So, the javadoc of ConcurrentHashMap at the class level and at the method level work together: put () is described inherently as an atomic operation, whilst putIfAbsent 's description is not atomic at all. It's explicitly described as a two-step process; it's even in the name! put, if absent - it's a two-step name.Frame Alert. This document is designed to be viewed using the frames feature. If you see this message, you are using a non-frame-capable web client. Link to Non-frame version. Creating Thread Safe ConcurrentHashSet is not possible before JDK 8 because of the java.util.concurrent package does not have a class called ConcurrentHashSet, but starting with JDK 8, the newly added keySet (the default) and newKeySet() methods to create a ConcurrentHashSet in Java that is supported by …Sep 8, 2009 · (See Java theory and practice: Concurrent collections classes). For example: synchronized(map) { for (Object obj: map) { // Do work here } } Using . new ConcurrentHashMap(); is probably a better choice as you won't need to lock the collection to iterate over it. Finally, you might want to consider a more functional programming approach. That is ... static ConcurrentHashMap map = new ConcurrentHashMap(32, 1); Where am I going wrong? My expectation? Or is there some bug in my code (the real software, not the above) that is causing this? I've gone over it repeatedly and am 99% sure I'm handling the locking correctly. I cannot even fathom a bug in ConcurrentHashMap or the JVM.ConcurrentHashMap : when u want multithreaded index based get/put, only index based operations are supported. Get/Put are of O(1) ConcurrentSkipListMap : More operations than just get/put, like sorted top/bottom n items by key, get last entry, fetch/traverse whole map sorted by key etc. Complexity is of O(log(n)), So put performance is not as great as …Hashtable is belongs to the Collection framework; ConcurrentHashMap belongs to the Executor framework. Hashtable uses single lock for whole data. ConcurrentHashMap uses multiple locks on segment level (16 by default) instead of object level i.e. whole Map. ConcurrentHashMap locking is applied only for updates. Jul 20, 2022 · Creating Thread Safe ConcurrentHashSet is not possible before JDK 8 because of the java.util.concurrent package does not have a class called ConcurrentHashSet, but starting with JDK 8, the newly added keySet (the default) and newKeySet() methods to create a ConcurrentHashSet in Java that is supported by ConcurrentHashMap. Aug 3, 2022 · Java 1.5 introduced Concurrent classes in the java.util.concurrent package to overcome this scenario. ConcurrentHashMap is the Map implementation that allows us to modify the Map while iteration. The ConcurrentHashMap operations are thread-safe. ConcurrentHashMap doesn’t allow null for keys and values. Add a comment. 3. The standard hashmap provides no concurrency protection whereas the concurrent hashmap does. Before it was available, you could wrap the hashmap to get thread safe access but this was coarse grain locking and meant all concurrent access got serialised which could really impact performance. Java is one of the most popular programming languages in the world, and a career in Java development can be both lucrative and rewarding. However, taking a Java developer course on...The java.util.ConcurrentHashMap is one of the most important classes of JDK. It was introduced in JDK 1.5 along with other concurrent collection classes like CopyOnWriteArrayList and BlockingQueue. Ever since then, it has been a workhorse in concurrent Java applications.Aug 9, 2011 · There's no built in type for ConcurrentHashSet because you can always derive a set from a map. Since there are many types of maps, you use a method to produce a set from a given map (or map class). Prior to Java 8, you produce a concurrent hash set backed by a concurrent hash map, by using Collections.newSetFromMap (map) In Java 8 (pointed out ... HashMap is a powerful data structure in Java used to store the key-pair values. It maps a value by its associated key. It allows us to store the null values and null keys. It is a non-synchronized class of Java collection. Whereas, ConcurrentHashMap is introduced as an alternative to the HashMap. The ConcurrentHashMap is a synchronized ...public class ConcurrentHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>, Cloneable, Serializable. A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to …A LoadingCache is more powerful than a HashMap and provide more functionality. – luk2302. Jan 13, 2022 at 13:18. LoadingCache offers a lot more functionality that you usually want for a cache that a simple HashMap does not: You can specify a maximum number of entries, you can specify an expire time for entries after which they …Improve this answer. Follow. answered May 28, 2016 at 1:21. assylias. 