Part 3
Interview questions on Concurrent Collections in java for experienced covered in this post:
What the the new methods introduced in ConcurrentHashMap to performing read and write operation?
-
There are 3 methods has introduced in ConcurrentHashMap which
are as follows.
1.
putIfAbsent():
If the key is present then there won’t be any change in collection.
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package simplifiedjava.crackedInterview; import java.util.concurrent.ConcurrentHashMap; public class ConcurrentHashMapPutIfAbsentDemo { public static void main(String[] args) {
ConcurrentHashMap<Integer, String> map
= new
ConcurrentHashMap<Integer, String>();
map.put(101,"Shweta");
map.put(102,"Shruti");
map.put(103,"Yogesh");
map.putIfAbsent(103,"Arpita"); // In this case 103 key was present
System.out.println(map);
map.putIfAbsent(104,"Ananya"); // In this case 104 key was not present.
System.out.println(map);
}
}
Output:
{101=Shweta, 102=Shruti, 103=Yogesh}
{101=Shweta, 102=Shruti, 103=Yogesh, 104=Ananya}
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2. remove(): This method won’t remove elements until and unless key and value match.
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package simplifiedjava.crackedInterview; import java.util.concurrent.ConcurrentHashMap; public class ConcurrentHashMapRemoveDemo {
public
static
void
main(String[] args) {
ConcurrentHashMap<Integer, String> map
= new
ConcurrentHashMap<Integer, String>();
map.put(101,"Shweta");
map.put(102,"Shruti");
map.put(103,"Yogesh");
map.remove(103,"Arpita"); // In this case 103 key is present but value was present.
System.out.println(map);
map.remove(103,"Yogesh"); // In this case key 103 and value Yogesh was present so
removed.
System.out.println(map);
} }
Output:
{101=Shweta, 102=Shruti, 103=Yogesh}
{101=Shweta, 102=Shruti} |
3.
replace(): This
method won’t replace value until and unless key and value match.
|
package simplifiedjava.crackedInterview; import java.util.concurrent.ConcurrentHashMap; public class ConcurrentHashMapReplaceDemo {
public
static
void
main(String[] args) {
ConcurrentHashMap<Integer, String> map
= new
ConcurrentHashMap<Integer, String>();
map.put(101,"Shweta");
map.put(102,"Shruti");
map.put(103,"Yogesh");
map.replace(103,"Arpita");
System.out.println(map);
map.replace(103,"Arpita","Updated Arpita");
System.out.println(map);
}
}
Output:
{101=Shweta, 102=Shruti, 103=Arpita}
{101=Shweta, 102=Shruti, 103=Updated Arpita} |
 
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What is CopyOnWriteArrayList? How does it work internally?
-
CopyOnWriteArrayList is Concurrent implementation of List
interface.
-
CopyOnWriteArrayList implements List interface so, all basic
functionality of List is applicable to CopyOnWriteArrayList.
-
It is a very costly object because every update operation
creates a clone copy of the same object.
-
When one thread is iterating CopyOnWriteArrayList same time
another thread is allowed to perform writes operation still we won’t get
ConcurrentModificationException.
-
Iterator of ArrayList performs remove operation. Iterator of
CopyOnWriteArrayList can not perform the remove operation. If we try to
remove then it will throw UnSupportedOperationException.
-
Iterator of CopyOnWriteArrayList is Fail-Safe.
-
Please refer to the below example.
|
package
simplifiedjava.crackedInterview; import java.util.concurrent.CopyOnWriteArrayList; public class CopyOnWriteArrayListDemo {
public
static
void
main(String[] args) {
CopyOnWriteArrayList<Integer> list
= new
CopyOnWriteArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
list.add(40);
list.add(50);
System.out.println(list);
}
}
Output: [10, 20, 30, 40, 50] |
What is ConcurrentModificationException?
-
When one thread is performing a read operation other threads
are not allowed to perform any operation on the same object still you are
attempting the same then it will throw ConcurrentModificationException.
-
Please refer to the below example.
|
package simplifiedjava.crackedInterview;
import
java.util.HashMap;
import
java.util.Iterator; import java.util.Map; public class ConcurrentModificationExceptionDemo {
public
static
void
main(String[] args) {
HashMap<Integer,String> m
= new
HashMap<Integer,String>();
m.put(101,"Shweta"); m.put(102,"Shruti");
Iterator itr
= m.entrySet().iterator();
while(itr.hasNext()) {
m.put(103, "Anjali");
Map.Entry<Integer, String> pair
= (Map.Entry<Integer, String>)itr.next();
System.out.println("Key = "+ pair.getKey()+ "\t Value = "+ pair.getValue());
}
}
}
Output:
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(Unknown Source)
at java.util.HashMap$EntryIterator.next(Unknown Source)
at java.util.HashMap$EntryIterator.next(Unknown Source)
at
simplifiedjava.crackedInterview.ConcurrentModificationExceptionDemo.main(ConcurrentModificationExceptionDemo.java:17)
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-
Java interview questions and answers all MNC - Click here
-
Basic core java interview questions and answers for freshers
- Click here
-
Core java interview questions for 3 years experience - Click here
-
Core java interview questions and answers for 3-5 years exp
- Click here
-
Core java interview questions and Answers for 5 - 7 Years exp
- Click here
-
Basic Java Interview Questions and Answers - Click here
-
Java interview questions and answers on oops - Click here
-
Java interview questions and answers on Strings - Click here
-
Java interview questions on exception handling - Click here
-
Interview questions on multithreading in java for experienced
- Click here
-
Interview questions on serialization in java for experienced
- Click here
-
Interview questions on inner class in java for experienced
- Click here
-
Interview questions on Collections in java for experienced
- Click here
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Thank you techies for visiting this blog. I hope you enjoyed this blog and got more technical knowledge. I have tried to cover all types of questions and provided examples tested on eclipse as much as I can. Guys, please don’t just mug up the questions and answers. Try to clear your concepts with examples. Try to write a code on eclipse once you read the concepts. It will help you to memorize the concepts for a very long time. Simultaneously you will be prepared for interview programs as well. It will help you to survive in the IT sector for a long time. It may be easy to crack an interview but it's really tough to survive in the IT industry with inadequate knowledge and skills. I have collected all the questions from actual interviews attended by my friends, colleagues, college mate and classmate. I have covered frequently asked questions as well as challenging questions. I have included many programs to understand the concept thoroughly. I will try to explain the concept with the help of a real-time program in eclipse. You can share more questions which are not covered in this blog or post. Always welcome your suggestions and queries. I will definitely try to resolve it. Please comment your queries and new set of questions under the comment sections. I will create a new post for those questions.
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