Q1. What are limitations of object Arrays?

The main limitations of Object arrays are

These are fixed in size ie once we created an array object there is

no chance of increasing or decreasing size based on our
requirement. Hence If we dont know size in advance , arrays are
not recommended to use

Arrays can hold only homogeneous elements.

There is no underlying data structure for arrays and hence no

readymade method support for arrays. Hence for every
requirement programmer has to code explicitly
To over come these problems collections are recommended to use

Q2. What are differences between arrays and collections?

Arrays

Collections

1. Arrays r fixed in size and hence once we

1. Collections are growable in nature and hence
created an array we are not allowed to increase or based on our requirement we can increase or
decrease the size based on our requirement.
decrease the size.

2. Memory point of view arrays are not

recommended to use

2. Memory point of view collections are

recommended to use.

3. Performance point of view arrays are

recommended to use

3. Performance point of view collections are not

recommended to use.

4. Arrays can hold only homogeneous elements 4. Collections can hold both homogeneous and
heterogeneous elements.

5. Arrays can hold both primitives as well as

objects

5. Collections can hold only objects.

6. For any requirement, there is no ready method 6. For every requirement ready made method
support compulsory programmer has to code
support is available. Being a programmer we

explicitly.

have to know how to use those methods and we

are not responsible to implement those.

Q3. what are differences between arrays and ArrayList?

Refer the answer of Q2
Q4. What are differences between arrays and Vector?
Refer the answer of Q2

Q5. What is Collection API ?

It defines set of classes and interfaces which can be used
for representing a group of objects as single entity
Q6. What is Collection framework?
It defines set of classes and inter faces which can be used
for representing a group of objects as single entity

Q7. What is difference between Collections and Collection?

Collection is an interface which can be used for representing a group
of individual objects as single entity and it acts as root interface of
collection frame work.
Collections is an utility class to define several utility methods for
Collection implemented class objects.
Q8. Explain about Collectiovvvn interface?

This interface can be used to represent a group of objects as a

single entity.

It acts as root interface for entire collection framework.

It defines the most commonly used methods which can be

applicable for any collection implemented class object

Q9. Explain about List interface?

List interface is a child interface of Collection interface. This can be used

to represent group of individual objects in as a single entity where

Duplicates are allowed

Insertion order is preserved

Q10. Explain about Set interface?

Set is a child interface of Collection interface. it can be used to represent
a group of individual objects as a single entity where

Duplicate objects are not allowed.

Insertion order is not preserved

Q11. Explain about SortedSet interface?

it is child interface of Set interface. it can be used to represent a group of
individual objects in to a single entity where

All the objects are arranged in some sorting order (Can be natural
sorting order or customizede).

Duplicates are not allowed.

Q12. Explain about NavigableSet ?

It is child interface of SortedSet and provides several utility methods for
navigation purposes

It doesnt allows duplicates

Insertion order is preserved

It is introduced in 1.6 version

Q13. Explain about Queue interface?

If we want to represent a group of individual objects prior to processing,
then we should go for Queue interface. It is child interface of Collection
interface.
It has introduced in 1.5 version.

Q14. Explain about Map interface?

Remember it is not a child Interface of Collection Interface and hence Map
and Collection Interfaces doesnt have any relationship.

It can be used for representing a group of Objects as key, value

pairs.

Both keys and values should be objects

Keys can t be duplicated but values can be duplicated.

it has introduced in 1.2 version

Q15. Explain about SortedMap ?

If we want to represent a group of objects as key value pairs where

all the entries are arranged according some sorting order of keys
then we should go for SortedMap.

It is child interface of Map.

It has introduced in 1.2 version

Q16. Explain about NavigableMap?

It is child interface of SortedMap and defines several method for

navigation purpose

It is introduced in 1.6 version

Q17. Explain about ArrayList class?

ArrayList is a Collection which can be used to represent a group of
objects as a single entity.

it is a implemented class for List interface

Introduced in 1.2 version

The underlying data structure is resizable or growable array.

Insertion order is preserved

Duplicates are allowed

Heterogeneous objects are allowed

null insertion is possible

This class implements RandomAccess , Serializable ,

Cloneable interfaces

Best choice for retrieval purpose and worst if our

frequent operation is insertion or deletion in the middle

Q18. What is RandomAccess Interface?

