Why HashMap can have only one null key?

Now you must be wondering why HashTable doesn't allow null and HashMap do? The answer is simple. In order to successfully store and retrieve objects from a HashTable, the objects used as keys must implement the hashCode method and the equals method. Since null is not an object, it can't implement these methods.

Why HashMap allows one null key and multiple null values?

In Hashmap one null key is allowed and multiple null values are allowed. Since hashcode of null is 0 so null key actually stored at index 0 in hashmap. Hashmap in java actsually created as Array of linkedlist where index in array is find out by hashing on key.

Why HashMap does not allow null key?

Hashtable does not allow null keys but HashMap allows one null key and any number of null values. There is a simple explanation behind that. put() method in hashmap does not call hashcode() when null is passed as key and null Key is handled as a special case.

What happens if two nulls stored in HashMap?

1.1) Does hashmap allows null key and value ? Yes, HashMap allows null key and null values. 1.2) Can a hashmap have multiple null keys and values? HashMap allows to store one null key and many null values i.e. many keys can have null value in java.

Can a map contain a null key?

A Map cannot contain duplicate keys and each key can map to at most one value. HashMap and LinkedHashMap allow null key and null value but TreeMap doesn't allow null key and null value.

is null key is allowed in hashmap job java interview question and answer

Can HashMap have multiple null keys?

HashMap allows one null key and multiple null values whereas Hashtable doesn't allow any null key or value. HashMap is generally preferred over HashTable if thread synchronization is not needed.

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.

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 HashMap take null?

HashMap allows to store one null key and many null values i.e. many keys can have null value in java. Hashtable does not allow to store null key or null value.

Which of the following may have one null key but can have multiple null values?

A HashMap contains values based on the key. It contains only unique elements. It may have one null key and multiple null values.

Why is HashMap not thread-safe?

Well, HashMap is not thread-safe. If multiple threads are accessing the same HashMap object and try to modify the structure of the HashMap (using put() or remove() method), it may cause an inconsistency in the state of HashMap .

Does HashSet allow null key?

Null values in HashSet − The HashSet object allows null values but, you can add only one null element to it. Though you add more null values if you try to print its contents, it displays only one null.

What is the difference between HashMap and ConcurrentHashMap?

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 null key and value is 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.

Why null is not allowed in TreeMap?

TreeMap sorts elements in natural order and doesn't allow null keys because compareTo() method throws NullPointerException if compared with null.

Why is Hashtable synchronized?

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 string is used as key in HashMap?

String is as a key of the HashMap

When you pass the key to retrieve its value, the hash code is calculated again, and the value in the position represented by the hash code is fetched (if both hash codes are equal).

Can HashMap have multiple values?

HashMap can be used to store key-value pairs. But sometimes you may want to store multiple values for the same key. For example: For Key A, you want to store - Apple, Aeroplane.

Can we store objects in HashMap?

Internal working. All instances of Entry class are stored in an array declard as 'transient Entry[] table' . For each key-value to be stored in HashMap, a hash value is calculated using the key's hash code. This hash value is used to calculate the index in the array for storing Entry object.

Can we make HashMap synchronized?

HashMap can be synchronized using the Collections. synchronizedMap() method. The synchronizedMap() method of java.

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.

Is Map a Hashtable?

Data Structures hash map Hashing

Hash map is a widely used efficient data structure that used to store data which can be searched in constant time O(1) . It is also referred as hash table, unordered map, dictionary, hash set and others. This data structure is implemented over an array that maps keys to values.

What is the difference between LinkedHashMap and HashMap?

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.

Is HashMap faster than ConcurrentHashMap?

The summary

If you choose a single thread access use HashMap , it is simply faster. For add method it is even as much as 3x more efficient. Only get is faster on ConcurrentHashMap , but not much.

Which is faster HashMap or LinkedHashMap?

HashMap as do not maintain any insertion order of its elements hence is faster as compare to TreeMap also do not sort its elements on the basis of its value so also faster than LinkedHashMap. LinkedHashMap is faster as compare to TreeMap but is slower than HashMap.