Java Collections Interview Questions
What is HashMap and Map?
Map is Interface and Hashmap is class that implements this interface.
What is the significance of ListIterator?
Or
What is the difference b/w Iterator and ListIterator?
Iterator : Enables you to cycle through a collection in the forward direction only, for obtaining or removing elements
ListIterator : It extends Iterator, allow bidirectional traversal of list and the modification of elements
Difference between HashMap and HashTable? Can we make hashmap synchronized?
1. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values
as key and value whereas Hashtable doesn’t allow nulls).
2. HashMap does not guarantee that the order of the map will remain constant over time.
3. HashMap is non synchronized whereas Hashtable is synchronized.
4. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.
Note on Some Important Terms
1)Synchronized means only one thread can modify a
hash table at one point of time. Basically, it means that any thread
before performing
an update on a hashtable will have to acquire a lock
on the object while others will wait for lock to be released.
2)Fail-safe is relevant from the context of
iterators. If an iterator has been created on a collection object and
some other thread tries to
modify the collection object "structurally”, a
concurrent modification exception will be thrown. It is possible for
other threads though to
invoke "set" method since it doesn’t modify the
collection "structurally”. However, if prior to calling "set", the
collection has been modified
structurally, "IllegalArgumentException" will be
thrown.
HashMap can be synchronized by
Map m = Collections.synchronizeMap(hashMap);
What is the difference between set and list?
A Set stores elements in an unordered way and does
not contain duplicate elements, whereas a list stores elements in an
ordered way but may
contain duplicate elements.
Difference between Vector and ArrayList? What is the Vector class?
Vector is synchronized whereas ArrayList is not.
The Vector class provides the capability to implement a growable array
of objects. ArrayList and
Vector class both implement the List interface. Both
classes are implemented using dynamically resizable arrays, providing
fast random access and
fast traversal. In vector the data is retrieved
using the elementAt() method while in ArrayList, it is done using the
get() method. ArrayList has no default
size while vector has a default size of 10. when
you want programs to run in multithreading environment then use concept
of vector because it is
synchronized. But ArrayList is not synchronized so,
avoid use of it in a multithreading environment.
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.
So Enumeration is used when ever we want to make Collection objects as Read-only.