If a collection class implements RandomAccess interface then we

can access any of its element with the same speed.

RandomAccess interface is marker interface and it dosent contains

any methods.

ArrayList and vector classes implements this interface.

Q19. Explain about LinkedList class?

LinkedList is a Collection implemented class which can be used
for representing a group of objects as a single entity.

LinkedList is the implemetation class for List interface

Introduced in 1.2 version

Underlying data Structure is

Allows duplicates

Insertion order is preserved

Allows heterogeneous objects

null insertion is possible

LinkedList class implements Seriallizable and Cloneable interface but

not RandomAccess interface

Best choice if frequent operation is insertion or deletion an objects

in middle but worst choice if frequent operation is retrieval.

DoubleLinkedList

Q20. Explain about Vector class?

Vector is a legacy collection class which can be used to represent a

group of objects.

Introduced in 1.0 version. it is legacy class

The underlying data structure is resizable or growable array.

Insertion order is preserved

Duplicates are allowed

Heterogeneous objects are allowed

It is a implemented class for List interface

null insertion is possible

Vector class implements RandomAccess

,Serializable,Cloneable interfaces

Best Choice if frequent operation is retrieval and worst choice if

frequent operation is insertion or deletion in the middle.

All methods present in Vector class are synchronized hence Vector

class object is thread safe.

Q21. What is difference between ArrayList and Vector?

ArrayList

Vector

1. No method is synchronized in the ArrayList

class

1. All methods in Vector are synchronized.

2. ArrayList object is not thread safe.

2. Vector is thread safe.

3. Relatively performance is high

3. Relatively performance is low

4. Introduced in 1.2 version and it is non legacy 4. Introduced in 1.0 version and it is legacy

Q22. How we can get synchronized version of ArrayList?

Collections class contains synchronizedList() method for this

Public static List synchronizedList(List l)
EX
ArrayList l= new ArrayList();
List l2=Collections.synchronizedList(l);
Similarly we can get synchronized versions of Set and Map objects by
the following methods.
Public static List synchronizedSet(Set s)
Public static List synchronizedMap(Map m)
Q23. What is difference between size and capacity of a Collection
Object?
size means number of objects present where as capacity means no of
objects it can accommodate.

Q24. What is difference between ArrayList and Linked List?

ArrayList

1. The underlying data structure is resizable or

growable array.

LinkedList

1. The underlying data structure is Double

Linked List.

2. This is Best choice if frequent operation is

2. This is Best choice if frequent operation is
retrieval and worst choice if frequent operation is insertion or deletion in the middle and worst
insertion or deletion in the middle.
choice if frequentoperation is retrieval .

3. This class implements Serializable , Cloneable 3. This class implements Serializable , Cloneable
and RandomAccess interfaces.
but not RandomAccess interface.

Q25. What are legacy classes and interfaces present in

Collections framework ?

Enumeration ---Interface

Dictonary ------Abstract class

Hashtable -----Concrete class

Properties -----Concrete class

Vector -----Concrete class

Stack -----Concrete class

Q26. what is difference Enumeration and Iterator?

Enumeration
1. It is legacy interface and introduced in 1.0
version
2Applicable only for legacy classes and it is not
universal cursor
3While iterating the elements we are not allowed
to remove the objects just we can perform only
readoperation
4By using elements() method we can get
Enumeration object

Iterator
1 It is non-legacy and introduced in 1.2 version
2Applicable for any Collection implemented
class object.
3While iterating we can perform removal also in
addition to read operation.

4. By using iterator() method we can get

Iterator
object

Q27. What are limitations of Enumeration?

While iterating the elements we are not allowed to perform

removal operation

It is applicable only for legacy classes and it is not a universal

cursor.

It can retrieve the elements only in forward direction

Q28. What is difference between enum and Enumeration?

An enum can be used to define a group of named constants .It has
introduced in 1.5 version
Ex
Class Beer{
KO,KF,RC,FO
}
Enumeration is cursor to retrieve Objects one by one from Collection
objects.
Q29. What is difference between Iterator and ListIterator?
o ListIterator is the child interface of the Iterator
o Iterator is the single direction cursor where as ListIterator is
bidirectional cursor.
o While iterating the elements by Iterator we can perform only
read and remove operations. But by using ListIterator we can
perform read,removal, replace and addition of new objects
also.
o Iterator is applicable for every Collecton implemented class
object but ListIterator is applicable only for List implemented
class objects.
o Iterator can be get by using iterator() of Collection interface
where as ListIterator can be get by using listIterator() method
of List interface
o both are introduced in 1.2 version
Q30. What is relation between ListIterator and Iterator?
ListIterator is child interface of Iterator
Q31. Explain about HashSet class?

