Top X Concurrenthashmap Questions From Coffee Interviews

The ConcurrentHashMap degree component subdivision of concurrent collections packet 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 in addition to synchronized HashMap. It implements Map in addition to ConcurrentMap (a sub-interface of Map) interface which allows y'all to shop key-value pairs. The degree is similar to HashMap or Hashtable but it's to a greater extent than scalable and the correct fit for concurrent Java application. Unlike Hashtable which achieves its thread-safety past times compromising the scalability, ConcurrentHashMap uses advanced techniques e.g. dividing the map into segments to rest thread-safe in addition to scalable at the same time.

Because of its performance in addition to scalability every bit good every bit thread-safety, it is the most pop choice of Map inwards concurrent Java applications.

In general, replacing synchronized collections amongst a concurrent collection tin dramatically improve the scalability of your Java application amongst a piffling risk, every bit advised inwards of the classic Java majority of all time, the Java Concurrency inwards Practice book past times Brian Goetz.

If y'all guide maintain non read this majority in addition to so I highly recommend it to every unmarried Java programmer. Many of y'all may honor it hard when y'all read it the kickoff fourth dimension but persist amongst it in addition to y'all volition reap the dividend inwards your career every bit Java programmer.

If y'all nonetheless honor concepts hard to empathise y'all tin likewise convey assistance from Heinz Kabutz's awesome course Java Concurrency inwards Practice Bundle, which is based on this book. Heinz is i of the Java champion in addition to first-class teacher who has gifted instruction skills. He explains hard concepts inwards such an slow linguistic communication in addition to examples which y'all tin correlate.

Anyway, let's start amongst this listing of Java interview questions based upon ConcurrentHashMap degree in addition to concepts only about it.

Java ConcurrentHashMap Interview Questions amongst Answers

Here are some of the best in addition to oftentimes asked Java ConcurrentHashMap interview questions. These questions are collected from the existent interview, hence, don't endure surprised if y'all guide maintain already seen them during interviews.

These questions volition non solely assistance y'all to gain good on interviews but likewise encourage y'all to larn to a greater extent than almost the concurrent hash map, which volition eventually assistance y'all inwards your solar daytime to solar daytime programming job.


1. What is ConcurrentHashMap inwards Java? (answer)
The java.util.concurrent.ConcurrentHashMap is a concurrent collection degree added on JDK 1.5 every bit a replacement of synchronized hash-based map implementations e.g. Hashtable in addition to synchronized HashMap. They offering amend performance in addition to scalability over their synchronized counterpart amongst piffling risk.


2. Does ConcurrentHashMap thread-safe inwards Java? (answer)
Yes, ConcurrentHashMap is thread-safe inwards Java, which way 2 thread tin modify the map without damaging its internal information construction e.g. array in addition to linked list. If y'all compare this to HashMap, which is non thread-safe, exposing HashMap to multiple threads may harm internal information structure in addition to may homecoming the map completely useless, where many links may become missing or pointing to incorrect elements.


3. How does ConcurrentHashMap accomplish thread-safety? (answer)
The java.util.ConcurrentHashMap achieves thread-safety past times dividing the map into segments in addition to locking solely the segment which requires instead of locking the whole map. So, yes, it achieves thread-safety using locking but it performs amend because different HashMap, it never locks the whole map. This technique is likewise known every bit lock stripping.

If y'all desire to larn to a greater extent than almost it, y'all tin likewise convey a expression at answer)
Yes, ConcurrentHashMap allows concurrent read without locking every bit reading performance doesn't require locking or thread-safety.


5. Can i thread read in addition to other writes on ConcurrentHashMap at the same time? (answer)
Yes, it's possible for a pocket-sized reveal of the writer. For example, if a write performance is modifying i segment of ConcurrentHashmap in addition to read performance is happening on other segments in addition to so a reader volition non block, but 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 in addition to retrieving values. The solely divergence inwards its implementation comes from concurrency perspective in addition to how it achievers thread-safety. It divides the map into several segments, past times default 16, likewise known every bit synchronization level.

