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Interview Questions and Answers for 'Map' - 64 question(s) found - Order By Newest

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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
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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

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q2. Difference between TreeMap and HashMap ?Core Java
Anonymous
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

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Very frequently asked across all types of companies.
  Q3. Difference between HashMap and Hashtable?Core Java
Anonymous
Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values.

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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
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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.

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Frequently asked in Tech Mahindra ( Based on 3 inputs )
  Q5. What is the difference between List, Set and Map ?Core Java
Anonymous
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.

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 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
Anonymous
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.

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 Q7. Which of the following collections stores its elements in insertion Order ?

a. HashMap
b. TreeMap
c. LinkedHashMap
d. LinkedMap
Core Java
Anonymous
Ans. LinkedHashMap

  Sample Code for LinkedHashMap

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 Q8. Why HashTable has been deprecated ?Core Java2016-07-08 10:01:58

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.

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 Q9. Can we use null as a key for a map collection ?Core Java
Anonymous
Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values.

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 Q10. Difference between Map and HashMap ?Core Java
Anonymous
Ans. Map is an interface where HashMap is the concrete class.

  Sample Code for map

  Sample Code for hashmap

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 Q11. What is a Property class ?Core Java
Anonymous
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

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 Q12. Can we add duplicate keys in a HashMap ? What will happen if we attempt to add duplicate values ?
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Ans. No, We cannot have duplicate keys in HashMap. If we attempt to do so , the previous value for the key is overwritten.

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 Q13. What is comparator interface used for ?Core Java
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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.

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 Q14. What are the configuration files in Hibernate ?Hibernate
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Ans. hibernate.cfg.xml ( Main Configuration File )

and *.hbm.xml files ( Mapping Files )

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 Q15. Which of the following collection maintain its elements in Natural Sorted order ?

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

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 Q16. 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.
Anonymous
Ans. The new element will replace the existing element.

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 Q17. Which interface does java.util.Hashtable implement?Core Java
Anonymous
Ans. Java.util.Map

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 Q18. Which interface provides the capability to store objects using a key-value pair?Core Java
Anonymous
Ans. java.util.map

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  Q19. What is Comparable Interface?Core Java
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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.

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 Q20. Why String is popular HashMap key in Java?Core Java
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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.

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 Q21. Which Java collection class can be used to maintain the entries in the order in which they were last accessed?
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Ans. LinkedHashMap

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 Q22. Which are the sorted collections ?Core Java
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Ans. TreeSet and TreeMap

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 Q23. How can we reverse the order in the TreeMap ?
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Ans. Using Collections.reverseOrder()

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

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 Q24. TreeMap orders the elements on which field ?
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Ans. Keys

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 Q25. How TreeMap orders the elements if the Key is a String ?
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Ans. As String implements Comparable, It refers to the String compareTo method to identify the order relationship among those elements.

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 Q26. Can we add heterogeneous elements into TreeMap ?
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Ans. No, Sorted collections don't allow addition of heterogeneous elements as they are not comparable.

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 Q27. Will it create any problem if We add elements with key as user defined object into the TreeMap ?
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Ans. It won't create any problem if the objects are comparable i.e we have that class implementing Comparable interface.

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  Q28. Can we have null keys in TreeMap ?Core Java
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Ans. No, results in exception.

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 Q29. Can value be null in TreeMap ?Core Java
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Ans. Yes.

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 Q30. Which interface TreeMap implements ?
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Ans. TreeMap implements NavigableMap, SortedMap, Serializable and Clonable.

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 Q31. What is a ConcurrentHashMap ?Core Java
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Ans. ConcurrentHashMap is a hashMap that allows concurrent modifications from multiple threads as there can be multiple locks on the same hashmap.

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 Q32. How is HashSet maintained in memory by Java ?Core Java
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Ans. HashSet is maintained as HashMap by Java with values of the HashSet as Keys of the HashMap and value of the HashMap as the constant PRESENT.

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 Q33. Name few Collections Map implementations ?
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Ans. AbstractMap
ConcurrentHashMap
ConcurrentSkipListMap
EnumMap
HashMap
IdentityHashMap
LinkedHashMap
SystemFlavorMap
TreeMap
WeakHashMap

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 Q34. How do we specify the criteria if it involves mapping between two entities or join between tables ?Hibernate
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Ans. The following code returns the list of Employee objects having employee name starting with A and Dept Name ( Department , Employee Mapped ).

session.createCriteria(Employee.class,"emp")
.createAlias("emp.department", "dept",Criteria.INNER_JOIN)
.add( Restrictions.like("name", "A%") )
.add(Restrictions.eq("dept.name","Finance")
.list();

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 Q35. Name few classes that implement Map interface ?
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Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement_82.html

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 Q36. Difference between HashMap and WeakHashMap ?
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Ans. WeakHashMap uses weak reference for keys, which means if the key object doesn't have any reference then both key/value mapping will become eligible for garbage collection.

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 Q37. Which tree is used to implement TreeMap in Java?
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Ans. Red Black Tree

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 Q38. If we add Enum constants to a sorted collection ( Treemap , TreeSet ), What will be the order in which they will be maintained ?

a. Sorted Collection wont maintain them in any order.
b. Insertion Order
c. Order in which constants are declared.
d. Natural Sorting Order.
Anonymous
Ans. Order in which constants are declared.

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 Q39. Which of the following is false ?

a. HashMap came before HashTable.
b. HashMap allows null values whereas Hashtable doesn’t allow null values.
c. HashTable and HashMap allow Key-Value pairs.
d. Hashtable is synchronized whereas HashMap is not.
Anonymous
Ans. HashMap came before HashTable.

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 Q40. Which of the collections allows null as the key ?

a. HashTable
b. HashMap
c. TreeMap
d. LinkedHashMap
Core Java
Anonymous
Ans. HashTable

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