## Introduction

Hash tables are an effective way of storing data in memory in an accessible and efficient manner. They rely on the use of hash functions to map data to a particular memory location. This allows for quicker search queries, which is why they are so popular in application development. Linked lists are linear data structures with each item connected to the next. We will be learning how to create a hash table by combining the two to create a hash table implemented with linked list in Java.

## What You Will Learn

- What is a Hash Table
- What is a Linked List
- How to implement a Hash Table using a Linked List in Java

## What is a Hash Table

A hash table is a data structure that stores a collection of key-value pairs. The idea behind it is that given the key to look up, you can quickly and efficiently find the associated data connected to it. It uses a hash function to calculate an index in an array, where the actual value is stored.

## What is a Linked List

Linked lists are linear data structures, where each data item is connected with the next data item. Each node contains a pointer or reference to the next node. It has several advantages, like dynamic memory allocation, easy insertion/deletion of nodes and reverse traversal of the linked list.

## How to Implement a Hash Table using a Linked List in Java

Now that we know the fundamentals of hash tables and linked lists, we can get to implementation.

### Step One: Create a hashTable Class

Firstly, we need to create a class which will contain all the elements of our hash table. The class should have properties for the array, the size, and a linked list for storing elements.

```
class HashTable {
private int size;
private LinkedList[] array;
public HashTable(int size) {
this.size = size;
array = new LinkedList[size];
//Initialize array with empty linked list objects
for(int i = 0; i < size; i++) {
array[i] = new LinkedList();
}
}
}
```

### Step Two: Create a Hash Method

The hash method will be responsible for mapping an item to a particular index in our array. The method takes a key as a parameter and should return an index of the array.

```
int hash(int key) {
return key % size;
}
```

The hash method should be able to take a variety of key types, so it's important to first convert the key to an integer if necessary.

### Step Three: Insert an Item into the Hash Table

We know our hash table will store items in the form of key-value pairs, so our insert method should have two parameters - a key and a value.

```
void insert(int key, int value) {
//Calculate the index of the array
int index = hash(key);
//Insert the item into the linked list at the index
array[index].add(value);
}
```

### Step Four: Retrieve an Item from the Hash Table

Retrieving an item from our hash table is just as easy as inserting it. We can use the same hash method to find the index of the key in the array and retrieve the item from the linked list at that index.

```
int retrieve(int key) {
//Calculate the index of the array
int index = hash(key);
//Retrieve the item from the linked list at that index
return array[index].get(value);
}
```

## FAQ

### What is the purpose of a hash table?

A hash table is used to store key-value pairs quickly and efficiently. It uses a hash function to calculate the index of an item in the array, and therefore can quickly find the associated data when a key is provided.

### What is the difference between a hash table and a linked list?

A hash table is a data structure used for storing key-value pairs, while a linked list is a data structure used for storing linear data. A hash table is more efficient for inserting and retrieving data, while a linked list has other advantages, like dynamic memory allocation and reverse traversal.

### How do you create a hash table in Java?

Creating a hash table in Java can be done by first creating a class with properties for the size and array, then using a hash function to map data to a particular memory location. After that, you can create methods for inserting and retrieving data, and a linked list can be used to store the data.

### Can you store different data types in a hash table?

Yes, you can store different data types in a hash table. You will just need to use a method convert the data type to an integer if necessary in order to calculate the index of the item in the array.

### What is the time complexity of hash tables?

The time complexity of hash tables can vary. Generally, the time required to search for an item in a hash table is O(1). However, insertion and deletion can take more time if the hash function does not generate well-distributed indices.