24 collections framework interview questions

24 Collections Framework Interview
Questions & Answers for Java
developers
Java Collections Framework contains most commonly asked Java interview questions.
A good understanding of Collections framework is required to understand and
leverage many powerful features of Java technology. Here are few important practical
questions which can be asked in a Core Java interview.

1. What is Java Collections API?

Java Collections framework API is a unified architecture for representing and
manipulating collections. The API contains Interfaces, Implementations &
Algorithm to help java programmer in everyday programming. In nutshell,
this API does 6 things at high level

o Reduces programming efforts. - Increases program speed and quality.
o Allows interoperability among unrelated APIs.
o Reduces effort to learn and to use new APIs.
o Reduces effort to design new APIs.
o Encourages & Fosters software reuse.

To be specific, There are six collection java interfaces. The most basic
interface is Collection. Three interfaces extend Collection: Set, List, and
SortedSet. The other two collection interfaces, Map and SortedMap, do not
extend Collection, as they represent mappings rather than true collections.

2. What is an Iterator?

Some of the collection classes provide traversal of their contents via a
java.util.Iterator interface. This interface allows you to walk through a
collection of objects, operating on each object in turn. Remember when using
Iterators that they contain a snapshot of the collection at the time the Iterator
was obtained; generally it is not advisable to modify the collection itself while
traversing an Iterator.

3. What is the difference between java.util.Iterator and
java.util.ListIterator?

Iterator : Enables you to traverse through a collection in the forward direction
only, for obtaining or removing elements ListIterator : extends Iterator, and
allows bidirectional traversal of list and also allows the modification of
elements.

4. What is HashMap and Map?

Map is Interface which is part of Java collections framework. This is to store
Key Value pair, and Hashmap is class that implements that using hashing
technique.

5. Difference between HashMap and HashTable? Compare Hashtable vs
HashMap?

Both Hashtable & HashMap provide key-value access to data. The Hashtable
is one of the original collection classes in Java (also called as legacy classes).
HashMap is part of the new Collections Framework, added with Java 2, v1.2.
There are several differences between HashMap and Hashtable in Java as
listed below

o 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).
o HashMap does not guarantee that the order of the map will remain
constant over time. But one of HashMap's subclasses is
LinkedHashMap, so in the event that you'd want predictable iteration
order (which is insertion order by default), you can easily swap out the
HashMap for a LinkedHashMap. This wouldn't be as easy if you were
using Hashtable.
o HashMap is non synchronized whereas Hashtable is synchronized.
o Iterator in the HashMap is fail-fast while the enumerator for the
Hashtable isn't. So this could be a design consideration.
6. What does synchronized means in Hashtable context?

Synchronized means only one thread can modify a hash table at one point of
time. 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.

7. What is fail-fast property?

At high level - Fail-fast is a property of a system or software with respect to its
response to failures. A fail-fast system is designed to immediately report any
failure or condition that is likely to lead to failure. Fail-fast systems are usually
designed to stop normal operation rather than attempt to continue a possibly-
flawed process.

When a problem occurs, a fail-fast system fails immediately and visibly.
Failing fast is a non-intuitive technique: "failing immediately and visibly"
sounds like it would make your software more fragile, but it actually makes it
more robust. Bugs are easier to find and fix, so fewer go into production.

In Java, Fail-fast term can be related to 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.

8. Why doesn't Collection extend Cloneable and Serializable?

From Sun FAQ Page: Many Collection implementations (including all of the
ones provided by the JDK) will have a public clone method, but it would be
mistake to require it of all Collections. For example, what does it mean to
clone a Collection that's backed by a terabyte SQL database? Should the
method call cause the company to requisition a new disk farm? Similar
arguments hold for serializable. If the client doesn't know the actual type of a
Collection, it's much more flexible and less error prone to have the client
decide what type of Collection is desired, create an empty Collection of this
type, and use the addAll method to copy the elements of the original collection
into the new one. Note on Some Important Terms

o 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.
o Fail-fast 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.
9. How can we make Hashmap synchronized?

