Top Ten Concurrenthashmap Questions From Coffee Interviews

The ConcurrentHashMap shape component subdivision of concurrent collections parcel added on JDK 1.5 which contains utility classes like BlockingQueue, CopyOnWriteArrayList, CopyOnWriteArraySet etc. It is a replacement of synchronized hash-based map implementations e.g. Hashtable together with synchronized HashMap. It implements Map together with ConcurrentMap (a sub-interface of Map) interface which allows you lot to shop key-value pairs. The shape is similar to HashMap or Hashtable merely it's to a greater extent than scalable and the correct fit for concurrent Java application. Unlike Hashtable which achieves its thread-safety yesteryear compromising the scalability, ConcurrentHashMap uses advanced techniques e.g. dividing the map into segments to stay thread-safe together with scalable at the same time.

Because of its performance together with scalability equally good equally thread-safety, it is the most pop selection of Map inward concurrent Java applications.

In general, replacing synchronized collections amongst a concurrent collection tin terminate dramatically improve the scalability of your Java application amongst a footling risk, equally advised inward of the classic Java majority of all time, the Java Concurrency inward Practice book yesteryear Brian Goetz.

If you lot receive got non read this majority together with so I highly recommend it to every unmarried Java programmer. Many of you lot may respect it hard when you lot read it the get-go fourth dimension merely persist amongst it together with you lot volition reap the dividend inward your career equally Java programmer.

If you lot notwithstanding respect concepts hard to sympathize you lot tin terminate likewise bring help from Heinz Kabutz's awesome course Java Concurrency inward Practice Bundle, which is based on this book. Heinz is 1 of the Java champion together with fantabulous teacher who has gifted teaching skills. He explains hard concepts inward such an slow linguistic communication together with examples which you lot tin terminate correlate.

Anyway, let's start amongst this listing of Java interview questions based upon ConcurrentHashMap shape together with concepts approximately it.

Java ConcurrentHashMap Interview Questions amongst Answers

Here are some of the best together with oft asked Java ConcurrentHashMap interview questions. These questions are collected from the existent interview, hence, don't hold out surprised if you lot receive got already seen them during interviews.

These questions volition non entirely help you lot to arrive at good on interviews merely likewise encourage you lot to larn to a greater extent than close the concurrent hash map, which volition eventually help you lot inward your 24-hour interval to 24-hour interval programming job.


1. What is ConcurrentHashMap inward Java? (answer)
The java.util.concurrent.ConcurrentHashMap is a concurrent collection shape added on JDK 1.5 equally a replacement of synchronized hash-based map implementations e.g. Hashtable together with synchronized HashMap. They offering ameliorate performance together with scalability over their synchronized counterpart amongst footling risk.


2. Does ConcurrentHashMap thread-safe inward Java? (answer)
Yes, ConcurrentHashMap is thread-safe inward Java, which agency 2 thread tin terminate modify the map without damaging its internal information construction e.g. array together with linked list. If you lot compare this to HashMap, which is non thread-safe, exposing HashMap to multiple threads may harm internal information structure together with may homecoming the map completely useless, where many links may become missing or pointing to incorrect elements.


3. How does ConcurrentHashMap compass thread-safety? (answer)
The java.util.ConcurrentHashMap achieves thread-safety yesteryear dividing the map into segments together with locking entirely the segment which requires instead of locking the whole map. So, yes, it achieves thread-safety using locking merely it performs ameliorate because dissimilar HashMap, it never locks the whole map. This technique is likewise known equally lock stripping.

If you lot desire to larn to a greater extent than close it, you lot tin terminate likewise bring a aspect at answer)
Yes, ConcurrentHashMap allows concurrent read without locking equally reading performance doesn't require locking or thread-safety.


5. Can 1 thread read together with other writes on ConcurrentHashMap at the same time? (answer)
Yes, it's possible for a modest position out of the writer. For example, if a write performance is modifying 1 segment of ConcurrentHashmap together with read performance is happening on other segments together with so a reader volition non block, merely if reader thread is likewise trying to read from the same segment than it volition block until the author is done.


6. How does ConcurrentHashMap piece of work internally? (answer)
The java.util.ConcurrentHashMap plant similar to HashMap when it comes to storing key/value pairs together with retrieving values. The entirely deviation inward its implementation comes from concurrency perspective together with how it achievers thread-safety. It divides the map into several segments, yesteryear default 16, likewise known equally synchronization level.

Because of this, concurrent get(), put(), contains() performance is possible because it never locks the whole map merely entirely the relevant segment is locked. Which agency readers tin terminate access the map concurrency amongst writers together with a express position out of writers tin terminate modify the map concurrently. The upshot is ameliorate throughput together with Scalability.

You tin terminate farther encounter The Complete Java MaseterClasss course of didactics on Udemy for details on the implementation of ConcurrentHashMap class. This course of didactics is of late updated for Java 11, the latest Java version equally well.

