
Hashtable: It maps key to value. You can use non-null value for key or value. It is part of group Map in collection.
Collections: It implements Polymorphic algorithms which operate on collections.
Collection: It is the root interface in the collection hierarchy.
What is difference between array & arraylist?
An ArrayList is resizable, where as, an array is not. ArrayList is a part of the Collection Framework. We can store any type of objects, and we can deal with only objects. It is growable. Array is collection of similar data items. We can have array of primitives or objects. It is of fixed size. We can have multi dimensional arrays.
Array: can store primitive            ArrayList: Stores object only
Array: fix size                            ArrayList: resizable
Array: can have multi dimensional
Array: lang                                ArrayList: Collection framework
Can you limit the initial capacity of vector in java?
Yes you can limit the initial capacity. We can construct an empty vector with specified initial capacity
public vector(int initialcapacity)
What method should the key class of Hashmap override?
The methods to override are equals() and hashCode().
What is the difference between Enumeration and Iterator?
The functionality of Enumeration interface is duplicated by the Iterator interface. Iterator has a remove() method while Enumeration doesn't. Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects, where as using Iterator we can manipulate the objects also like adding and removing the objects.

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