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 Q121. 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));
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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 Q122. 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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 Q123. 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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 Q124. 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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 Q125. 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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 Q126. 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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 Q127. Name few Concurrent Collection Classes ?
Ans. ConcurrentHashMap
ConcurrentLinkedDeque
ConcurrentLinkedQueue
ConcurrentMap
ConcurrentNavigableMap
ConcurrentSkipListMap
ConcurrentSkipListSet

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 Q128. Name few Collections Map implementations ?
Ans. AbstractMap
ConcurrentHashMap
ConcurrentSkipListMap
EnumMap
HashMap
IdentityHashMap
LinkedHashMap
SystemFlavorMap
TreeMap
WeakHashMap

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 Q129. What is ConcurrentModificationException ?Core Java
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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 Q130. Name few interfaces that extends Collection Interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-interfaces-that.html

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 Q131. Name few classes that extends Collections class ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-inherit.html

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 Q132. Name few abstract classes of Java collections framework ?
Ans. http://www.buggybread.com/2015/02/java-collections-abstract-classes.html

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 Q133. 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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 Q134. 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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 Q135. 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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 Q136. 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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 Q137. 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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 Q138. Name few classes that implement Collection interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement.html

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 Q139. Name few Hash collections classes and interfaces ?
Ans. http://www.buggybread.com/2015/01/java-hash-collections-classes-and.html

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 Q140. Name few Queue classes and interfaces ?
Ans. http://www.buggybread.com/2015/01/java-collections-queue-classes-and.html

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 Q141. 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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 Q142. Which sorting algorithm is used by Collections.sort() in Java ?Core Java
Ans. The sorting algorithm is a modified mergesort. This algorithm offers guaranteed n log(n) performance.

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 Q143. 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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 Q144. Which tree is used to implement TreeMap in Java?
Ans. Red Black Tree

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 Q145. What is collision in hash collections ?Core Java
Ans. Collision is the situation when two different elements have the same Hash Code.

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 Q146. What will the following code result ?

List> intList = new ArrayList>();

intList.add(Optional.empty());
intList.add(Optional.of(2));
intList.add(Optional.of(3));
intList.add(Optional.of(4));

System.out.println(intList.get(null));
Ans. Compile time error at last line as the get method expect argument of type native int.

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 Q147. What will the following code result ? Will it compile ?

List> intList = new ArrayList>();

intList.add(Optional.empty());
intList.add(Optional.of(2));
intList.add(Optional.of(3));
intList.add(Optional.of(4));

System.out.println(intList.get((Integer)null));
Ans. Yes but the last line will throw NullPointerException upon execution.

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 Q148. What is a BlockingQueue?Data Structures
Ans. BlockingQueue is a Queue that supports operations that wait for the queue to become non-empty when retrieving and removing an element, and wait for space to become available in the queue when adding an element.

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 Q149. Do you know of any facility to convert between Scala and Java Util Collections ?Scala
Ans. We can use JavaConversions which is a part of Scala Library.

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 Q150. 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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