Top X Coffee Concurrenthashmap Interview Questions As Well As Answers

The ConcurrentHashMap degree component of concurrent collections added on JDK 1.5 e.g. 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 yous to shop cardinal value pairs. The degree is similar to HashMap or Hashtable simply it's to a greater extent than scalable in addition to the correct fit for concurrent Java application. Unlike Hashtable which achieves its thread-safety yesteryear compromising 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 equally good equally thread-safety, it is the most pop selection of Map inwards concurrent Java applications. In general, replacing synchronized collections alongside a concurrent collection tin dramatically improve the scalability of your Java application alongside picayune risk, equally advised on Java Concurrency inwards Practice.

Java ConcurrentHashMap Interview Questions alongside Answers

Here are roughly of the best in addition to oftentimes asked Java ConcurrentHashMap interview questions. These questions are collected from the existent interview, hence, don't hold out surprised if yous receive got already seen them during interviews. These questions volition non solely aid yous to produce good on interviews simply too encourage yous to larn to a greater extent than virtually the concurrent hash map, which volition eventually aid yous inwards your 24-hour interval to 24-hour interval programming job.

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


is ConcurrentHashMap thread-safe inwards Java? (answer)
Yes, ConcurrentHashMap is thread-safe inwards Java, which agency ii thread tin modify the map without damaging its internal information construction e.g. array in addition to linked list. If yous compare this to HashMap, which is non thread-safe, exposing HashMap to multiple threads may impairment internal information construction in addition to may homecoming the map completely useless, where many links may decease missing or pointing to incorrect elements.


How does ConcurrentHashMap orbit thread-safety? (answer)
The java.util.ConcurrentHashMap achieves thread-safety yesteryear 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 simply it performs improve because it never locks the whole map. This technique is too known equally lock stripping.


Can multiple threads read from ConcurrentHashMap same time? (answer)
Yes, ConcurrentHashMap allows concurrent read without locking equally reading functioning doesn't require locking or thread-safety.

Can ane thread read in addition to other writes on ConcurrentHashMap at the same time? (answer)
Yes, it's possible for a pocket-size position out of the writer. For example, if a write functioning is modifying ane segment of ConcurrentHashmap in addition to read functioning is happening on other segments in addition to so a reader volition non block, simply if reader thread is too trying to read from the same segment than it volition block until the author is done.


How does ConcurrentHashMap operate internally? (answer)
The java.util.ConcurrentHashMap industrial plant similar to HashMap when it comes to storing key/value pairs in addition to retrieving values. The solely departure inwards its implementation comes from concurrency perspective in addition to how it achievers thread-safety. It divides the map into several segments, yesteryear default 16, too known equally synchronization level.

Because of this, concurrent get(), put(), contains() functioning is possible because it never locks the whole map simply solely relevant segment is locked. Which agency readers tin access the map concurrency alongside writers in addition to a express position out of writers tin modify the map concurrently. The resultant is improve throughput in addition to Scalability. See Java Concurrency inwards Practice for details on the implementation of ConcurrentHashMap class.

Here is a diagram which explains how a segment looks similar inwards a ConcurrentHashMap of Java, basically it's cipher simply a mini hash tabular array alongside a bucket in addition to linked listing of hash entries inwards 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 advise yous reading Java Concurrency inwards Practice to larn to a greater extent than virtually how concurrency is handled yesteryear ConcurrentHashMap inwards Java.


How produce yous atomically update a value inwards ConcurrentHashMap? (answer)
If yous desire to atomically update an existing value inwards ConcurrentHashMap, yous tin purpose the replace() component of concurrentHashMap. It accepts both onetime value in addition to novel value in addition to solely updates the map if the existing value inwards the map matches alongside the onetime value provided, this agency the map is non concurrently modified during its call. If the existing value is changed in addition to non matches alongside the onetime value in addition to so supercede neglect alongside returning false. You tin purpose telephone vociferation upwards the replace() method inwards spell loop until yous 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 too see Java SE 8 for Really Impatient for roughly code examples of atomically updating an existing value inwards ConcurerntHashMap.

How produce yous take a mapping spell iterating over ConcurrentHashMap? (answer)
You tin purpose an Iterator to take a mapping from ConcurrentHashMap inwards 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 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}

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 too 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.



What volition come about if yous add together a novel mapping inwards ConcurrentHashMap spell ane thread is iterating over it? (answer)
This is ane of the tricky questions related to ConcurrentHashMap. Since iterator's of ConcurrentHashMap are weekly consistent in addition to fail-safe they volition non neglect alongside ConcurrentModificationException simply it's too possible that they won't come across whatever modification ane time iteration started. Even though it's implementation dependent, JDK to a greater extent than often than non creates a separate re-create of ConcurrentHashMap for iteration, instead of iterating over master copy copy.


Can yous locomote yesteryear 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, so ConcurrentHashMap IS-A Map. Also, yous tin shop an object of ConcurrentHashMap into a Map variable equally shown below:

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

Though, this means, yous may non receive got access to methods declared inwards the java.util.concurrent.ConcurrentHashMap degree e.g. forEachKey() or forEachValue() method added inwards JDK 8.


That's all virtually roughly of the frequently asked questions virtually ConcurrentHashMap on Java interviews. These questions volition non solely aid yous to produce good on your project interview simply too encourage yous to larn to a greater extent than virtually ConcurrentHashMap. Influenza A virus subtype H5N1 expert in addition to corporation cognition 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 yous should hold out comfortable alongside day-to-day operations alongside 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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