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

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Q31. Can value be null in TreeMap ?

Core Java

Ans. Yes.

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Q32. Which interface TreeMap implements ?

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

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Q33. 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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Q34. Name few Collections Map implementations ?

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

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Q35. 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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Q36. 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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Q37. 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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Q38. Which tree is used to implement TreeMap in Java?

Ans. Red Black Tree

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Q39. 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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Q40. 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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Q41. 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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Q42. Effective Java 6 , TreeMap implements ...

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

Ans. SortedMap Interface

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Q43. Which of the following is a valid flow in Hadoop ?

a. Input -> Reducer -> Mapper -> Combiner -> -> Output
b. Input -> Mapper -> Reducer -> Combiner -> Output
c. Input -> Mapper -> Combiner -> Reducer -> Output
d. Input -> Reducer -> Combiner -> Mapper -> Output

Ans. Input -> Mapper -> Combiner -> Reducer -> Output

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Q44. MapReduce was devised by ...

a. Apple
b. Google
c. Microsoft
d. Samsung

Ans. Google

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Q45. Which of the following is not a phase of Reducer ?

a. Map
b. Reduce
c. Shuffle
d. Sort

Ans. Map

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Q46. What is the input to the Reduce function ?

a. One Key and One Value
b. Multiple Keys and Multiple associated Values
c. Multiple Keys and One associated values with each
d. One key and associated values.

Ans. One key and associated values.

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Q47. Which of the following is the implementation language for Map Reduce Framework ?

a. Big Data
b. Hadoop
c. Java
d. C++

Ans. Java

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Recently asked in Sophos

Q48. Write a Method to get a map of words and their count by passing in the string

Core Java

Ans. http://javasearch.buggybread.com/CodeSnippets/searchCodeSamples.php?keyword=Method+to+get+a+map+of+words&category=code

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Q49. What is unmodifiable set / unmodifiable map ? How can we create one ?

Ans. It is an unmodifiable / read only view of the underlying collection. We can use

Collections.unmodifiableSet(Set set)
Collections.unmodifiableMap(Map map)
Collections.unmodifiableCollection(Collection collection)

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Q50. What is IdentityHashMap ?

Core Java

Ans. IdentityHashMap is a class that implements AbstractMap and provides a data structure with Elements having Key Value pair, just like HashMap. It is similar to HashMap except that it uses reference equality when comparing elements.

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Q51. What is the difference between filter and filter-mappings xml tag within deployment descriptor ?

Java EE

Ans. filter tag speficies the filter name and respective class for handling filter action whereas filter mapping maps the filter to the url patterns ( specifying the url that needs to be intercepted )

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Q52. Difference between concurrentHashMap and HashTable ?

Core Java

Ans. HashTable locks the complete collection to provide synchronization whereas ConcurrentHashMap only locks segments to achieve synchronization and hence better efficient and faster.

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Q53. isnt the use of HashTable and ConcurrentHashMap the same, i.e providing synchronized map collection ?

Core Java

Ans. Yes, they both aim at providing synchronized access to the Map collection. The only difference is in their implementation. ConcurrentHashMap came after HashTable and hence technically more efficient as it doesn't lock the complete map while accessing it.

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Q54. Difference between Mutex and Semaphore?

Operating System

^{This question was recently asked at 'Veritas'.This question is still unanswered. Can you please provide an answer.}

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Q55. What is the difference between CountingSemaphore and a CountDownLatch?

Operating System

^{This question is still unanswered. Can you please provide an answer.}

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Q56. What is the difference between Java SE Map and Apache Commons MultiMap ? How can we implement functionality similar to multimap using Java SE map ?

Core Java

Ans. We can have duplicate keys within MultiMap whereas we cannot have duplicate keys within Java Util Map.

We can have a Map with value as a "collection of values" instead of single value to have a similar function as Multimap.

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Q57. If a Map needs to be accessed simultaneously by multiple threads, which collection class should be used ?

Core Java

Ans. ConcurrentHashMap

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Q58. Create a custom hashmap using synchronization.

Core Java

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Q59. What is the difference between TreeSet and TreeMap ?

Core Java

Ans. TreeSet contains only values as elements whereas TreeMap contains Key value pairs.

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Q60. How to sort objects based on one of the field ?

Core Java

Ans. Using comparable and comparator and sorted collections like TreeSet or TreeMap.

or

use stream api from java 8 onwards which internally refers to comparable and comparator through lambda expressions

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