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 Q31. What are the pre-requisite for the collection to perform Binary Search ?
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Ans. 1. Collection should have an index for random access.
2. Collection should have ordered elements.

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   Like         Discuss         Correct / Improve     java   collections   search algorithm   search   binary search   at&t      intermediate


 Q32. 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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 Q33. 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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 Q34. If I try to add Enum constants to a TreeSet, What sorting order will it use ?
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Ans. Tree Set will sort the Values in the order in which Enum constants are declared.

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   Like         Discuss         Correct / Improve     java   collections   set   treeset   enum   sorting   technical lead


 Q35. What will be the output of this code ?

Set mySet = new HashSet();
mySet.add("4567");
mySet.add("5678");
mySet.add("6789");
for(String s: mySet){
System.out.println(s);
}
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Ans. It will print 4567,5678 and 6789 but Order cannot be predicted.

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 Q36. What will be the output of this code ?

Set mySet = new HashSet();
mySet.add("4567");
mySet.add("5678");
mySet.add("6789");
System.out.println(s.get(0));
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Ans. This will give compile time error as we cannot retrieve the element from a specified index using Set. Set doesn't maintain elements in any order.

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 Q37. What is ConcurrentModificationException ?Core Java
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Ans. This is the exception that is thrown when we try to modify the non concurrent collection class while iterating through it.

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 Q38. 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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 Q39. 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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Frequently asked in high end product companies. Frequently asked in Deloitte.
  Q40. How is Hashmap internally implemented in Java ?Core Java2016-12-06 08:49:37

Ans. https://www.quora.com/How-is-Hashmap-in-Java-implemented-internally-What-are-the-pros-and-cons-to-use-it-What-are-the-complexities-it-provides-for-insert-delete-and-lookup/answer/Anshudeep-Bajpai?srid=iwnK

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 Q41. Java doesn't provide exclusive access to memory like C/C++ and other lower level languages ? What are the advantanges and disadvantages ?Core Java2017-05-20 16:22:05

Ans. Yes, doesn't provide exclusive access as we cannot allocate and deallocate memory exclusively as Java internally manages it. The advantage of this is that it relieves the coder for such tasks and helps protect from many bugs that may get introduced with imperfect coding. Moreover as java garbage collector collects all unclaimed memory or objects, it helps the application from memory leaks.

On the flip side , as coder doesn't have extensive excess to memory , it is upto java to decide on the state for programming construct and data storage and hence may introduce some security risks. For example - Java keeps string literals in string pool and there is no exclusive way to remove it and hence may stay and sensitive data in string pool may introduce security issues. Moreover when we overwrite a value or object for a variable / reference, it is upto java to purge those values and hence it may stay in memory for a while till java decide that it is no longer referenced and hence should be removed and hence makes it vulnerable for inappropriate access.

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 Q42. What is meant by "Vector is synchronized and ArrayList isn't " ?Core Java2017-06-08 07:32:02

Ans. It means that only 1 thread can access have access to Vector at a time and no parallel access is allowed whereas Array List allows parallel access by multiple threads.

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   Like         Discuss         Correct / Improve     vectors  arraylist  collections  list  synchronization  synchronized


 Q43. Which collection stores element in insertion order ?Core Java2018-01-10 09:48:36

Ans. LinkedList maintains the insertion order in collection.

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 Q44. Which interface does java.util.Hashtable implement?Core Java
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Ans. Java.util.Map

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   Like         Discuss         Correct / Improve     java   collections   hashtable   map      basic        rare


Frequently asked.
  Q45. What is an Iterator?Core Java
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Ans. Iterator is an interface that provides methods to iterate over any Collection.

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   Like         Discuss         Correct / Improve     java   collections   iterator     Asked in 11 Companies      basic        frequent


 Q46. Which interface provides the capability to store objects using a key-value pair?Core Java
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Ans. java.util.map

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 Q47. What is a finalize method ?Core Java
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Ans. finalize() method is called just before an object is destroyed.

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  Q48. what is the difference between collections class vs collections interface ?Core Java
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Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.

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Usually asked only to fresh graduates and less experienced developers.
  Q49. What is a daemon thread? Give an Example ?
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Ans. These are threads that normally run at a low priority and provide a basic service to a program or programs when activity on a machine is reduced. garbage collector thread is daemon thread.

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  Q50. Advantage of Collection classes over Arrays ?Core Java
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Ans. Collections are re-sizable in nature. We can increase or decrease the size as per recruitment.
Collections can hold both homogeneous and heterogeneous data's.
Every collection follows some standard data structures.
Collection provides many useful built in methods for traversing,sorting and search.

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  Q51. 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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 Q52. Explain Set Interface?Core Java
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Ans. It is a collection of element which cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.

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 Q53. What is the difference between the size and capacity of a Vector?Core Java
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Ans. The size is the number of elements actually stored in the vector, while capacity is the maximum number of elements it can store at a given instance of time.

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 Q54. Can we use Ordered Set for performing Binary Search ?
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Ans. We need to access values on the basis of an index in Binary search which is not possible with Sets.

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 Q55. 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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 Q56. Is it legal to initialize List like this ?


LinkedList l=new LinkedList();
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Ans. No, Generic parameters cannot be primitives.

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 Q57. Which of the following syntax are correct ?a. LinkedList<Integer> l=new LinkedList<int>();b. List<Integer> l=new LinkedList<int>();c. LinkedList<Integer> l=new LinkedList<Integer>();d. List<Integer> l = new LinkedList<Integer>();Core Java
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Ans. All are correct.

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 Q58. Which are the sorted collections ?Core Java
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Ans. TreeSet and TreeMap

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Very Frequently asked. Favorite question in walkins and telephonic interviews. Usually among first few questions. Asked in different variants. Must know for intermediate and expert professionals.Among Top 10 frequently asked questions.
  Q59. What is rule regarding overriding equals and hashCode method ?Core Java
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Ans. A Class must override the hashCode method if its overriding the equals method.

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 Q60. What is the difference between Collection and Collections ?Core Java
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Ans. Collection is an interface whereas Collections is a utility class.

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