Interview Questions and Answers for 'Maps' | Search Interview Question

Interview Questions and Answers for 'Maps' - 67 question(s) found - Order By Newest

Frequently asked to fresh graduates and less experienced developers. Among the first few questions in many interviews.

Q1. What are the difference between composition and inheritance in Java?

Core Java

Ans. Composition - has-a relationship between objects.
Inheritance - is-a relationship between classes.

Composition - Composing object holds a reference to composed objects and hence relationship is loosely bound.
Inheritance - Derived object carries the base class definition in itself and hence its tightly bound.

Composition - Used in Dependency Injection
Inheritance - Used in Runtime Polymorphism

Composition - Single class objects can be composed within multiple classes.
Inheritance - Single class can only inherit 1 Class.

Composition - Its the relationship between objects.
Inheritance - Its the relationship between classes.

Sample Code for inheritance

Sample Code for composition

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java java concepts inheritance object oriented programming (oops) oops concepts composition object oriented programming (oops) oops concepts inheritance object oriented programming (oops) inheritance vs composition object oriented programming (oops) oops concepts Asked in 29 Companies basic frequent}

Try 5 Question(s) Test

Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.

Q2. Difference between TreeMap and HashMap ?

Core Java

Ans. They are different the way their elements are stored in memory. TreeMap stores the Keys in order whereas HashMap stores the key value pairs randomly.

Sample Code for treemap

Sample Code for hashmap

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections map treemap hashmap treemap vs hashmap Asked in 31 Companies basic frequent}

Try 1 Question(s) Test

Very frequently asked across all types of companies.

Q3. Difference between HashMap and Hashtable?

Core Java

Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashmap map hashtable hashmap vs hashtable Asked in 14 Companies basic frequent}

Try 2 Question(s) Test

Advanced level question frequently asked in US based companies. Recently asked in EMC and Intuit.

Q4. Can you provide some implementation of a Dictionary having large number of words ?

Solution

Ans. Simplest implementation we can have is a List wherein we can place ordered words and hence can perform Binary Search.

Other implementation with better search performance is to use HashMap with key as first character of the word and value as a LinkedList.

Further level up, we can have linked Hashmaps like ,

hashmap {
a ( key ) -> hashmap (key-aa , value (hashmap(key-aaa,value)
b ( key ) -> hashmap (key-ba , value (hashmap(key-baa,value)
....................................................................................
z( key ) -> hashmap (key-za , value (hashmap(key-zaa,value)
}

upto n levels ( where n is the average size of the word in dictionary.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashmap binary search search algorithm advanced architecture data structure Asked in 6 Companies frequent}

Try 1 Question(s) Test

Frequently asked in high end product companies. Frequently asked in Deloitte.

Q5. How is Hashmap internally implemented in Java ?

Core Java

Ans. https://medium.com/javarevisited/internal-working-of-hashmap-in-java-97aeac3c7beb#:~:text=HashMap%20internally%20uses%20HashTable%20implementation,the%20entries%20into%20the%20map.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve hashmap collections hashmap internal implementation Asked in 20 Companies expert frequent}

Q6. There are two objects a and b with same hashcode. I am inserting these two objects inside a hashmap.

hMap.put(a,a);
hMap.put(b,b);

where a.hashCode()==b.hashCode()

Now tell me how many objects will be there inside the hashmap?

Core Java

Ans. There can be two different elements with the same hashcode. When two elements have the same hashcode then Java uses the equals to further differentation. So there can be one or two objects depending on the content of the objects.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java hashcode map hashmap object reference advanced Asked in 1 Companies}

Q7. What is a ConcurrentHashMap ?

Core Java

Ans. ConcurrentHashMap is a hashMap that allows concurrent modifications from multiple threads as there can be multiple locks on the same hashmap.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashmap map concurrenthashmap concurrenthashmap concurrency general electric ge Asked in 32 Companies rare}

Q8. Why HashTable has been deprecated ?

Core Java

Ans. HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap. The only difference between the HashTable and HashMap is that Hashtable is synchronized whereas HashMap is not. Most of the synchronized collections have been deprecated and their raw alternative have been presented as preferred.Synchronization has a cost. Using synchronized collection in places where there is no need of it leads to useless utilization of resources. As these collections are rarely used in a static context or shared among threads, Java might have thought it better to just provide the raw collection and let developers implement synchronization if he feels the need to do so. HashMap is now presented as the default and the preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable. Because of this, Java thought it right to deprecate the use of HashTable.'

Synchronization has a cost. Using synchronized collection at a place where there is hardly any need of it would means useless utilization of resources. As these collections are rarely used in static context or shared among threads, Java might have thought it better to just provide the raw collection and let developer implement synchronization if he feels the need to do so.