324k 83 663 786. Add a comment. -1. ConcurrentHashMap class provides a constructor to create a ConcurrentHashMap from a given Map. Map<String, String> myMap = getHashMap (); TestClass.getSingleton () .setConcurrentHashMap (new …Aug 7, 2022 · Recommended Initialization. Based on the above analysis, I will suggest to use the constructor parameters wisely to reduce the number of unnecessary objects and improve the map’s performance. A good approach can be having initialization like this: ConcurrentHashMap<String, Integer> instance = new ConcurrentHashMap<String, Integer> (16, 0.9f, 1); Java : ConcurrentHashMap (並列処理用マップ) - API使用例 . 更新日：2023年06月17日 公開日：2023年06月05日. English. Tweet. ConcurrentHashMap (Java SE 20 & JDK 20) の使用例まとめです。 だいたいのメソッドを網羅済みです。 API仕様のおともにどうぞ。 概要. 取得の完全な同時性および予想される高い更新平行性を ...if (null == componentData) { data.putIfAbsent(componentName, new ConcurrentHashMap ());Aug 15, 2023 · Detailed Explanation: Code demonstrates a performance comparison between different types of thread-safe maps: HashTable, synchronizedMap, and ConcurrentHashMap. It measures the time it takes to perform a set of operations (addition and retrieval of entries) on each of these map implementations using multiple threads. Jan 8, 2024 · 4.2. Performance. While both Hashtable and ConcurrentHashMap guarantee thread safety, they differ in performance due to their underlying synchronization mechanisms. Hashtable locks the entire table during a write operation, thereby preventing other reads or writes. This could be a bottleneck in a high-concurrency environment. The ConcurrentHashMap is very similar to the java.util.HashTable class, except that ConcurrentHashMap offers better concurrency than HashTable or synchronizedMap does. ConcurrentHashMap does not lock the Map while you are reading from it. Additionally,ConcurrentHashMap does not lock the entire Mapwhen writing to it. import java.util.concurrent.ConcurrentHashMap; class Main { public static void main(String[] args) { ConcurrentHashMap<String, Integer> numbers = new ConcurrentHashMap<>(); numbers.put("One", 1); …In this tutorial, we’ll see what are the possibilities to create thread-safe HashSet instances and what is the equivalent of the ConcurrentHashMap for HashSet. …Jul 24, 2022 · ConcurrentHashMap is a Java class that enables multiple threads to access the map at the same point of time. It is a part of the Java Collections Framework, and it provides a way for applications to efficiently handle concurrent updates to the map. The ConcurrentHashMap class provides several methods for doing basic operations on the map, such ... Aug 13, 2019 · 2. Java ConcurrentHashMap Example. The following example demonstrates how ConcurrentHashMap is used in a multi-threaded context. The program creates two writer threads and 5 reader threads working on a shared instance of a ConcurrentHashMap. The writer thread randomly modifies the map (put and remove). ConcurrentHashMap was introduced in JDK 1.5 as an enhancement of HashMap that supports high concurrency for retrievals as well as updates. HashMap isn’t …ConcurrentHashMap Iterators. In contrast to Hashtable, which uses “fail-fast” iterators, ConcurrentHashMap employs “weakly consistent” iterators. These iterators …Aug 15, 2018 ... 在多线程环境下，使用HashMap进行put操作时存在丢失数据的情况，为了避免这种bug的隐患，强烈建议使用ConcurrentHashMap代替HashMap。import java.util.concurrent.ConcurrentHashMap; class Main { public static void main(String[] args) { ConcurrentHashMap<String, Integer> numbers = new ConcurrentHashMap<>(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // remove method with single parameter int value ... ConcurrentHashMap uses multiple buckets to store data. This avoids read locks and greatly improves performance over a HashTable.Both are thread safe, but there are obvious performance wins with ConcurrentHashMap.. When you read from a ConcurrentHashMap using get(), there are no locks, contrary to the HashTable for which all operations are …static ConcurrentHashMap map = new ConcurrentHashMap(32, 1); Where am I going wrong? My expectation? Or is there some bug in my code (the real software, not the above) that is causing this? I've gone over it repeatedly and am 99% sure I'm handling the locking correctly. I cannot even fathom a bug in ConcurrentHashMap or the JVM.Java ConcurrentHashMap merge() Method . The merge() method of ConcurrentHashMap class merge sets If the specified key is not already associated with a (non-null) value, associates it with the given value.ConcurrentHashMap uses multiple buckets to store data. This