Interview questions on Collections in java for experienced

December 31, 2022

 Part 7

Interview questions on Collections in java for experienced covered in this post:

    Interview questions on Collections classes in java for experienced


        What are the characteristics of Map Interface?

    -          Map represents the group of objects as a key-value pair.

    -          Map is not a child interface of the Collection interface.

    -          Both keys and values are objects only.

    -          Characteristics of the map are as follows.

    1.       Underlying data structure is hashtable.

    2.       Insertion order is not preserved.

    3.       Inertion order is based on hashcode keys.

    4.       Duplicates keys are not allowed.

    5.       Values can be duplicated.

    6.       Heterogeneous (Different types of Object) objects are allowed for both keys and values.

    7.       Null is allowed for keys only once.

    8.       Null is allowed for values multiple times.

    9.       Hashmap is Serializable because Hashmap implements a Serializable interface.

    10.   Hashmap is Clonable because Hashmap implements a Clonable interface.

    11.   Hashmap has not implemented RandomAccess.

     

        What is the default size of Map?

    -          Default size of the map is 16.


     

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        What is EntrySet?

    -          A map is a group of key-value pair.Each key-value pair is called an Entry. Set of Entry is called EntrySet.

    -          Entry is an inner interface of Map Interface.

    -          Inside entry actual key and value pair will be stored.

    -          While iterating the map object we have to place iterator on Entry, not on a map.

    -          Without an existing Map object there is no chance of existing Entry object.

    -          Please refer to the below image for entry set.

     


        What is keySet?

    -          A map is a group of key-value pairs. The Set of keys are called keyset.

    -          Graphical representation of the key set.

     

     

        What is HashMap?

    -          HashMap is an implementation class of Map interface.

    -          Underlying data structure of HashMap is Hashtable.

    -          HashMap stores an object in the form of key and value pair.

     

        Explain the internal working of HashMap?

    -          Hashmap internally stores mapping in the form of Map.Entry object which contains key and value.

    -          Hashmap works on hashing principle.

    -          We use the put() method to insert elements in hashmap and get() method to retrieve elements from the hashmap object.

    -          Internal working of inserting an element in hashmap.

    1.       When we use the put() method to insert key and value objects into a hashmap, internally hashmap uses hashing technique to identify the bucket reference.

    2.       Once the bucket is identified object is stored in Map.Entry object in the form of key and value.

    3.       It may happen multiple objects can have the same hashcode so multiple objects will be stored in the same bucket.

    4.       If one bucket has multiple objects then those objects are stored in the form of a LinkedList.

    5.       We must override hashcode() and equals() method.

    6.       Hashcode() method calculates the hashcode as an integer value and the equals() method is used to identify the key if the key is matching then it will replace the existing element otherwise that element will be stored next to the current element.

    -            Internal working of retrieving element in hashmap.

    1.       When you want to retrieve an object from hashmap then you can call the get() method and pass the key object.

    2.       Hashmap generates the hashcode for that key and finds the appropriate bucket.

    3.       If there is only one element available in that bucket then simply that element will be returned.

    4.       If we want to retrieve an object from this linked list, we need an extra check to search for the correct element, this is done by the equals() method.

    -          Please refer to the below example.

    package simplifiedjava.crackedInterview; 

    import java.util.HashMap; 

    public class HashMapDemo { 

          public static void main(String[] args) {           

                HashMap<Integer, String> hMap = new HashMap<Integer, String>();

                hMap.put(1, "One");

                hMap.put(10, "Ten");

                hMap.put(100, "Hundred");

                hMap.put(1000, "Thousand");           

                System.out.println(hMap);          

                System.out.println(hMap.get(100));

          }

    }

    Output:

    {1=One, 100=Hundred, 1000=Thousand, 10=Ten}

    Hundred 

     -          Graphical Representation of Hashmap.

     

        What will happen if two different objects have the same hashcode?

    -          If the two different object have same hashcode, bucket location would be same that time collision will occur in HashMap Since HashMap uses LinkedList to store object internally, this entry (object of Map.Entry contains key and value) will be stored in LinkedList.

     

        What happen if hashmap object gets full?

    -          Default size of the hashmap is 16 and the load factor is 75%.

    -          If the load factor is filled 75% then internally hashmap size gets changed like other data structures like ArrayList, vector etc.

    -          Hashmap re-size itself by creating a new bucket array of size twice the previous size of hashmap and then start putting all old elements into the new bucket array this process is called rehashing.


    • Java interview questions and answers all MNC - Click here
    • Basic core java interview questions and answers for freshers - Click here
    • Core java interview questions for 3 years experience - Click here
    • Core java interview questions and answers for 3-5 years exp - Click here
    • Core java interview questions and Answers for 5 - 7 Years exp - Click here
    • Basic Java Interview Questions and Answers - Click here
    • Java interview questions and answers on oops - Click here
    • Java interview questions and answers on Strings - Click here
    • Java interview questions on exception handling - Click here
    • Interview questions on multithreading in java for experienced - Click here
    • Interview questions on serialization in java for experienced - Click here

    • Interview questions on inner class in java for experienced - Click here
    • Interview questions on Collections in java for experienced - Click here


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