Core Java - Interview Questions and Answers for 'Collection' | Search Interview Question

Interview Questions and Answers - Order By Newest

Q151. What is a weak reference ?

Core Java

Ans. A weak reference is a reference that gets removed on the next garbage collection cycle.

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Q152. What are we doing here

list.stream().collect(Collectors.groupingBy(ClassA::getElement1,
Collectors.averagingDouble(ClassA::getelement2)));

Core Java

Ans. We are averaging elements of a collection by element1 grouped by element1

* list is the reference of collection
* element1 is the member element returned by getElement1 method of ClassA
* element2 is the member element returned by getElement2 method of ClassA

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Q153. What will the following code return

list.stream().collect(Collectors.groupingBy(ClassA::getElement1,
Collectors.averagingDouble(ClassA::getElement2)));

Core Java

Ans. Map<DataType1,DataType2> where DataType1 is the data type of element1 ( returned by getelement1 method ) and DataType2 is the data type of element2 ( returned by getElement2 method )

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Q154. 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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Q155. 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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Q156. Which memory segment is cleaned by Garbage Collection - stack or heap ?

Core Java

Ans. Heap as objects are stored ink heap.

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Q157. What are the different ways to avoid multi Threading related problems in Java ?

Core Java

Ans. Synchronization,
Concurrent classes,
Volatile keyword,
Implementing concurrent Lock interface,
Immutable classes

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Q158. Why Concurrent Collection Classes are fail-fast in Java ?

Core Java

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

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Q159. Does Java SE has Immutable collections ?

Core Java

Ans. Yes wef from Java 9

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Q160. Isnt the use of HashTable and ConcurrentHashMap the same, i.e providing synchronized map collection ?

Core Java

Ans. Both provide the thread safety but the actual difference come when talk about performance. CHM gives best performance when no of writer threads are less in number.

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Q161. What will happen if there are no collections in java ?

Core Java

Ans. Collections in Java is nothing but a library implementation for data structures and algorithm. If it's not available , we might have to include some other library or provide our own implementation.

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Q162. If you override hashcode to always return true, how a hash based collection will behave?

Core Java

Ans. At that bucket, it will form a linked list depending on what equals method evaluates for that object.

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Q163. Difference between List and ArrayList ?

Core Java

Ans. List is an interface whereas ArrayList is an implementation of List.

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Q164. difference between ArrayList and array ?

Core Java

Ans. ArrayList is a variable length collection class whereas arrays are fixed length primitive structure.

We can use generics with arraylist but not with arrays.

We can store primitive data types within arrays but can't with ArrayList. In ArrayList that needs to be converted to Wrapper objects.

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Q165. Can we override behavior of a collection class ? How ?

Core Java

Ans. Yes. We can do that.

1. We can create own own implementation class extending the collection class and then override the behavior method.

2. We can override the behavior at the time of instantiation of class as following

List<MyType> list = new ArrayList<MyType>() {
public boolean add(MyType mt) {
super.add(mt);
Collections.sort(list, comparator);
return true;
}
};

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Q166. Write a program to make an array act as a set.

Core Java

Ans. public Set convertArrayToList(T array[]) {
Set set = new HashSet<>(Arrays.asList(array));
return set;
}

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Q167. How can we create an arraylist of unique values ?

Core Java

Ans. We can put the value in a set to enforce uniqueness and then dum those value into an arraylist.

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Q168. What is the different between collection and Stream Api ?

Core Java

Ans. data under collection are actually stored in memory so that they can be retrieved when needed whereas data in streams are not stored and hence we need to construct it again when needed.

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Q169. Collections is a / an ..

Core Java

a. interface

b. abstract class

c. final class

d. util class

Ans.d. util class

Q170. What will be the output of exceuting main method ?

public static void main(String[] args){
      List list = new ArrayList();
      list.add(1);
      list.add(2);
      list.add(3);
      System.out.println(list);
   }

Core Java

a. 1,2,3

b. Order cannot be determined

c. compilation error

d. 3,2,1

Ans.a. 1,2,3

Q171. What will be the output upon executing main method ?

public static void main(String[] args){
      Set set = new HashSet();
      set.add(1);
      set.add(2);
      set.add(3);
      System.out.println(set);
   }

Core Java

a. 1,2,3

b. Order cannot be determined

c. Compilation error

d. 3,2,1

Ans.b. Order cannot be determined

Q172. Which of the following methods are used by Java Garbage Collection Mechanism ?

Core Java

a. final

b. finally

c. finalize

d. All of the above