Here is a diagram which explains how a segment looks similar inward a ConcurrentHashMap of Java, basically it's zilch merely a mini hash tabular array amongst a bucket together with a linked listing of hash entries inward instance of collision:

Since the Iterator returned yesteryear ConcurrentHashMap is weakly consistent, the recent concurrency modification may or may non hold out visible to it. There is no guarantee offered on such operation.

I  likewise advise you lot joining course Java Concurrency inward Practice Bundle, to larn to a greater extent than close how concurrency is handled yesteryear ConcurrentHashMap inward Java.



7. How arrive at you lot atomically update a value inward ConcurrentHashMap? (answer)
If you lot desire to atomically update an existing value inward ConcurrentHashMap, you lot tin terminate work the replace() move of concurrentHashMap.

It accepts both onetime value together with novel value together with entirely updates the map if the existing value inward the map matches amongst the onetime value provided, this agency the map is non concurrently modified during its call.

If the existing value is changed together with non matches amongst the onetime value together with so supervene upon neglect amongst returning false. You tin terminate work telephone phone the replace() method inward acre loop until you lot succeed, equally shown below:

ConcurrentMap<String, Long> populationByCities = new ConcurrentHashMap<>(); do{   Long currentValue = populationByCities.get("New York");   Long newValue = currentValue == null ? 1 : currentValue + 1; }while(!populationByCities.replace("New York", currentValue, newValue));

You tin terminate likewise see Java SE 8 for Really Impatient for some code examples of atomically updating an existing value inward ConcurerntHashMap.

8. How arrive at you lot withdraw a mapping acre iterating over ConcurrentHashMap? (answer)
You tin terminate work an Iterator to withdraw a mapping from ConcurrentHashMap inward Java equally shown below:

Map<String, Integer> bookAndPrice = novel ConcurrentHashMap<>(); bookAndPrice.put("Effective Java", 42); bookAndPrice.put("Head First Java", 29); bookAndPrice.put("Java Concurrency inward Practice", 33); bookAndPrice.put("Head First Design Patterns", 41);  System.out.println("before removing : " + bookAndPrice); Iterator<String> iterator = bookAndPrice.keySet().iterator();  while(iterator.hasNext()){   if(iterator.next().contains("Java")){   iterator.remove(); } }   System.out.println("after removing : " + bookAndPrice);  Output earlier removing : {Java Concurrency inward Practice=33,  Head First Design Patterns=41, Effective Java=42, Head First Java=29} after removing : {Head First Design Patterns=41}

9. Does Iterator of ConcurrentHashMap is fail-safe or fail-fast? (answer)
Iterator of ConcurrentHashMap is a fail-safe iterator which agency it volition non throw a ConcurrentModificationException, thus, eliminating the demand to lock the map during iteration.

The Iterator returned yesteryear ConcurrentHashMap are likewise weakly consistent which agency if the Map is modified during iteration, it may or may non reverberate the recent modification. Generally, it creates a re-create of collection earlier iterating.



10. What volition hap if you lot add together a novel mapping inward ConcurrentHashMap acre 1 thread is iterating over it? (answer)
This is 1 of the tricky questions related to ConcurrentHashMap. Since iterator's of ConcurrentHashMap are weekly consistent together with fail-safe they volition non neglect amongst ConcurrentModificationException merely it's likewise possible that they won't encounter whatever modification 1 time iteration started. Even though it's implementation dependent, JDK mostly creates a separate re-create of ConcurrentHashMap for iteration, instead of iterating over master copy copy.


11. Can you lot overstep an object of ConcurrentHahsMap when a Map is expected? (answer)
Yes because ConcurrentHashMap implements java.util.concurrent.ConcurrentMap interface, which extends java.util.Map interface, thence ConcurrentHashMap IS-A Map. Also, you lot tin terminate shop an object of ConcurrentHashMap into a Map variable equally shown below:

Map<String, Integer> bookAndPrice = new ConcurrentHashMap<>();

Though, this means, you lot may non receive got access to methods declared inward the java.util.concurrent.ConcurrentHashMap shape e.g. forEachKey() or forEachValue() method added inward Java 8.


That's all close some of the frequently asked questions close ConcurrentHashMap on Java interviews. These questions volition non entirely help you lot to arrive at good on your project interview merely likewise encourage you lot to larn to a greater extent than close ConcurrentHashMap.

Influenza A virus subtype H5N1 proficient together with enterprise cognition of ConcurrentHashMap is expected from both junior together with senior Java developer given its importance together with usability inward every Java application.

At to the lowest degree you lot should hold out comfortable amongst day-to-day operations amongst ConcurrentHashMap together with sympathize how the internal implementation works, peculiarly when compared to other thread-safe map implementations e.g. Hashtable together with Synchronized HashMap.


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Thanks for reading this article so far. If you lot similar these interview questions together with respect my explanations useful together with so delight percentage amongst your friends together with colleagues. If you lot receive got whatever enquiry or incertitude together with so delight driblet a comment.