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 doesn’t 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	LinkedHashSet
1The Underlying datastructure is Hashtable	1The underlying datastructure is combination of LinkedList and Hashtable
2Insertion Order is not preserved	2 Insertion order is preserved.
3Introduced in 1.2 version	3 Introduced in 1.4 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	Set
1Insertion Order is preserved	1Insertion Order is not preserved
2Duplicate Objects are allowed	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

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

Q40. What are differences between Comparable and Comparator?

Comparable	Comparator
1This can be used for natural sorting order	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)
4 It is marker interface	4 It is not a marker interface.