As HashMap has been presented as default and preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable, Java thought it right to deprecate the use of HashTable.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve hashtable synchronized collections Why synchronized collections have been deprecated Why HashTable has been deprecated HashTable vs HashMap expert}

Try 3 Question(s) Test

Frequently asked in Indian Service Companies. Tech Mahindra ( Based on 3 inputs ) and Infosys ( 3 inputs )

Q9. What is the difference between List, Set and Map ?

or

What are the different Java Collections Interfaces ?

Core Java

Ans. List - Members are stored in sequence in memory and can be accessed through index.
Set - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates.
Map - Contains Key , Value pairs.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections list set map list vs set vs map Asked in 8 Companies basic frequent}

Try 1 Question(s) Test

Q10. Can we use null as a key for a map collection ?

Core Java

Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashmap map hashtable yes-no Asked in 1 Companies}

Q11. What is comparator interface used for ?

Core Java

Ans. The purpose of comparator interface is to compare objects of the same class to identify the sorting order. Sorted Collection Classes ( TreeSet, TreeMap ) have been designed such to look for this method to identify the sorting order, that is why class need to implement Comparator interface to qualify its objects to be part of Sorted Collections.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections treemap treeset comparator Asked in 2 Companies Intermediate}

Try 2 Question(s) Test

Q12. Which of the following collections stores its elements in insertion Order ?

a. HashMap
b. TreeMap
c. LinkedHashMap
d. LinkedMap

Core Java

Ans. LinkedHashMap

Sample Code for LinkedHashMap

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve linkedhashmap collections java map}

Q13. Difference between Map and HashMap ?

Core Java

Ans. Map is an interface where HashMap is the concrete class.

Sample Code for map

Sample Code for hashmap

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashmap map basic frequent}

Q14. What is a Property class ?

Core Java

Ans. The properties class is a subclass of Hashtable that can be read from or written to a stream.

Sample Code for Load Properties using Property Class

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashtable map synchronization synchronized}

Q15. What is Comparable Interface?

Core Java

Ans. It is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections comparable interface Asked in 7 Companies intermediate frequent}

Try 1 Question(s) Test

Q16. Why String is popular HashMap key in Java?

Core Java

Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesnt need to be calculated again. This makes it a great candidate for key in a Map and its processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java string class string immutable immutability hashmap immutable immutability hashcode hash code advanced Asked in 2 Companies expert frequent}

Q17. Can we add duplicate keys in a HashMap ? What will happen if we attempt to add duplicate values ?

Ans. No, We cannot have duplicate keys in HashMap. If we attempt to do so , the previous value for the key is overwritten.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashmap map hashtable}

Try 1 Question(s) Test

Q18. Which are the sorted collections ?

Core Java

Ans. TreeSet and TreeMap

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections treemap treeset basic interview question basic frequent}

Q19. What are the configuration files in Hibernate ?

Hibernate

Ans. hibernate.cfg.xml ( Main Configuration File )

and *.hbm.xml files ( Mapping Files )

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve hibernate configuration mapping files}

Q20. Which of the following collection maintain its elements in Natural Sorted order ?

a. HashMap
b. TreeMap
c. LinkedHashMap
d. LinkedMap

Ans. TreeMap

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve treemap collections java sorted collections map}

Q21. If we try to add duplicate key to the HashMap, What will happen ?

a. It will throw an exception.
b. It won't add the new Element without any exception.
c. The new element will replace the existing element.
d. Compiler will identify the problem and will throw an error.

Ans. The new element will replace the existing element.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve hashmap map collections java}

Q22. Which interface does java.util.Hashtable implement?

Core Java

Ans. Java.util.Map

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashtable map basic rare}

Q23. Which interface provides the capability to store objects using a key-value pair?

Core Java

Ans. java.util.map

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections map basic frequent}

Q24. Which Java collection class can be used to maintain the entries in the order in which they were last accessed?

Ans. LinkedHashMap

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashmap linkedhashmap}

Q25. How can we reverse the order in the TreeMap ?

Ans. Using Collections.reverseOrder()

Map tree = new TreeMap(Collections.reverseOrder());

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections map treemap}

Try 1 Question(s) Test

Q26. TreeMap orders the elements on which field ?

Ans. Keys

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections map treemap}

Q27. How TreeMap orders the elements if the Key is a String ?

Ans. As String implements Comparable, It refers to the String compareTo method to identify the order relationship among those elements.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections comparable interface treemap compareto}

Q28. Can we add heterogeneous elements into TreeMap ?

Ans. No, Sorted collections don't allow addition of heterogeneous elements as they are not comparable.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections comparable interface treemap}

Q29. Will it create any problem if We add elements with key as user defined object into the TreeMap ?

Ans. It won't create any problem if the objects are comparable i.e we have that class implementing Comparable interface.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections treemap comparable interface}

Q30. Can we have null keys in TreeMap ?

Core Java

Ans. No, results in exception.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections treemap Asked in 6 Companies Basic frequent}

Search Interview Questions

Help us and Others Improve. Please let us know the questions asked in any of your previous interview.