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

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Q131. What is the trade off between using Collections.synchronizedList() on an ArrayList and a CopyOnWriteArrayList?

Core Java

Ans. Synchronized List locks the whole list to provide synchronization and thread safety during the read or write operation, while, CopyOnWriteArrayList doesn’t lock the whole list during these operations.
The CopyOnWriteArrayList class works according to its name i.e. copy-on-write which performs different actions for reading and write operations. For every write operation (add, set, remove, etc), it makes a new copy of the elements in the list. and for the read operations (get, iterator, listIterator, etc), it works on a different copy. So there is no additional overhead during a read operation and its read operation is faster than Collections.SynchronizedList(). Thus, COWAL is better for reading operation than Synchronized List.

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Q132. How can we destroy an object ?

Core Java

Ans. We cannot explicitly initiate destruction of an object as no destroy method is available in java. Only Garbage collection can destroy an object.

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Q133. When does an object cleaned or destroyed by garbage collection ?

Core Java

Ans. When it is no longer referenced or it looses all references which were earlier pointing to it.

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Q134. What happens when you assign a pre initialized reference some other object ?

Core Java

Ans. Now the reference points to a new object in memory. If that was the only reference for the previous object , it will be marked for garbage collection.

Foe example -

Object obj = new Object();
obj = new Object();

object created in first line will be eligible for garbage collection after line 2 as it looses all it's handlers.

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Q135. How would you explain the statement "Java Programs are collection of objects" ?

Core Java

Ans. Java Programs are collection of objects that communicates with each other to get a task accomplished. To add to those objects, there are common spaces ( static i.e common for objects belonging to a class ) that are used too.

We can visualize objects as departments of an organization in real world. Just like Task gets initiated in one department and then files are moved across different departments to get work done. In a similar fashion, a task is initiated in one object ( having main method ) and then information ( through POJOs / DTOs ) is moved across objects to accomplish a task.

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Q136. Why iterators of an array list are fail fast ?

Core Java

Ans. Because it's access isn't synchronized and hence access / modification by multiple threads may lead to inconsistent state.

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Q137. What problem we could have with ArrayList which aren't possible with Vectors ?

Core Java

Ans. ArrayLists aren't synchronized and hence doesn't allow synchronized access. As multiple threads can access an arraylist in parallel, it may result in an inconsistent state.

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Q138. What is the quickest way to find count of duplicate elements in an arraylist, without using iteration or loops ?

Core Java

Ans. We can copy the elements to a Set and then find the difference of count between ArrayList and Set. As Set don't allow duplicates , they will be removed in the set.

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Q139. In Java, would you like to see an explicit destructor?

Core Java

Ans. Yes , a gc method to explicitly call garbage collection immediately on the object. It will provide better flexibility over application that require optimal usage of memory.

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Q140. What does the following initialization mean ?

ArrayList<LinkedList> traversalPaths = new ArrayList<LinkedList>();

What could be the use of such a collection.

Core Java

Ans. Initialize an ArrayList that will hold LinkedLists i.e every element of the arraylist will be a linked list.

Such collection could be used in algorithms that require first random access and then sequential traversal. For example - Storing traversal paths for a graph wherein we can start from any vertex. Implementing dictionary with each arraylist element holding staring with character and then linked list holding duplicate words.

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Q141. Can we use primitive types with Collection classes ?

Core Java

Ans. No, As collection classes involve use of Generics, they cannot accept primitive types.

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Q142. Which objects are eligible for garbage collection ?

Core Java

Ans. Objects that have lost their reference i.e. they cannot be accessed/used by the program anymore.

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Q143. Which Data structure can be used for creating Queue ?

Data Structure

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Q144. What is a BlockingQueue ?

Core Java

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Q145. Which immutable classes have you worked with ? Can you name some immutable Collections ? How to make an immutable collection ?

Core Java

Ans. string and wrapper class objects

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Q146. Which of the two - Arrays or LinkedList - is a better data structure for implementing Queue ? and Why ?

Data Structure

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Q147. What is collection framework in Java ?

Core Java

Ans. The Java collections framework (JCF) is a set of classes and interfaces that implement commonly reusable collection data structures. Although referred to as a framework, it works in a manner of a library. The JCF provides both interfaces that define various collections and classes that implement them.

Sample Code for Map

Sample Code for HashMap

Sample Code for Treemap

Sample Code for set

Sample Code for hashset

Sample Code for treeset

Sample Code for list

Sample Code for arraylist

Sample Code for linkedlist

Sample Code for queue

Sample Code for priorityqueue

Sample Code for concurrenthashmap

Sample Code for vector

Sample Code for stack

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Q148. When do you use a Vector?

Core Java

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Q149. Can we modify a list while iterating it ?

Core Java

Ans. No,we cannot.I t will give concurrentModificationExceptin error. It can be resolved by using ConcurrentClasses like ConcurrentHashMap,CopyOnWriteArrayList,BlockingQueue etc which are fail-safe and wont give exception.

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Q150. Which are the thread safe / synchronized / concurrent classes in java collections ?

Core Java

Ans. Stack
Properties
Vector
BlockingQueue
ConcurrentMap
ConcurrentNavigableMap

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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

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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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