The underlying data structure is Hashtable

null values are accepted

duplicates are not allowed

insertion order is based on hashcode of the object hence insertion

order is not preserved

best suitable if frequent operation is search operations

HashSet class implements Serializable and Cloneable

it is implementation class for Set interface

heterogeneous objects are allowed

it is introduced in 1.2 version

Q32. If we are trying to insert duplicate values in Set what will

happen?
If we are trying to insert duplicate objects to the HashSet , we wont get
any compile time or run time errors just the add(Object o) returns false
and it doesnt add that object.
Q33. What is LinkedHashSet?
It is the child class of HashSet. The main difference between HashSet and
LinkedHashSet is:
In the case of HashSet insertion order is not preserved , but in the case of
LinkedHashSet insertion will be preserved.
Q34. Differences between HashSet and LinkedHashSet?

HashSet
1The Underlying datastructure is Hashtable

LinkedHashSet
1The underlying datastructure is combination of
LinkedList and Hashtable

2Insertion Order is not preserved

2

Insertion order is preserved.

Introduced in 1.4 version

3Introduced in 1.2 version

Q35. What are major enhancements in 1.4 version of collection

frame work?
LinkedHashSet
LinkedHashMap
IdentityHashMap
Q36. Explain about TreeSet?
It is Collection object which can be used to represent a group of objects
according to some sorting order.

The underlying datastructure is Balanced tree

Duplicates are not allowed

All objects are stored according to some sorting order hence

insertion order is not preserved

Heterogeneous objects are not allowed violation leads to

ClassCastException

For an Empty TreeSet as firs element null value can be inserted but
after inserting that first value if we are trying to insert any other
objects then we will get NullPointerException

For an non empty TreeSet if we are trying to inser null value at run
time u will get NullPointerException

Q37. What are differences between List and Set interfaces?

List
1Insertion Order is preserved
2Duplicate Objects are allowed

Set
1Insertion Order is not preserved
2

Duplicate Objects are not allowed

3The implemented classes are

ArrayList,LinkedList , Vector and Stack classes 3 The implemented classes are HashSet,
LinkedHashSet and Tree

Q38. What is Comparable interface?

This interface can be used for defining natural sorting order of the
objects.

It is present in java.lang package

It contains a method public int compareTo(Object obj1)

Q39. What is Comparator interface?

This interface can be used for implementing customized sorting

order.

It is present in java.util package

It contains two methods

o public int compare(Object ,Object)
o public boolean equals(Object)

Q40. What are differences between Comparable and Comparator?

Comparable
1This can be used for natural sorting order

Comparator
1This can be used
for implementing customized sorting

2This interface present in java.lang package

2

This is present in java.util package

3Contains only one method:

public int compareTo(Object obj1)

3 It contains two methods.

public int compare(Object ,Object)
public Boolean equals(Object)

It is marker interface

4 It is not a marker interface.

Q41. What is difference between HashSet and TreeSet?

HashSet
1The underlying data structure is Hashtable
2Heterogeneous objects are allowed

TreeSet
1The underlying data structure is balanced tree
2
Heterogeneous objects are not allowed
bydefalut

3Insertion order is not preserved and it is based

on hashcode of the objects
3 Insertion order is not preserved and all the
objects are inserted according to some sorting
order.
4null insertion is possible
4 As the first element only null insertion is
possible and in all other cases we will get
NullPointerException

Q42. What is Entry interface?

It is inner interface of Map.
In the Map each key value pair is considered as Entry object.
interface Map{
//more code here
interface Entry{
Object getKey()
Object getValue()
Object setValue(Object new)
}
}
Q43. Explain about HashMap?
It is a Map Object which can be used used to represent a group of
objects as key-value pairs.

The underlying data structure is Hashtable

Duplicaes keys are not allowed duplicate values are allowed

Insertion order is not preserved because insertion is based on

hashcode of keys.