HashMap can be synchronized by Map m =
Collections.synchronizedMap(hashMap);

10. Where will you use Hashtable and where will you use HashMap?

There are multiple aspects to this decision: 1. The basic difference between a
Hashtable and an HashMap is that, Hashtable is synchronized while HashMap
is not. Thus whenever there is a possibility of multiple threads accessing the
same instance, one should use Hashtable. While if not multiple threads are
going to access the same instance then use HashMap. Non synchronized data
structure will give better performance than the synchronized one. 2. If there is
a possibility in future that - there can be a scenario when you may require to
retain the order of objects in the Collection with key-value pair then HashMap
can be a good choice. As one of HashMap's subclasses is LinkedHashMap, so
in the event that you'd want predictable iteration order (which is insertion
order by default), you can easily swap out the HashMap for a
LinkedHashMap. This wouldn't be as easy if you were using Hashtable. Also
if you have multiple thread accessing you HashMap then
Collections.synchronizedMap() method can be leveraged. Overall HashMap
gives you more flexibility in terms of possible future changes.

11. Difference between Vector and ArrayList? What is the Vector class?

Vector & ArrayList both classes are implemented using dynamically resizable
arrays, providing fast random access and fast traversal. ArrayList and Vector
class both implement the List interface. Both the classes are member of Java
collection framework, therefore from an API perspective, these two classes are
very similar. However, there are still some major differences between the two.
Below are some key differences

o Vector is a legacy class which has been retrofitted to implement the
List interface since Java 2 platform v1.2
o Vector is synchronized whereas ArrayList is not. Even though Vector
class is synchronized, still when you want programs to run in
multithreading environment using ArrayList with
Collections.synchronizedList() is recommended over Vector.
o ArrayList has no default size while vector has a default size of 10.
o The Enumerations returned by Vector's elements method are not fail-
fast. Whereas ArraayList does not have any method returning
Enumerations.
12. What is the Difference between Enumeration and Iterator interface?

Enumeration and Iterator are the interface available in java.util package. The
functionality of Enumeration interface is duplicated by the Iterator interface.
New implementations should consider using Iterator in preference to
Enumeration. Iterators differ from enumerations in following ways:

1.Enumeration contains 2 methods namely hasMoreElements() &
nextElement() whereas Iterator contains three methods namely hasNext(),
next(),remove().
2.Iterator adds an optional remove operation, and has shorter method
names. Using remove() we can delete the objects but Enumeration
interface does not support this feature.
3.Enumeration interface is used by legacy classes. Vector.elements() &
Hashtable.elements() method returns Enumeration. Iterator is returned by
all Java Collections Framework classes. java.util.Collection.iterator()
method returns an instance of Iterator.
2. Why Java Vector class is considered obsolete or unofficially deprecated?
or Why should I always use ArrayList over Vector?

You should use ArrayList over Vector because you should default to non-
synchronized access. Vector synchronizes each individual method. That's
almost never what you want to do. Generally you want to synchronize a whole
sequence of operations. Synchronizing individual operations is both less safe
(if you iterate over a Vector, for instance, you still need to take out a lock to
avoid anyone else changing the collection at the same time) but also slower
(why take out a lock repeatedly when once will be enough)?

Of course, it also has the overhead of locking even when you don't need to. It's
a very flawed approach to have synchronized access as default. You can
always decorate a collection using Collections.synchronizedList - the fact that

Vector combines both the "resized array" collection implementation with the
"synchronize every operation" bit is another example of poor design; the
decoration approach gives cleaner separation of concerns.

Vector also has a few legacy methods around enumeration and element
retrieval which are different than the List interface, and developers (especially
those who learned Java before 1.2) can tend to use them if they are in the code.
Although Enumerations are faster, they don't check if the collection was
modified during iteration, which can cause issues, and given that Vector might
be chosen for its syncronization - with the attendant access from multiple
threads, this makes it a particularly pernicious problem. Usage of these
methods also couples a lot of code to Vector, such that it won't be easy to
replace it with a different List implementation.

