What is the difference between Hashtable and ConcurrentHashMap in Java?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.
What is difference between HashMap and ConcurrentHashMap in Java?HashMap is non-Synchronized in nature i.e. HashMap is not Thread-safe whereas ConcurrentHashMap is Thread-safe in nature. HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously.
Why is ConcurrentHashMap faster than Hashtable in Java?Answer: ConcurrentHashMap is introduced in Java 1.5. ConcurrentHashMap uses multiple buckets to store data. This avoids read locks and greatly improves performance over a HashTable.
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.
Which is better Hashtable or ConcurrentHashMap?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 .
#12 - Difference b/w ConcurrentHashMap
Is ConcurrentHashMap slower than HashMap?Only modifying operations on ConcurrentHashMap are synchronized. Hence, add or remove operations on ConcurrentHashMap are slower than on HashMap . The read operations on both, ConcurrentHashMap and HashMap , give same performance as read operations on both maps are not synchronized.
Does Hashtable allow null values?HashMap allows one null key and multiple null values whereas Hashtable doesn't allow any null key or value.
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.
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.
Why do we need ConcurrentHashMap in Java?The advantages of using ConcurrentHashMap are as follows: It provides very high concurrency in a multi-threaded environment. The read operation can be very fast when the write operation is done with a lock. It provides No object-level Locking.
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.
Is ConcurrentHashMap thread safe?ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications. At a time any number of threads are applicable for a read operation without locking the ConcurrentHashMap object which is not there in HashMap.
How many threads access ConcurrentHashMap?A ConcurrentHashMap has internal final class called Segment so we can say that ConcurrentHashMap is internally divided in segments of size 32, so at max 32 threads can work at a time.
What is a Hashtable in Java?A Hashtable is an array of a list. Each list is known as a bucket. The position of the bucket is identified by calling the hashcode() method. A Hashtable contains values based on the key. Java Hashtable class contains unique elements.
Can we make HashMap synchronized?HashMap can be synchronized using the Collections. synchronizedMap() method. The synchronizedMap() method of java.
Why ConcurrentHashMap is fail safe?This is because, they operate on the clone of the collection, not on the original collection and that's why they are called fail-safe iterators. Iterator on CopyOnWriteArrayList, ConcurrentHashMap classes are examples of fail-safe Iterator.
What is difference between iterator and ListIterator?An Iterator is an interface in Java and we can traverse the elements of a list in a forward direction whereas a ListIterator is an interface that extends the Iterator interface and we can traverse the elements in both forward and backward directions.
What is concurrent exception in Java?The ConcurrentModificationException occurs when an object is tried to be modified concurrently when it is not permissible. This exception usually comes when one is working with Java Collection classes. For Example - It is not permissible for a thread to modify a Collection when some other thread is iterating over it.
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.
Does Hashtable allow duplicate values?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 the difference between HashMap and Hashtable class?1) HashMap is non synchronized. It is not-thread safe and can't be shared between many threads without proper synchronization code. Hashtable is synchronized. It is thread-safe and can be shared with many threads.
What is load factor in ConcurrentHashMap?ConcurrentHashMap() Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).
Does ConcurrentHashMap uses object level lock?ConcurrentHashMap was introduced in JDK 5. There is no locking at the object level,The locking is at a much finer granularity.
Is ArrayList thread-safe?Vectors are synchronized. Any method that touches the Vector 's contents is thread safe. ArrayList , on the other hand, is unsynchronized, making them, therefore, not thread safe.
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