Heterogeneous objects are allowed for both keys and values

null key is allowed only once

null values are allowed multiple times

Introduced in 1.2 version

Q44. Explain about LinkedHashMap?

It is child class of HashMap. It is exactly same as HashMap except the
following difference.
In the case of HashMap the insertion order is not preserved but in the
case of LinkedHashMap insertion order is preserved. Introduced in 1.4
version
Q45. Differences between HashMap and LinkedHashMap ?

HashMap
1.The underlying data structure is Hashtable

LinkedHashMap
1.The underlying data structure is a combination
of Hashtable and linkedlist

2.Insertion order is not preserved and it is based

on hashcode of keys
2

Insertion order is preserved

3.Introduced in 1.2 version

3 Introduced in 1.4 version.

Q46. Differences between HashMap and Hashtable?

HashMap
1.The underlying data structure is Hashtable
2.No method is synchronized and hence
HashMap object is not thread safe

Hashtable
1.The underlying data structure of Hashtable
2 .All methods are synchronized and hence it is
thread safe

3.Performance is high
3. Performance is low

4.null insertion is possible for both keys and

values

4. null insertion is not possible for both key and

value violation leads to NullPointerException

5.Introduced in 1.2 version and it is non legacy

5. Introduced in 1.0 version and it is legacy

Q47. What is IdentityHashMap?

It is exactly same as HashMap except the following difference.
In the HashMap JVM uses equals() method to identify duplicate keys
but in the case of IdentityHashMap JVM uses == operator for this.

Q48. What is difference between HashMap and IdentityHashMap?

Refer Q47 for the answer.
Q49. What is WeakHashMap?
It is exactly same as HashMap except the following difference.
In case of HashMap an Object is not eligible for garbage collection if it is
associated with HashMap even though it dosent have any external
references. ie HashMap dominates garbage collector.
But in case of WeakHashMap , if an Object is not having any external
references then it is always eligible for garabage collectoion even though
it is associated with weakHashMap. ie garbage collector dominates
WeakHashMap
Q50. What is difference between HashMap and WeakHashMap?
Refer Q49 for the answer.

Q51. What is TreeMap?

TreeMap can be used to store a group of objects as key-value pairs where

all the entries are arranged according to some sorting order of keys.

The underlying data structure is RED-BLACK Tree

Duplicates keys are not allowed but values can be duplicated.

Insertion order is not preserved because insertion is based on some

sorting order

If we are depending on Natural sorting order then keys should be

homogeneous(violation leads to ClassCastException) but values
need not homogeneous

In case of customized sorting order we can insert heterogeneous

keys and values

For empty TreeMap as first entry with null values are allowed but
after inserting that entry if we are trying to insert any other entry
we will get NullPointerException

For non empty TreeMap if we are trying to insert null keys we will
get NullPointerException

There are no restrictions for null values.

Q52. What is Hashtable

Hashtable is a legacy Map and can be used to store objects as key
value pairs.

The underlying data sturucture is Hashtabe

Duplicates keys are not allowed but duplicate values are allowed

null insertion is not possible for both keys and values

all methods are synchronized

insertion order is not preserved because it is based on hashcode of

keys

heterogeneous Objects are allowed for both keys and values

introduced in 1.0 version it is legacy class

Q53. What is PriorityQueue?

It represents a data structure to hold group of individual objects prior to
processing based on some priority .it can be natural sorting order and it
can be customized sorting order described by Comparator.
It is the implementation class of Queue interface.

Insertion order is not preserved because here insertion is done

based on some sorting order

Duplicates are not allowed

null insertion is not possible even as first element also

If we are depending on natural sorting order Objects should be

homogeneous violation leads to ClassCastException

If we are depending on customized sorting order Objects can be

heterogeneous also.

Q54. What is Arrays class?

It is utility class for arrays.

It defines several utility methods for arrays like sorting an array or

searching an element in array

present in java.util package

Q55. We are planning to do an indexed search in a list of objects.

Which of the two Java collections should you use: ArrayList or
LinkedList?
ArrayList
Q56. Why ArrayList is faster than Vector?
All methods present in the Vector are synchronized and hence any
method can be executed by only one thread at a time. It slows down the
execution.
But in ArrayList, no method is synchronized and hence multiple thread
are allowed execute simultaneously which speed up the execution.