Because of this, concurrent get(), put(), contains() performance is possible because it never locks the whole map but solely the relevant segment is locked. Which way readers tin access the map concurrency amongst writers in addition to a express reveal of writers tin modify the map concurrently. The resultant is amend throughput in addition to Scalability.

You tin farther come across The Complete Java MaseterClasss class on Udemy for details on the implementation of ConcurrentHashMap class. This class is lately updated for Java 11, the latest Java version every bit well.

Here is a diagram which explains how a segment looks similar inwards a ConcurrentHashMap of Java, basically it's nil but a mini hash tabular array amongst a bucket in addition to a linked listing of hash entries inwards instance of collision:

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

I  likewise propose y'all joining course Java Concurrency inwards Practice Bundle, to larn to a greater extent than almost how concurrency is handled past times ConcurrentHashMap inwards Java.



7. How gain y'all atomically update a value inwards ConcurrentHashMap? (answer)
If y'all desire to atomically update an existing value inwards ConcurrentHashMap, y'all tin piece of work the replace() component subdivision of concurrentHashMap.

It accepts both quondam value in addition to novel value in addition to solely updates the map if the existing value inwards the map matches amongst the quondam value provided, this way the map is non concurrently modified during its call.

If the existing value is changed in addition to non matches amongst the quondam value in addition to so supersede neglect amongst returning false. You tin piece of work telephone telephone the replace() method inwards piece loop until y'all succeed, every bit 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 likewise see Java SE 8 for Really Impatient for some code examples of atomically updating an existing value inwards ConcurerntHashMap.

8. How gain y'all withdraw a mapping piece iterating over ConcurrentHashMap? (answer)
You tin piece of work an Iterator to withdraw a mapping from ConcurrentHashMap inwards Java every bit shown below:

Map<String, Integer> bookAndPrice = novel ConcurrentHashMap<>(); bookAndPrice.put("Effective Java", 42); bookAndPrice.put("Head First Java", 29); bookAndPrice.put("Java Concurrency inwards 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 inwards 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 way it volition non throw a ConcurrentModificationException, thus, eliminating the demand to lock the map during iteration.

The Iterator returned past times ConcurrentHashMap are likewise weakly consistent which way 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 occur if y'all add together a novel mapping inwards ConcurrentHashMap piece i thread is iterating over it? (answer)
This is i of the tricky questions related to ConcurrentHashMap. Since iterator's of ConcurrentHashMap are weekly consistent in addition to fail-safe they volition non neglect amongst ConcurrentModificationException but it's likewise possible that they won't come across whatever modification in i lawsuit iteration started. Even though it's implementation dependent, JDK by in addition to large creates a separate re-create of ConcurrentHashMap for iteration, instead of iterating over master copy.


11. Can y'all move past times 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, y'all tin shop an object of ConcurrentHashMap into a Map variable every bit shown below:

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

Though, this means, y'all may non guide maintain access to methods declared inwards the java.util.concurrent.ConcurrentHashMap degree e.g. forEachKey() or forEachValue() method added inwards Java 8.


That's all almost some of the frequently asked questions almost ConcurrentHashMap on Java interviews. These questions volition non solely assistance y'all to gain good on your project interview but likewise encourage y'all to larn to a greater extent than almost ConcurrentHashMap.

H5N1 skillful in addition to venture noesis of ConcurrentHashMap is expected from both junior in addition to senior Java developer given its importance in addition to usability inwards every Java application.

At to the lowest degree y'all should endure comfortable amongst day-to-day operations amongst ConcurrentHashMap in addition to empathise how the internal implementation works, particularly when compared to other thread-safe map implementations e.g. Hashtable in addition to Synchronized HashMap.


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Thanks for reading this article so far. If y'all similar these interview questions in addition to honor my explanations useful in addition to so delight portion amongst your friends in addition to colleagues. If y'all guide maintain whatever query or dubiousness in addition to so delight drib a comment.

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