Difference between HashMap and HashSet in java

One of the common interview question is "What is difference between HashMap and HashSet"..Before we actually see differences,let me give you brief introduction of both.

HashMap

HashMap implements Map interface which maps key to value.It is not synchronized and is not thread safe.Duplicate keys are not allowed and null keys as well as values are allowed.

HashMap<Interger,String> employeeHashmap=new HashMap<Integer,String>();
employeeHashmap.put(1,"Arpit");
employeeHashmap.put(2,"John");  

HashSet

Hashtable implements Set interface which does not allow duplicate value.It is not synchronized and is not thread safe.

Hashtable<String> employeeSet=new Hashtable<String>();
employeeSet.add("Arpit");
employeeHashmap.add("john");  

add method is used to add element to HashSet.If It return true then element is added successfully but if it return false then you are trying to insert duplicate value.

public boolean add(Object o)

One main thing about HashSet is objects which we are going to add in HashSet must implement Hashcode() and equals() method so that we can check for duplicate values.If we are adding custom objects to HashSet then we must override() Hashcode() and equals() method according to our need.If we do not override then object will take default implementation which may not desirable.

HashMap vs HashSet:

Parameter	HashMap	HashSet
Interface	This is core difference among them.HashMap implements Map interface	HashSet implement Set interface
Method for storing data	It stores data in a form of key->value pair.So it uses put(key,value) method for storing data	It uses add(value) method for storing data
Duplicates	HashMap allows duplicate value but not duplicate keys	HashSet does not allow duplicate values.
Performance	It is faster than hashset as values are stored with unique keys	It is slower than HashMap
HashCode Calculation	In hash map hashcode value is calculated using key object	In this,hashcode is calculated on the basis of value object.Hashcode can be same for two value object so we have to implement equals() method.If equals() method return false then two objects are different.

Difference between Hashtable and HashMap in java

One of the common interview question is "What are differences between Hashtable and HashMap".When I started using them,I used any of them irrespective of their differences.Afterwards i found noticeable differences between them which can affect your performance of application. .Before we actually see differences,let me give you brief introduction of both.

HashMap

HashMap implements Map interface which maps key to value.It is not synchronized and is not thread safe.Duplicate keys are not allowed and null keys as well as value are allowed.

HashMap<Interger,String> employeeHashmap=new HashMap<Integer,String>();
employeeHashmap.put(1,"Arpit");
employeeHashmap.put(2,null);  // will work fine

Hashtable

Hashtable implements Map interface which maps key to value.It is synchronized and thread safe.Duplicate keys are not allowed and null key is not allowed.

Hashtable<Interger,String> employeeHashmap=new Hashtable<Integer,String>();
employeeHashmap.put(1,"Arpit");
employeeHashmap.put(2,null);  //not allowed and will throw NullPointer exception at run time

Hashtable vs HashMap:

Parameter	Hashtable	HashMap
ThreadSafe	Yes	No
Synchronized	Yes	No
Performance	Due to theadSafe and Synchronized,it is often slower than HashMap	In single threaded environment, it is much faster than Hashtable.So if you do not work in multi thread environment ,then hashMap is recommended
Null key	Do not allow	Allows null key as well as values
Fail fast	enumeration in hashtable is not fail fast	Iterator in hashMap is fail fast
Extends	It extends Dictionary class which is quite old	It extends AbstractMap class
Alternative	No alternative	You can use ConcurrentHashMap for multi thread environment

Some important points need to be discussed.

• Synchonized meaning only one thread can modify one table  at one point of time.When any thread perform update operation on hashtable then it acquires lock on it and other threads have to wait for lock to be released.
• Fail-fast iterator means if one thread is iterating over hashmap and other thread trying to modify hashmap structurally it will throw ConcurrentModification Exception and fail immediately.Structurally modification means inserting or deleting elements that can change structure of map.

Can we synchronize HashMap?

Yes,We can synchonized a HashMap also with the help of Collections.synchonizedMap(hashmap) so HashMap can be synchronized by

Map map=Collections.synchonizedMap(hashmap)