HashSet & HashMap & HashTable in Java

HashSet and HashMap

1) Implementation: HashMap implements Map interface and HashSet implements Set interface.

2) Duplicates: HashSet does’t allow duplicate values. HashMap store key, value pairs and it does not allow duplicate keys. If key is duplicate then old key is replaced with new value.

3) Number of objects during storing objects : HashMap requires two objects put(K key, V Value) to add an element to HashMap object, while HashSet requires only one object add(Object o) .

4) Dummy value : In HashMap no concept of dummy value, HashSet internally uses HashMap to add elements. In HashSet, the argument passed in add(Object) method serves as key K. Java internally associates dummy value for each value passed in add(Object) method.

5) Storing or Adding mechanism : HashMap internally uses hashing to store or add objects, HashSet internally uses HashMap object to store or add the objects.

6) Faster:HashSet is slower then HashMap,

7) Insertion HashMap use put() method for storing data, While in HashSet use add() method for add or storing data.

8) Example: HashSet is a set, e.g. {1, 2, 3, 4, 5, 6, 7},

HashMap is a key -> value pair(key to value) map, e.g. {a -> 1, b -> 2, c -> 2, d -> 1}

HashMap and HashTable

There are several differences between HashMap and Hashtable in Java:

1. Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.

2. Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.

3. One of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.