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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.
  Q61. What is rule regarding overriding equals and hashCode method ?Core Java
Ans. A Class must override the hashCode method if its overriding the equals method.

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   Like         Discuss         Correct / Improve     java   collections   hashcode  hash code   equals   collections     Asked in 44 Companies      intermediate        frequent

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 Q62. What is the difference between Collection and Collections ?Core Java
Ans. Collection is an interface whereas Collections is a utility class.

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 Q63. How can we reverse the order in the TreeMap ?
Ans. Using Collections.reverseOrder()

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

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 Q64. TreeMap orders the elements on which field ?
Ans. Keys

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 Q65. 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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   Like         Discuss         Correct / Improve     java   collections   comparable interface   treemap   compareto


 Q66. 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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 Q67. 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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  Q68. Can we have null keys in TreeMap ?Core Java
Ans. No, results in exception.

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


 Q69. Can value be null in TreeMap ?Core Java
Ans. Yes.

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 Q70. Which interface TreeMap implements ?
Ans. TreeMap implements NavigableMap, SortedMap, Serializable and Clonable.

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 Q71. What will be the output of this code ?

Set mySet = new TreeSet();
mySet.add("4567");
mySet.add("5678");
mySet.add("6789");
for(String s: mySet){
System.out.println(s);
}
Ans. 4567
5678
6789

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 Q72. public enum Day {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY
}

public class Test {
Set mySet = new HashSet();
mySet.add(Day.MONDAY);
mySet.add(Day.SUNDAY);
mySet.add(Day.SATURDAY);

for(Day d: mySet){
System.out.println(d);
}
}
Ans. SUNDAY
MONDAY
SATURDAY

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   Like         Discuss         Correct / Improve     enum   set   collections   hashset   coding   code

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 Q73. 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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   Like         Discuss         Correct / Improve     ebay  java   collections   hashset   hashmap      expert


 Q74. What is ConcurrentLinkedDeque ?Core Java
Ans. It's a Collections concrete class that provides implementation of a Double Ended queue that allows concurrent insertion, removal, and access operations that can be executed safely across multiple threads.

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 Q75. What is CopyOnWriteArrayList ?
Ans. Its a type of ArrayList in which all Write operations , i.e add and set are performed by creating a new copy. This array never changes during the lifetime of the iterator, so it never throws ConcurrentModificationException

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 Q76. What are the advantages and disadvantages of CopyOnWriteArrayList ?Core Java
Ans. This collections class has been implemented in such a manner that it can never throw ConcurrentModificationException. As it performs update and write operations by creating a new copy of ArrayList, It's slower compared to ArrayList.

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 Q77. Name few Concurrent Collection Classes ?
Ans. ConcurrentHashMap
ConcurrentLinkedDeque
ConcurrentLinkedQueue
ConcurrentMap
ConcurrentNavigableMap
ConcurrentSkipListMap
ConcurrentSkipListSet

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 Q78. Name few Collections Map implementations ?
Ans. AbstractMap
ConcurrentHashMap
ConcurrentSkipListMap
EnumMap
HashMap
IdentityHashMap
LinkedHashMap
SystemFlavorMap
TreeMap
WeakHashMap

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 Q79. Name few interfaces that extends Collection Interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-interfaces-that.html

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 Q80. Name few classes that extends Collections class ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-inherit.html

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 Q81. Name few abstract classes of Java collections framework ?
Ans. http://www.buggybread.com/2015/02/java-collections-abstract-classes.html

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 Q82. Name few classes that implement Set interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement_78.html

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 Q83. Name few classes that implement iterable interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement_45.html

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 Q84. 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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 Q85. Name few classes that implement List interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement_35.html

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 Q86. Name few classes that implement Queue interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement_1.html

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 Q87. Name few classes that implement Collection interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement.html

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 Q88. Name few Hash collections classes and interfaces ?
Ans. http://www.buggybread.com/2015/01/java-hash-collections-classes-and.html

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 Q89. Name few Queue classes and interfaces ?
Ans. http://www.buggybread.com/2015/01/java-collections-queue-classes-and.html

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 Q90. What are few iterator classes provided by Java ?
Ans. http://www.buggybread.com/2015/01/java-iterator-classes-and-interfaces.html

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