Review everything you need to know about hash tables.

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Hash Tables in Java

# Introduction
Before we solve any challenges regarding Hast Tables, it is necessary to look at the definitions of **`HashMap`** and **`HashSet`** and see how they are different. Both are implemented in Java using the **Hash Table** class, which is why it is also a common misconception that these two structures are the same, but they are very different from each other.

 ## 🔍 HashMap?
`HashMap` is a collection that contains all the `key-value` pairs; it *maps* the values to keys. There is a built-in class available in Java for `HashMap`, implemented by using **`Map`** interface. 
It provides the basic functionality of hashing along with some helper functions that help in the process of insertion, deletion, and search.

Some of the key features of `HashMap` are given below:

- A `HashMap` stores `key-value` pairs (examples given below) to map a key to the value:  
$$abc->123$$
$$ xyz->456$$
- `HashMap` cannot contain duplicate keys. It can, however, have duplicate values. 
- `HashMap` also allows multiple **null** values and only one **null** key
- This mechanism does **not** support synchronous operations and is **not** thread-safe.

# 🔍 HashSet?
`HashSet` class is implemented in Java using **`Set`** interface. It is also built in the same way as `HashMap`, i.e., using the Hash Table class, but it is still quite different from the `HashMap` class. Some of the key features of `HashSet` are listed below:

- `HashSet` also stores values in an unordered way, using hashing, but this happens in the backend. On the backend, the `HashSet` class is implemented using the `HashMap` class. The `value` that we add to the `HashSet` is then added to the `HashMap` as a `key`, corresponding to a dummy `value` Object. The retrieval remains $O(1)$
- HashSet is a class which implements the `Set` interface and this interface only stores values, not a `key-value` pair.
- `HashSet` restricts storing multiple **null** values and only allows **one** **null** value in the whole table
- `HashSet` does not allow storing duplicate values as a **set** can only contain unique elements
- Just like `HashMap`, `HashSet` also does **not** support synchronous operations 




This lesson will discuss the difference between HashMap and HashSet and look at some of their key features. 

HashMap v/s HashSet

This lesson will discuss the difference between HashMap and HashSet and look at some of their key features.

HashMap vs HashSet

Introduction