Despite all above reasons Sun may never officially deprecate Vector class.
(Read details Deprecate Hashtable and Vector)

3. What is an enumeration?

An enumeration is an interface containing methods for accessing the
underlying data structure from which the enumeration is obtained. It is a
construct which collection classes return when you request a collection of all
the objects stored in the collection. It allows sequential access to all the
elements stored in the collection.

4. 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.

5. Where will you use Vector and where will you use ArrayList?

The basic difference between a Vector and an ArrayList is that, vector is
synchronized while ArrayList is not. Thus whenever there is a possibility of
multiple threads accessing the same instance, one should use Vector. While if
not multiple threads are going to access the same instance then use ArrayList.
Non synchronized data structure will give better performance than the
synchronized one.

6. What is the importance of hashCode() and equals() methods? How they
are used in Java?

The java.lang.Object has two methods defined in it. They are - public boolean
equals(Object obj) public int hashCode(). These two methods are used heavily
when objects are stored in collections.

There is a contract between these two methods which should be kept in mind
while overriding any of these methods.

The Java API documentation describes it in detail. The hashCode() method
returns a hash code value for the object. This method is supported for the
benefit of hashtables such as those provided by java.util.Hashtable or
java.util.HashMap. The general contract of hashCode is:

Whenever it is invoked on the same object more than once during an
execution of a Java application, the hashCode method must consistently return
the same integer, provided no information used in equals comparisons on the
object is modified. This integer need not remain consistent from one execution
of an application to another execution of the same application. If two objects
are equal according to the equals(Object) method, then calling the hashCode
method on each of the two objects must produce the same integer result. It is
not required that if two objects are unequal according to the
equals(java.lang.Object) method, then calling the hashCode method on each of
the two objects must produce distinct integer results. However, the
programmer should be aware that producing distinct integer results for
unequal objects may improve the performance of hashtables. As much as is
reasonably practical, the hashCode method defined by class Object does return
distinct integers for distinct objects. The equals(Object obj) method indicates
whether some other object is "equal to" this one. The equals method
implements an equivalence relation on non-null object references:

It is reflexive: for any non-null reference value x, x.equals(x) should return
true.

It is symmetric: for any non-null reference values x and y, x.equals(y) should
return true if and only if y.equals(x) returns true.

It is transitive: for any non-null reference values x, y, and z, if x.equals(y)
returns true and y.equals(z) returns true, then x.equals(z) should return true.

It is consistent: for any non-null reference values x and y, multiple invocations
of x.equals(y) consistently return true or consistently return false, provided no
information used in equals comparisons on the objects is modified. For any
non-null reference value x, x.equals(null) should return false. The equals
method for class Object implements the most discriminating possible
equivalence relation on objects; that is, for any non-null reference values x and
y, this method returns true if and only if x and y refer to the same object (x ==
y has the value true).

Note that it is generally necessary to override the hashCode method whenever
this method is overridden, so as to maintain the general contract for the
hashCode method, which states that equal objects must have equal hash codes.

A practical Example of hashcode() & equals(): This can be applied to
classes that need to be stored in Set collections. Sets use equals() to enforce
non-duplicates, and HashSet uses hashCode() as a first-cut test for equality.

Technically hashCode() isn't necessary then since equals() will always be used
in the end, but providing a meaningful hashCode() will improve performance
for very large sets or objects that take a long time to compare using equals().

7. What is the difference between Sorting performance of Arrays.sort() vs
Collections.sort() ? Which one is faster? Which one to use and when?

Many developers are concerned about the performance difference between
java.util.Array.sort() java.util.Collections.sort() methods. Both methods have
same algorithm the only difference is type of input to them. Collections.sort()
has a input as List so it does a translation of List to array and vice versa which
is an additional step while sorting. So this should be used when you are trying
to sort a list. Arrays.sort is for arrays so the sorting is done directly on the
array. So clearly it should be used when you have a array available with you
and you want to sort it.

8. What is java.util.concurrent BlockingQueue? How it can be used?

Java has implementation of BlockingQueue available since Java 1.5. Blocking
Queue interface extends collection interface, which provides you power of
collections inside a queue. Blocking Queue is a type of Queue that
additionally supports operations that wait for the queue to become non-empty
when retrieving an element, and wait for space to become available in the
queue when storing an element. A typical usage example would be based on a
producer-consumer scenario. Note that a BlockingQueue can safely be used
with multiple producers and multiple consumers. An ArrayBlockingQueue is a
implementation of blocking queue with an array used to store the queued
objects. The head of the queue is that element that has been on the queue the
longest time. The tail of the queue is that element that has been on the queue
the shortest time. New elements are inserted at the tail of the queue, and the
queue retrieval operations obtain elements at the head of the queue.
ArrayBlockingQueue requires you to specify the capacity of queue at the
object construction time itself. Once created, the capacity cannot be increased.
This is a classic "bounded buffer" (fixed size buffer), in which a fixed-sized
array holds elements inserted by producers and extracted by consumers.
Attempts to put an element to a full queue will result in the put operation
blocking; attempts to retrieve an element from an empty queue will be
blocked.

9. Set & List interface extend Collection, so Why doesn't Map interface
extend Collection?

Though the Map interface is part of collections framework, it does not extend
collection interface. This is by design, and the answer to this questions is best
described in Sun's FAQ Page: This was by design. We feel that mappings are
not collections and collections are not mappings. Thus, it makes little sense for
Map to extend the Collection interface (or vice versa). If a Map is a
Collection, what are the elements? The only reasonable answer is "Key-value
pairs", but this provides a very limited (and not particularly useful) Map
abstraction. You can't ask what value a given key maps to, nor can you delete

the entry for a given key without knowing what value it maps to. Collection
could be made to extend Map, but this raises the question: what are the keys?
There's no really satisfactory answer, and forcing one leads to an unnatural
interface. Maps can be viewed as Collections (of keys, values, or pairs), and
this fact is reflected in the three "Collection view operations" on Maps
(keySet, entrySet, and values). While it is, in principle, possible to view a List
as a Map mapping indices to elements, this has the nasty property that deleting
an element from the List changes the Key associated with every element
before the deleted element. That's why we don't have a map view operation on
Lists.

10. 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 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.

11. What is the difference between ArrayList and LinkedList? (ArrayList vs
LinkedList.)

java.util.ArrayList and java.util.LinkedList are two Collections classes used
for storing lists of object references Here are some key differences:

o ArrayList uses primitive object array for storing objects whereas
LinkedList is made up of a chain of nodes. Each node stores an
element and the pointer to the next node. A singly linked list only has
pointers to next. A doubly linked list has a pointer to the next and the
previous element. This makes walking the list backward easier.
o ArrayList implements the RandomAccess interface, and LinkedList
does not. The commonly used ArrayList implementation uses primitive
Object array for internal storage. Therefore an ArrayList is much faster
than a LinkedList for random access, that is, when accessing arbitrary
list elements using the get method. Note that the get method is
implemented for LinkedLists, but it requires a sequential scan from the
front or back of the list. This scan is very slow. For a LinkedList,
there's no fast way to access the Nth element of the list.
o Adding and deleting at the start and middle of the ArrayList is slow,
because all the later elements have to be copied forward or backward.
(Using System.arrayCopy()) Whereas Linked lists are faster for inserts
and deletes anywhere in the list, since all you do is update a few next
and previous pointers of a node.
o Each element of a linked list (especially a doubly linked list) uses a bit
more memory than its equivalent in array list, due to the need for next
and previous pointers.
o ArrayList may also have a performance issue when the internal array
fills up. The arrayList has to create a new array and copy all the

elements there. The ArrayList has a growth algorithm of (n*3)/2+1,
meaning that each time the buffer is too small it will create a new one
of size (n*3)/2+1 where n is the number of elements of the current
buffer. Hence if we can guess the number of elements that we are
going to have, then it makes sense to create a arraylist with that
capacity during object creation (using construtor new
ArrayList(capacity)). Whereas LinkedLists should not have such
capacity issues.

