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

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Q31. Why String is popular HashMap key in 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.

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Q32. 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.

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Q33. Which Java collection class can be used to maintain the entries in the order in which they were last accessed?

Ans. LinkedHashMap

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Q34. 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.

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Q35. 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.

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Q36. Which are the sorted collections ?

Core Java

Ans. TreeSet and TreeMap

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Q37. How can we reverse the order in the TreeMap ?

Ans. Using Collections.reverseOrder()

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

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Q38. TreeMap orders the elements on which field ?

Ans. Keys

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Q39. 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.

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Q40. Can we add heterogeneous elements into TreeMap ?

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

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Q41. 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.

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Q42. Can we have null keys in TreeMap ?

Core Java

Ans. No, results in exception.

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Q43. Can value be null in TreeMap ?

Core Java

Ans. Yes.

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Q44. Which interface TreeMap implements ?

Ans. TreeMap implements NavigableMap, SortedMap, Serializable and Clonable.

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Q45. 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.

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Q46. What are the configuration files in Hibernate ?

Hibernate

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

and *.hbm.xml files ( Mapping Files )

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Q47. How is HashSet maintained in memory by Java ?

Core Java

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

Q48. 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

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Q49. Name few Collections Map implementations ?

Ans. AbstractMap
ConcurrentHashMap
ConcurrentSkipListMap
EnumMap
HashMap
IdentityHashMap
LinkedHashMap
SystemFlavorMap
TreeMap
WeakHashMap

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Q50. How do we specify the criteria if it involves mapping between two entities or join between tables ?

Hibernate

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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Q51. Name few classes that implement Map interface ?

Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement_82.html

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Q52. Difference between HashMap and WeakHashMap ?

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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Q53. Which tree is used to implement TreeMap in Java?

Ans. Red Black Tree

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Q54. Which of the following collection maintain its elements in Natural Sorted order ?

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

Ans. TreeMap

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Q55. 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

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Q56. 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.

Ans. Order in which constants are declared.

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Q57. 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.

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Q58. Which of the following is false ?

a. HashMap came before HashTable.
b. HashMap allows null values whereas Hashtable doesnâ€™t allow null values.
c. HashTable and HashMap allow Key-Value pairs.
d. Hashtable is synchronized whereas HashMap is not.

Ans. HashMap came before HashTable.

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Q59. Which of the collections allows null as the key ?

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

Core Java

Ans. HashMap

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Q60. Effective Java 6 , TreeMap implements ...

a. Map Interface
b. SortedMap Interface
c. NavigableMap Interface
d. SortedNavigableMap Interface

Ans. SortedMap Interface

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