avoids read locks and greatly improves performance over a HashTable.Both are thread safe, but there are obvious performance wins with ConcurrentHashMap.. When you read from a ConcurrentHashMap using get(), there are no locks, contrary to the HashTable for which all operations are …Are you a beginner in Java programming and looking for ways to level up your skills? One of the best ways to enhance your understanding of Java concepts is by working on real-world...Yes; per the Javadoc for ConcurrentHashMap.computeIfAbsent (...): The entire method invocation is performed atomically, so the function is applied at most once per key. That's really the whole point of the method. However, to be clear, the lock is not completely specific to that one key; rather, ConcurrentHashMap typically works by …When a task is launched, it is stored in a cached based on ConcurrentHashMap, When a client ask for a task, if the task in the cache is done, a new one is launched and cached; if it is running, the task is retrieved from the cache and passed to the client. Here is a excerpt of the code: private static volatile TaskLauncher instance;However most often you get more benign effects. The below code should put 2000 entries in each map from multiple threads. But for the HashMap, there will be consistently fewer entries than 2000 in the map after the operation, since some of the puts will clash with each other and their result will be lost. public class BreakingMap { public ...2. Thread Safe HashSet Using ConcurrentHashMap Factory Method. Firstly we’ll look at the ConcurrentHashMap class that exposed the static newKeySet () method. Basically, this method returns an instance that respects the java.util.Set interface and allows the usage of standard methods like add (), contains (), etc. This can be created simply as:ConcurrentHashMap (at least in Sun's current implementation) works by dividing the underlying map into a number of separate buckets. Getting an element does not require any locking per se, but it does use atomic/volatile operations, which implies a memory barrier (potentially very costly, and interfering with other possible optimisations).Jul 24, 2022 · ConcurrentHashMap is a Java class that enables multiple threads to access the map at the same point of time. It is a part of the Java Collections Framework, and it provides a way for applications to efficiently handle concurrent updates to the map. The ConcurrentHashMap class provides several methods for doing basic operations on the map, such ... A ConcurrentHashMap is one implementation of the ConcurrentMap interface, and it’s one of the thread-safe collections that Java provides. It is backed by a …A ConcurrentHashMap can be used as a scalable frequency map (a form of histogram or multiset) by using LongAdder values and initializing via computeIfAbsent. For example, to add a count to a ConcurrentHashMap<String,LongAdder> freqs, you can use freqs.computeIfAbsent (key, k -> new LongAdder ()).increment (); if (null == componentData) { data.putIfAbsent(componentName, new ConcurrentHashMap ());A ConcurrentHashMap can be used as a scalable frequency map (a form of histogram or multiset) by using LongAdder values and initializing via computeIfAbsent. For example, to add a count to a ConcurrentHashMap<String,LongAdder> freqs, you can use freqs.computeIfAbsent (key, k -> new LongAdder ()).increment (); Aug 9, 2011 · There's no built in type for ConcurrentHashSet because you can always derive a set from a map. Since there are many types of maps, you use a method to produce a set from a given map (or map class). Prior to Java 8, you produce a concurrent hash set backed by a concurrent hash map, by using Collections.newSetFromMap (map) In Java 8 (pointed out ... A ConcurrentHashMap has no order. There is no "first" and no "last". There is no "first" and no "last". 2) A ConcurrentHashMap provides an entrySet which is "weakly consistent", so that if the content of the Map changes during iteration you might see the changes, you might not:If you’re interested in mastering Java web development, choosing the right course is crucial. With so many options available, it can be overwhelming to determine which one suits yo...Learn how to use the Java ConcurrentHashMap class, a thread-safe map that supports full concurrency of retrievals and high expected concurrency for updates. See the …But in ConcurrentHashMap, the locking happens at bucket level so the chance of giving concurrent modification exception is not present. So to answer you question here, checking size of ConcurrentHashMap doesn't help because , it keeps chaining based on the operations or modification code that you write on the map.See full list on geeksforgeeks.org ConcurrentHashMap可以做到读取数据不加锁，并且其内部的结构可以让其在进行写操作的时候能够将锁的粒度保持地尽量地小，允许多个修改操作并发进行，其关键在于使用了 锁分段技术 。