Gain insights into Java Collections Framework. Delve into lists, sets, and hashmaps. Learn about data modification, sorting, thread-safety, and internal workings for efficient usage.

Collection is one of the most important topics for Java programming. Without the proper knowledge of every collection type in Java, it becomes difficult for you to decide which option to pick in a particular scenario.

This course will give you a great understanding of all collection types available in the Collections Framework like lists, linked lists, sets, and hashmaps. 

After completing this course, you will know how data can be modified within a collection, how to sort a collection, and how it can be made thread-safe. You will also cover the internal working of each Java collection so you can work with them more efficiently.

Collections in Java

**HashMap** is a class in the `java.utils` package that implements the **Map** interface. It is used to store the key-value pair. Let's suppose we need to store the stock prices of some companies. In this case, we can use a **HashMap**. The company name will be the key and the stock price will be the value. 

Some of the features of HashMap are:
1. The keys should be unique.
2. HashMap allows only one `null` key.
3. The values can be null or duplicate.
4. The keys are stored in random order.

## Creating a HashMap

There are four different constructors available to create a HashMap in Java.

### Using the no-arg constructor
The simplest way to create a **HashMap** is by using the no-arg constructor. This constructor creates a **HashMap** with an initial capacity of 16 and **load factor** of 0.75.

Below is the code syntax to create a **HashMap**. It states that the key is a String type and the _value_ is an Integer type.

```Java
Map<String, Integer> map = new HashMap<>();
```

> Load factor is a number that defines when a Map should be resized. If the load factor is 0.75, then it means that the Map should be resized when it is 75 percent full.

### Using the constructor that takes initial capacity
We can also provide the initial capacity while creating the **HashMap**. If we are already aware that our **HashMap** will contain more than 16 elements, then it is better to provide some initial capacity as it reduces the number of resizes. In this case, also, the default load factor (0.75) is used.

### Using the constructor that takes initial capacity and load factor
We can also provide initial capacity with the load factor while creating the **HashMap**. If we don't want frequent resizing then we can set the load factor to a higher number.

### Using the constructor that takes another Map as a parameter
We can also create a **HashMap** using another Map by passing it to the constructor. The newly created **HashMap** will have the same size as that of the passed Map, whereas the load factor will default to **0.75**

## Inserting into a HashMap

Let’s discuss all the methods that we can use to insert a key-value pair in a  **HashMap**.

### Using the `put()` method
We can use the `put(K key, V value)` method to insert a key-value pair in the **HashMap**. If the key is not present, then a new key-value pair will be added. If the key is already present, then the value will be updated.

### Using the `putIfAbsent()` method
The `putIfAbsent(K key, V value)` method inserts a key-value pair only if it is not already present in the Map. If the key is already present then its value will not be updated. This method was added in Java 8.

### Using the `putAll()` method
The `putAll(Map<? extends K, ? extends V> m)` method copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.

The below example shows **HashMap** working properly.



**HashMap** is a class in the `java.utils` package that implements the **Map** interface. It is used to store the key-value pair. Let's suppose we need to store the stock prices of some companies. In this case, we can use a **HashMap**. The company name will be the key and the stock price will be the value. 

Some of the features of HashMap are:
1. The keys should be unique.
2. HashMap allows only one `null` key.
3. The values can be null or duplicate.
4. The keys are stored in random order.

# Creating a HashMap

There are four different constructors available to create a HashMap in Java.

## Using the no-arg constructor
The simplest way to create a **HashMap** is by using the no-arg constructor. This constructor creates a **HashMap** with an initial capacity of 16 and **load factor** of 0.75.

Below is the code syntax to create a **HashMap**. It states that the key is a String type and the _value_ is an Integer type.

```Java
Map<String, Integer> map = new HashMap<>();
```

> Load factor is a number that defines when a Map should be resized. If the load factor is 0.75, then it means that the Map should be resized when it is 75 percent full.

## Using the constructor that takes initial capacity
We can also provide the initial capacity while creating the **HashMap**. If we are already aware that our **HashMap** will contain more than 16 elements, then it is better to provide some initial capacity as it reduces the number of resizes. In this case, also, the default load factor (0.75) is used.

## Using the constructor that takes initial capacity and load factor
We can also provide initial capacity with the load factor while creating the **HashMap**. If we don't want frequent resizing then we can set the load factor to a higher number.

## Using the constructor that takes another Map as a parameter
We can also create a **HashMap** using another Map by passing it to the constructor. The newly created **HashMap** will have the same size as that of the passed Map, whereas the load factor will default to **0.75**

# Inserting into a HashMap

Let’s discuss all the methods that we can use to insert a key-value pair in a  **HashMap**.

## Using the `put()` method
We can use the `put(K key, V value)` method to insert a key-value pair in the **HashMap**. If the key is not present, then a new key-value pair will be added. If the key is already present, then the value will be updated.

## Using the `putIfAbsent()` method
The `putIfAbsent(K key, V value)` method inserts a key-value pair only if it is not already present in the Map. If the key is already present then its value will not be updated. This method was added in Java 8.

## Using the `putAll()` method
The `putAll(Map<? extends K, ? extends V> m)` method copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.

The below example shows **HashMap** working properly.



Let's look at how to create a HashMap and insert elements into it.

Let's Begin

Lists in Java

LinkedList in Java

CopyOnWriteArrayList in Java

Sets in Java

TreeSet in Java

HashMap In Java

TreeMap in Java

LinkedHashMap in Java

Additional Map Implementations

Arrays Class

Collections Class

HashMap: Creation and Insertion

Creating a HashMap