> For the complete documentation index, see [llms.txt](https://feifeizheng.gitbook.io/leetcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://feifeizheng.gitbook.io/leetcode/data-structure/hashset-and-hashmap-and-hashtable-in-java.md).

# 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`](http://java.sun.com/javase/7/docs/api/java/util/HashMap.html) and [`Hashtable`](http://java.sun.com/javase/7/docs/api/java/util/Hashtable.html) in Java:

1. `Hashtable` is [synchronized](https://stackoverflow.com/questions/1085709/what-does-synchronized-mean), 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`](http://java.sun.com/javase/7/docs/api/java/util/LinkedHashMap.html), 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`.