它使用了多个锁来控制对hash表的不同部分进行的修改。对于JDK1.7版本的实现 ... Learn to improve the performance of ConcurrentHashMap by optimizing the constructor arguments in a sensible manner.. 1. ConcurrentHashMap class. The …Apr 4, 2023 · ConcurrentHashMap and ConcurrentSkipListMap are powerful, thread-safe map implementations in Java. Choosing the right concurrent map depends on your application’s specific requirements. May 31, 2017 ... 不同的是，Java 8的ConcurrentHashMap作者认为引入红黑树后，即使哈希冲突比较严重，寻址效率也足够高，所以作者并未在哈希值的计算上做过多设计，只是将 ...I've tried all of the returned Enumerations and Sets from the documentation, but I can't make it work because I'm still too new to Java to figure it out. Please show me how. Please show me how. (I know what I'm doing is bad practice as many great stackers have shown me here and here , but all I'm trying to do is quickly get a working prototype …You cannot retrieve the original key from a HashMap, without just doing a brute-force sequential search like this: K keyToLookup = ...; K originalKey = map.keySet().stream().filter(keyToLookup::equals).findAny().orElse(null); Option 1: Embed key in value. Workaround for a HashMap is of course to have the key object as part of …In this tutorial, we will learn everything about the Java ConcurrentHashMap class with examples. We will also see the real-world example of ConcurrentHashMap.. The ConcurrentHashMap in Java is a part of the Java Collection Framework which provides a concurrent version of the standard HashMap. ConcurrentHashMap可以做到读取数据不加锁，并且其内部的结构可以让其在进行写操作的时候能够将锁的粒度保持地尽量地小，允许多个修改操作并发进行，其关键在于使用了 锁分段技术 。它使用了多个锁来控制对hash表的不同部分进行的修改。对于JDK1.7版本的实现 ... ConcurrentHashMap<Integer, Integer> map = new ConcurrentHashMap(1); for(int i=0; i<10; i++){. map.put(i,i); } //print map. I set the initial capacity of ConcurrentHashMap to 1, and after that I put 10 values into it. So in my opinion, the ConcurrentHashMap will only receive the first and deny the rest of 9; however, …You should be using the putIfAbsent methods of ConcurrentMap. ConcurrentMap<String, AtomicLong> map = new ConcurrentHashMap<String, AtomicLong> (); public long addTo (String key, long value) { // The final value it became. long result = value; // Make a new one to put in the map. AtomicLong newValue = new …Jul 10, 2020 ... The Concurrent hash maps · SynchronizedMap A thread-safe hash map from the JDK. It can be created by calling Collections.synchronizedMap on java.ConcurrentHashMap (int initialCapacity, float loadFactor, int concurrencyLevel) Creates a new, empty map with an initial table size based on the given number of elements ( …Aug 15, 2018 ... 在多线程环境下，使用HashMap进行put操作时存在丢失数据的情况，为了避免这种bug的隐患，强烈建议使用ConcurrentHashMap代替HashMap。Within a HashMap, both keys and values can be set to null without issue. However, in a ConcurrentHashMap, assigning null values to either keys or values will ...How do you make a HashMap concurrent? How to Synchronize HashMap in Java. Synchronize HashMap – ConcurrentHashMap. Our first choice should always be using the ...No, ConcurrentHashMap does not make a fresh copy of underlying data structure. ConcurrentHashMap is a segmented map, number of segments are based on concurrency level. And when you write to a segment, then it gets locked until write is complete. It uses lock stripping for finer grained access and does not copy the map.... java/util/concurrent/ConcurrentHashMap.html. Page 5. 5. • Provides efficient concurrent operations on key/value pairs via OO & functional programming APIs. • A ...So while ConcurrentHashMap.newKeySet() looks like doing the same as Collections.newSetFromMap(new ConcurrentHashMap<>()), there is the semantic difference that the latter says you shouldn’t use the map afterwards while the former is part of a feature which is designed to interact with the map.. See …Yes, ConcurrentHashMap uses a multitude of locks (by default, 16 of them), each lock controls one segment of the hash. When setting data in a particular segment, the lock for that segment is obtained. When getting data, a volatile read is used. If the volatile read results in a miss, then the lock for the segment is obtained for a last attempt ...Jan 5, 2021 · Iterators produced by a ConcurrentHashMap are weakly consistent. That is: they are guaranteed to traverse elements as they existed upon construction exactly once, and may (but are not guaranteed to) reflect any modifications subsequent to construction. The last bullet-point is pretty important, an iterator returns a view of the map at some ... ConcurrentHashMap is divided into different segments based on concurrency level. So different threads can access different segments concurrently in java. Can threads read the segment of ConcurrentHashMap locked …Concrete class in Java is the default class and is a derived class that provides the basic implementations for all of the methods that are not already implemented in the base class...