- Where is Hashtable used?
- Why is null not allowed in ConcurrentHashMap?
- How does a HashMap work internally?
- Why Hashtable not allow null but HashMap allow?
- Which is faster HashMap or Hashtable?
- Is Hashtable obsolete?
- What hashing means?
- Is HashMap thread safe?
- How do you prove a HashMap is not thread safe?
- Why is Hashtable performance slow?
- Does HashMap use Hashtable internally?
- Which is better ArrayList or HashMap?
- Is ArrayList thread safe?
- Which data structure HashMap uses internally?
- When would you use a HashMap?
Where is Hashtable used?
Before we get into the definition of Hash Tables, it is good to introduce WHY to use Hash tables.
Hash tables are good for doing a quick search on things.
For instance if we have an array full of data (say 100 items).
If we knew the position that a specific item is stored in an array, then we could quickly access it..
Why is null not allowed in ConcurrentHashMap?
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. … get(key) returns null , you can’t detect whether the key explicitly maps to null vs the key isn’t mapped.
How does a HashMap work internally?
HashMap internally stores mapping in the form of Map. Entry object which contains both key and value object. When you want to retrieve the object, you call the get() method and again pass the key object.
Why Hashtable not allow null but HashMap allow?
HashMap , allows storing one null key and multiple null values. The reason for allowing only one null key is because keys in a HashMap has to be unique. … This is because the objects used as keys in a Hashtable implements the hashCode() and equals() methods for their storage and retrieval.
Which is faster HashMap or Hashtable?
Performance : HashMap is much faster and uses less memory than Hashtable as former is unsynchronized . Unsynchronized objects are often much better in performance in compare to synchronized object like Hashtable in single threaded environment.
Is Hashtable obsolete?
HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap.
What hashing means?
Hashing is the process of converting a given key into another value. A hash function is used to generate the new value according to a mathematical algorithm. The result of a hash function is known as a hash value or simply, a hash.
Is HashMap thread safe?
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. … HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value.
How do you prove a HashMap is not thread safe?
The hash operation of the hashMap is non-thread safe, and generally there will be data loss. Size will generally be smaller than the number you put in. Note that different thread keys cannot be repeated. 49600<50000, proving that hashMap is not thread safe.
Why is Hashtable performance slow?
Hashtable is slow due to added synchronization. HashMap is traversed by Iterator. Hashtable is traversed by Enumerator and Iterator. Iterator in HashMap is fail-fast.
Does HashMap use Hashtable internally?
Both HashMap and HashSet use a hash table (note the lower case!) which is a name for a general way of writing a certain kind of data structure.
Which is better ArrayList or HashMap?
The ArrayList has O(n) performance for every search, so for n searches its performance is O(n^2). The HashMap has O(1) performance for every search (on average), so for n searches its performance will be O(n). While the HashMap will be slower at first and take more memory, it will be faster for large values of n.
Is ArrayList thread safe?
Any method that touches the Vector ‘s contents is thread safe. ArrayList , on the other hand, is unsynchronized, making them, therefore, not thread safe. With that difference in mind, using synchronization will incur a performance hit. So if you don’t need a thread-safe collection, use the ArrayList .
Which data structure HashMap uses internally?
HashMap contains an array of the nodes, and the node is represented as a class. It uses an array and LinkedList data structure internally for storing Key and Value. There are four fields in HashMap. Before understanding the internal working of HashMap, you must be aware of hashCode() and equals() method.
When would you use a HashMap?
Maps are used for when you want to associate a key with a value and Lists are an ordered collection. Map is an interface in the Java Collection Framework and a HashMap is one implementation of the Map interface. HashMap are efficient for locating a value based on a key and inserting and deleting values based on a key.