What is difference between synchronized HashMap and a Hashtable?

Hashtable vs SynchronizedHashMap

Hashtable doesn't allow even a single null key and null values. Synchronized HashMap allows one null key and any number of null values.

What is difference between Hashtable and HashMap and ConcurrentHashMap?

1) Hashtable is belongs to the Collection framework; ConcurrentHashMap belongs to the Executor framework. 2) 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. 3) ConcurrentHashMap locking is applied only for updates.

What does synchronized mean in HashMap?

Synchronization means controlling the access of multiple threads to any shared resource. A synchronized resource can be accessed by only one thread at a time. HashMap can be synchronized using the Collections. synchronizedMap() method.

What is synchronization in Hashtable?

Hashtable is synchronized. It ensures that no more than one thread can access the Hashtable at a given moment of time. The thread which works on Hashtable acquires a lock on it to make the other threads wait till its work gets completed. 2) HashMap allows one null key and any number of null values.

Why HashMap is faster than Hashtable?

HashMap is not synchronized, therefore it's faster and uses less memory than Hashtable. Generally, unsynchronized objects are faster than synchronized ones in a single threaded application.

What is difference between synchronized HashMap and a Hashtable?||Concurrency Interview Question

What is the difference between HashMap Hashtable and HashSet?

Hashtable and HashMap both implement Map , HashSet implements Set , and they all use hash codes for keys/objects contained in the sets to improve performance. Hashtable is a legacy class that almost always should be avoided in favor of HashMap .

How do you make a Hashtable synchronized?

The operations on hashtable may not be synchronized. for example, if Thread t1 access the hastable and check for key and at the same time Thread t2 checks for key, before t1 executes put. now two threads are inside the if block and overwriting of key-value happens. so synchronized block is necessary.

Is HashSet synchronized?

1) Both HashMap and HashSet are not synchronized which means they are not suitable for thread-safe operations unitl unless synchronized explicitly. This is how you can synchronize them explicitly: HashSet: Set s = Collections.

Why HashMap is non-synchronized?

HashMap is non-synchronized. It is not thread-safe and can't be shared between many threads without proper synchronization code whereas Hashtable is synchronized. It is thread-safe and can be shared with many threads.

Can HashMap have null key?

Hashtable does not allow null keys or values, while HashMap allows null values and 1 null key.

Is TreeMap synchronized?

The implementation of a TreeMap is not synchronized. This means that if multiple threads access a tree set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by using the Collections.

Can we replace Hashtable with ConcurrentHashMap?

Is it safe to replace the Hashtable instances with ConcurrentHashmap instances for performance gain? In most cases it should be safe and yield better performance. The effort on changing depends on whether you used the Map interface or Hashtable directly.

Does ConcurrentHashMap allow null values?

The main reason that nulls aren't allowed in ConcurrentMaps (ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that may be just barely tolerable in non-concurrent maps can't be accommodated. The main one is that if map.

What is the difference between collections synchronizedMap M and ConcurrentHashMap?

synchronizedMap() requires each thread to acquire a lock on the entire object for both read/write operations. By comparison, the ConcurrentHashMap allows threads to acquire locks on separate segments of the collection, and make modifications at the same time.

Does synchronized HashMap throws ConcurrentModificationException?

A retrieval operation will return the value inserted by the most recent completed insert operation. A lock is required for read operation too in SynchronizedHashMap. ConcurrentHashMap doesn't throw a ConcurrentModificationException if one thread tries to modify it while another is iterating over it.

What is the difference between synchronization and concurrency?

The word synchronization generally means sharing data between multiple processors or threads, while concurrency refers to a measure of– or the art of improving– how effectively an application allows multiple jobs required by that application (e.g. serving web page requests from a web server) to run simultaneously.

Can ConcurrentHashMap throws ConcurrentModificationException?

ConcurrentHashMap does not throw ConcurrentModificationException if the underlying collection is modified during an iteration is in progress. Iterators may not reflect the exact state of the collection if it is being modified concurrently. It may reflect the state when it was created and at some moment later.

Can HashMap have duplicate keys?

HashMap stores key, value pairs and it does not allow duplicate keys. If the key is duplicate then the old key is replaced with the new value.

What does synchronized mean in Java?

Synchronization in java is the capability to control the access of multiple threads to any shared resource. In the Multithreading concept, multiple threads try to access the shared resources at a time to produce inconsistent results. The synchronization is necessary for reliable communication between threads.

What is the difference between HashMap and LinkedHashMap?

The key difference between HashMap and LinkedHashMap is order. Elements of a HashMap are not in order, totally random, whereas elements of LinkedHashMap are ordered. The entries of a LinkedHashMap are in key insertion order, which is the order in which the keys are inserted in the Map.

What is the difference between HashMap and Hashtable class?

One of the major differences between HashMap and Hashtable is that HashMap is non-synchronized whereas Hashtable is synchronized, which means Hashtable is thread-safe and can be shared between multiple threads but HashMap can not be shared between multiple threads without proper synchronization.

Does Hashtable allow duplicate keys?

Hashtable Features

It does not accept duplicate keys. It stores key-value pairs in hash table data structure which internally maintains an array of list.

What is difference between list and set?

List is an ordered sequence of elements whereas Set is a distinct list of elements which is unordered. List <E>: An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted.