Collections FAQs

Q41. What is difference between HashSet and TreeSet?

HashSet	TreeSet
1The underlying data structure is Hashtable	1The underlying data structure is balanced tree
2Heterogeneous objects are allowed	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	LinkedHashMap
1.The underlying data structure is Hashtable	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	Hashtable
1.The underlying data structure is Hashtable	1.The underlying data structure of Hashtable
2.No method is synchronized and hence HashMap object is not thread safe	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.

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