<Java 8+> Although original question was raised during time of Java 7, it is worth to note behavioral change after Java 8. Starting from Java 8+, implementation of ConcurrentHashMap changed drastically.. The major change in implementation of ConcurrentHashMap was, instead of dividing into segments, we are using first entry of …Because the elements of a ConcurrentHashMap are not ordered in any particular way, and may be processed in different orders in different parallel executions, the correctness of supplied functions should not depend on any ordering, or on any other objects or values that may transiently change while computation is in progress; and except for forEach actions, …Are you interested in learning Java programming but worried about the cost of courses? Look no further. In this full course guide, we will explore various free resources that can h...ConcurrentHashMap is a thread-safe alternative of HashMap class. The synchronizedMap() is a method in Collections class and was introduced in Java version 1.2. It returns a synchronized Map backed by the specified Map that we provide in the parameter. // synchronizedMap () method public static Map<K, V> synchronizedMap (Map<K, V> map) ConcurrentHashMap and ConcurrentSkipListMap are powerful, thread-safe map implementations in Java. Choosing the right concurrent map depends on your application’s specific requirements.Software that uses Java coding is considered a binary, or executable, file that runs off of the Java platform. The SE portion stands for Standard Edition, which is commonly install...ConcurrentHashMap. 1. Lock mechanism -Locks the portion, Concurrent hashmap allows concurrent read and write. So performance is relatively better than a synchronized map. 2. Null key or Value - It doesn't allow null as a key or value. If you use it will throw java.lang. NullPointerException at Runtime. 3.DeadLock In ConcurrentHashMap. Following code when run, never terminates and stuck in endless loop. I am not sure where it is getting stuck. Interesting thing is when I change AaAa to AAAA every thing works fine as expected. public class Test { public static void main (String [] args) { Map<String, Integer> map = new …Java ConcurrentHashMap class is part of the Concurrency Collection Classes. It’s a hash table implementation, which supports concurrent retrieval and …Mar 9, 2017 · ConcurrentHashMap has been a very popular data structure these days. In this talk, we will walk through the new features that got introduced in Java 8. Java 8 introduced the forEach, search, and ... Transform ConcurrentHashMap into a sorted list in Java. 1. Sorting a LinkedHashMap in Java. 2. Sorting LinkedHashMap by key java. 6. Sorting Keys with same Values -->LinkedHashMap. 1. Problem sorting ConcurrentHashMap by values using java.util.Collections.sort() in Java. Hot Network QuestionsInterview Questions. In “ How HashMap works in Java “, we learned the internals of HashMap or ConcurrentHashMap class and how they fit into the whole concept. But when the interviewer asks you about HashMap related concepts, he does not stop only on the core concept. The discussion usually goes in multiple directions to understand whether ...
Improve this answer. Follow. answered May 28, 2016 at 1:21. assylias. 324k 83 663 786. Add a comment. -1. ConcurrentHashMap class provides a constructor to create a ConcurrentHashMap from a given Map. Map<String, String> myMap = getHashMap (); TestClass.getSingleton () .setConcurrentHashMap (new …. Bah stock price
A Map providing thread safety and atomicity guarantees.. Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread. This interface is a …Jul 20, 2022 · Creating Thread Safe ConcurrentHashSet is not possible before JDK 8 because of the java.util.concurrent package does not have a class called ConcurrentHashSet, but starting with JDK 8, the newly added keySet (the default) and newKeySet() methods to create a ConcurrentHashSet in Java that is supported by ConcurrentHashMap. The hashcode () method of ConcurrentHashMap class Returns the hash code value for this Map, i.e., the sum of, for each key-value pair in the map, key.hashCode () ^ value.hashCode (). 14. public Enumeration<K> keys () The keys () method of ConcurrentHashMap class returns an enumeration of the keys in this table. 