What is the difference between HashMap and Hashtable?

Hashmap vs Hashtable

It is thread-safe and can be shared with many threads. 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.

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 .

Which is faster HashMap or 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.

Should I use HashMap or Hashtable Java?

When to use HashMap and Hashtable? HashMap should be preferred over Hashtable for the non-threaded applications. In simple words , use HashMap in unsynchronized or single threaded applications . We should avoid using Hashtable, as the class is now obsolete in latest Jdk 1.8 .

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.

14.11 HashMap and HashTable in 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.

Why null key is allowed in HashMap?

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.

Can we have null key in HashMap?

For HashMap, it allows one null key and there is a null check for keys, if the key is null then that element will be stored in a zero location in Entry array. We cannot have more than one Null key in HashMap because Keys are unique therefor only one Null key and many Null values are allowed.

Can we add null key in HashMap?

HashMap is similar to HashTable, but it is unsynchronized. It allows to store the null keys as well, but there should be only one null key object and there can be any number of null values. This class makes no guarantees as to the order of the map.

What is a Hashtable used for?

A hash table uses a hash function to compute an index, also called a hash code, into an array of buckets or slots, from which the desired value can be found. During lookup, the key is hashed and the resulting hash indicates where the corresponding value is stored.

What is the difference between HashMap and TreeMap?

HashMap allows a single null key and multiple null values. TreeMap does not allow null keys but can have multiple null values. HashMap allows heterogeneous elements because it does not perform sorting on keys.

How does HashMap differ from Hashtable Mcq?

HashMap Vs HashTable In Java :

HashMap is not synchronized and therefore it is not thread safe. HashTable is internally synchronized and therefore it is thread safe. HashMap allows maximum one null key and any number of null values. HashTable doesn't allow null keys and null values.

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.

Why HashMap is fast?

The reason that HashMap is faster than HashSet is that the HashMap uses the unique keys to access the values. It stores each value with a corresponding key and we can retrieve these values faster using keys during iteration. While HashSet is completely based on objects and therefore retrieval of values is slower.

What is the difference between TreeSet and SortedSet?

TreeSet maintains an object in sorted order. SortedSet maintains an object in sorted order.

Does Java Map allow duplicates?

The map implementations provided by the Java JDK don't allow duplicate keys. If we try to insert an entry with a key that exists, the map will simply overwrite the previous entry.

Can I store object as a key in HashMap?

Answer to your question is yes, objects of custom classes can be used as a key in a HashMap.

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.

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.

What is 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.

Is HashMap synchronized?

HashMap is similar to HashTable in java. The main difference between HashTable and HashMap is that HashTable is synchronized but HashMap is not synchronized.

What is hash bucket in Java?

HashMap stores elements in so-called buckets and the number of buckets is called capacity. When we put a value in the map, the key's hashCode() method is used to determine the bucket in which the value will be stored. To retrieve the value, HashMap calculates the bucket in the same way – using hashCode().

What is hash collision in Java?

A collision, or more specifically, a hash code collision in a HashMap, is a situation where two or more key objects produce the same final hash value and hence point to the same bucket location or array index.

Does Java have a class called Hashtable?

Java Hashtable class is one of the oldest members of Java Collection Framework. It is an implementation of mathematical hash table data structure. In Java hashtable internally contains buckets where the key/value pairs are stored. Hashtable is pretty similar to HashMap .