![]() Because of this factor, hashtable has significant performance problems. Like all “early” collections (from Java 1.0), a hashtable is synchronized (almost all methods are marked as synchronized). This collection was created earlier than the Java Collection Framework, but was later included in it. Hashtable in Java Hashtable class is the implementation of a hash table data structure. The result of applying Hash Function to an Object calls hashCode. The good hash function minimizes probability of collisions. Different objects may have the same hash code.If two hash codes are different, the objects are definitely not equal.Two equal objects have the same hash codes.A particular object has the particular hash code.Hash functions could be different, but they all submit certain properties: Usually that kind of function converts a big piece of data into a small integer value. What is hashing? It is a rule that maps the Object into a set of characters (code). Hash Table consists of an array to keep data and hashing for generation an index where an element should be located. So, insertion and search operations are fast independently on the data size. If the key is known, access to the needed data is very fast. Hashtable as a data structure Hashtable is a data structure where data is stored in an array format. The most significant difference between them: Hashtable is synchronized while HashMap is not. ![]() In Java hashtable internally contains buckets where the key/value pairs are stored. It is an implementation of mathematical hash table data structure. HashMap.put(someObject, someValue) //it will throw the ConcurrentModificationExceptionįor HashTable Enumeration: Enumeration keys = hashTable.keys() įor (Enumeration e = v.elements() e.hasMoreElements() e.Java Hashtable class is one of the oldest members of Java Collection Framework. ![]() Syncronized →Being synchronized means that operation is thread safe, so only one thread can access the table at a time.įail safe – Fail-fast means when you try to modify the content when you are iterating, it will throw ConcurrentModification Exception and fail immediately.įor HashMap Itration: Set keys = hashMap.keySet() Note – You can synchonized a HashMap with the help of Collections.synchonizedMap(hashmap) Map map=Collections.synchonizedMap(hashmap) Structurally modification means deleting or inserting element which could effectively change the structure of map.Īllows one null key and any number of null valuesĮnumeration in Hashtable is not fail fast.However, if prior to calling “set”, the collection has been modified structurally, “ IllegalArgumentException” will be thrown. It is possible for other threads though to invoke “set” method since it doesn’t modify the collection “structurally”. If an iterator has been created on a collection object and some other thread tries to modify the collection object “structurally”, a concurrent modification exception will be thrown. Fail-safe is relevant from the context of iterators. ![]() Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released. Synchronized means only one thread can modify a hash table at one point of time.But this is not a guaranteed behavior and will be done by JVM on best effort. Iterator in the Hashtable is fail-safe because enumerator for the Hashtable is not throw ConcurrentModificationException if any other Thread modifies the map structurally by adding or removing any element except Iterator‘s own remove() method.HashMap is non synchronized whereas Hashtable is synchronized.HashMap does not guarantee that the order of the map will remain constant over time.( HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls). The HashMap class is roughly equivalent to Hashtable, except that it is non synchronized and permits nulls.
0 Comments
Leave a Reply. |