15. Aug 12, 2022 · The java.util.Concurrent.ConcurrentHashMap class achieves thread-safety by dividing the map into segments, the lock is required not for the entire object but for one segment, i.e one thread requires a lock of one segment. In ConcurrentHashap the read operation doesn’t require any lock. Java is one of the most popular programming languages in the world, and a career in Java development can be both lucrative and rewarding. However, taking a Java developer course on...Java ConcurrentHashMap compute ()方法及示例. ConcurrentHashMap类 的 compute (Key, BiFunction) 方法用于计算指定键和其当前映射值的映射（如果没有找到当前映射，则为空）。. 该方法用于原子化地更新ConcurrentHashMap中给定键的值。. 如果重映射函数抛出一个异常，该异常将被 ... Dec 4, 2015 · static ConcurrentHashMap map = new ConcurrentHashMap(32, 1); Where am I going wrong? My expectation? Or is there some bug in my code (the real software, not the above) that is causing this? I've gone over it repeatedly and am 99% sure I'm handling the locking correctly. I cannot even fathom a bug in ConcurrentHashMap or the JVM. ConcurrentHashMap : when u want multithreaded index based get/put, only index based operations are supported. Get/Put are of O(1) ConcurrentSkipListMap : More operations than just get/put, like sorted top/bottom n items by key, get last entry, fetch/traverse whole map sorted by key etc. Complexity is of O(log(n)), So put performance is not as great as …In this tutorial, we will learn everything about the Java ConcurrentHashMap class with examples. We will also see the real-world example of ConcurrentHashMap.. The ConcurrentHashMap in Java is a part of the Java Collection Framework which provides a concurrent version of the standard HashMap. In Java 8 you can use computeIfAbsent. public void put (String string) { // all most thread safe. map.computeIfAbsent (string, k -> new ArrayList<> ()) .add ("add something"); } Note this is still not thread safe as the ArrayList is not thread safe, so what you need is. public void put (String string) { List<String> list = map.computeIfAbsent ...The ConcurrentHashMap is a clever implementation that divides the Map into regions and locks them individually so that you can have concurrent, thread safe, reads and writes of the map without external locking.Jul 24, 2022 · ConcurrentHashMap is a Java class that enables multiple threads to access the map at the same point of time. It is a part of the Java Collections Framework, and it provides a way for applications to efficiently handle concurrent updates to the map. The ConcurrentHashMap class provides several methods for doing basic operations on the map, such ... Jun 8, 2023 ... import java.util.List;. import java.util.Map;. import java.util.concurrent.ConcurrentHashMap;. import java.util.concurrent.Executors;. public ...The contains () method of Java.util.ConcurrentHashMap is used to check whether if some key maps into the specified value in this table. It is a legacy method of performing a particular task. The operation is similar to containsValue () Method of ConcurrentHashMap. Syntax: Parameters: The method takes just one parameter Value …Jul 17, 2018 · A ConcurrentHashMap's computeIfPresent method is a possibility. From the javadocs: If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value. Feb 20, 2023 · The remove () method works in a concurrent environment, which means that multiple threads can access and modify the ConcurrentHashMap instance at the same time, without any data race or synchronization issues. When a remove () operation is performed, the method first acquires the lock on the segment of the map corresponding to the given key. import java.util.concurrent.ConcurrentHashMap; class Main { public static void main(String[] args) { ConcurrentHashMap<String, Integer> numbers = new ConcurrentHashMap<>(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // remove method with single parameter int value ... It doesn't make sense to have a Map<String, new SomeType ()>. You need to simply write the type of the second parameter. To paraphrase, you're creating a single new ConcurrentHashMap<K, V> (), which can hold multiple inner maps later. By the way, Java 7 has a more concise syntax now (the "diamond"):The "Concurrent" prefix used with some classes in this package is a shorthand indicating several differences from similar "synchronized" classes. For example java.util.Hashtable and Collections.synchronizedMap (new HashMap ()) are synchronized. But ConcurrentHashMap is "concurrent". A concurrent collection is thread-safe, but not …import java.util.concurrent.ConcurrentHashMap; class Main { public static void main(String[] args) { ConcurrentHashMap<String, Integer> numbers = new ConcurrentHashMap<>(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // remove method with single parameter int value ... .