13. Where will you use ArrayList and Where will you use LinkedList?
Or Which one to use when (ArrayList / LinkedList).

Below is a snippet from SUN's site. The Java SDK contains 2 implementations
of the List interface - ArrayList and LinkedList. If you frequently add
elements to the beginning of the List or iterate over the List to delete elements
from its interior, you should consider using LinkedList. These operations
require constant-time in a LinkedList and linear-time in an ArrayList. But you
pay a big price in performance. Positional access requires linear-time in a
LinkedList and constant-time in an ArrayList.

14. What is performance of various Java collection
implementations/algorithms? What is Big 'O' notation for each of
them ?

Each java collection implementation class have different performance for
different methods, which makes them suitable for different programming
needs.

o Performance of Map interface
implementations
Hashtable
An instance of Hashtable has two parameters that affect its
performance: initial capacity and load factor. The capacity is the
number of buckets in the hash table, and the initial capacity is simply
the capacity at the time the hash table is created. Note that the hash
table is open: in the case of a "hash collision", a single bucket stores
multiple entries, which must be searched sequentially. The load factor
is a measure of how full the hash table is allowed to get before its
capacity is automatically increased. The initial capacity and load factor
parameters are merely hints to the implementation. The exact details as
to when and whether the rehash method is invoked are
implementation-dependent.

HashMap

This implementation provides constant-time [ Big O Notation is O(1) ]
performance for the basic operations (get and put), assuming the hash
function disperses the elements properly among the buckets.
Iteration over collection views requires time proportional to the
"capacity" of the HashMap instance (the number of buckets) plus its
size (the number of key-value mappings). Thus, it's very important not
to set the initial capacity too high (or the load factor too low) if
iteration performance is important.

TreeMap
The TreeMap implementation provides guaranteed log(n) [ Big O
Notation is O(log N) ] time cost for the containsKey, get, put and
remove operations.

LinkedHashMap
A linked hash map has two parameters that affect its performance:
initial capacity and load factor. They are defined precisely as for
HashMap. Note, however, that the penalty for choosing an excessively
high value for initial capacity is less severe for this class than for
HashMap, as iteration times for this class are unaffected by capacity.

o Performance of Set interface
implementations
HashSet
The HashSet class offers constant-time [ Big O Notation is O(1) ]
performance for the basic operations (add, remove, contains and size),
assuming the hash function disperses the elements properly among the
buckets. Iterating over this set requires time proportional to the sum of
the HashSet instance's size (the number of elements) plus the
"capacity" of the backing HashMap instance (the number of buckets).
Thus, it's very important not to set the initial capacity too high (or the
load factor too low) if iteration performance is important.

TreeSet
The TreeSet implementation provides guaranteed log(n) time cost for
the basic operations (add, remove and contains).

LinkedHashSet
A linked hash set has two parameters that affect its performance: initial
capacity and load factor. They are defined precisely as for HashSet.

Note, however, that the penalty for choosing an excessively high value
for initial capacity is less severe for this class than for HashSet, as
iteration times for this class are unaffected by capacity.

o Performance of List interface
implementations
LinkedList
- Performance of get and remove methods is linear time [ Big O
Notation is O(n) ] - Performance of add and Iterator.remove methods is
constant-time [ Big O Notation is O(1) ]

ArrayList
- The size, isEmpty, get, set, iterator, and listIterator operations run in
constant time. [ Big O Notation is O(1) ]
- The add operation runs in amortized constant time [ Big O Notation
is O(1) ] , but in worst case (since the array must be resized and
copied) adding n elements requires linear time [ Big O Notation is O(n)
]
- Performance of remove method is linear time [ Big O Notation is
O(n) ]
- All of the other operations run in linear time [ Big O Notation is O(n)
]. The constant factor is low compared to that for the LinkedList
implementation.

24 collections framework interview questions

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