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

**IdentityHashMap** is another type of Map that implements Map, Serializable, and Cloneable interfaces and extends the AbstractMap class. The main feature of this map is that while storing elements, the equality of the keys is checked on the basis of reference instead of the equals method. What this means is that if we have two keys, **key1** and **key2**, then **key1** will be considered equal to **key2** if key1 == key2. In other words, if both the keys reference the same object.

This means that **IdentityHashMap** intentionally violates Map’s general contract which mandates the use of the equals method when comparing objects. This class is designed for use only in rare cases wherein reference-equality semantics are required.

Some of the features of **IdentityHashMap** are:

1. The IdentityHashMap stores the elements in random order. 
2. The **IdentityHashMap** allows a single null key.
3. The **IdentityHashMap** is not thread-safe.

## Difference between HashMap and IdentityHashMap

Let's discuss some of the differences between a HashMap and IdentityHashMap.

1. IdentityHashMap uses reference equality to compare keys and values while HashMap uses object equality to compare keys and values.
2.  IdentityHashMap does not use the `hashCode()` method. Instead it uses `System.identityHashCode()` to find the bucket location.
3. IdentityHashMap does not require keys to be immutable as it does not rely on the `equals()` and `hashCode()` methods.
To safely store the object in HashMap, keys must be immutable.
4. The default initial capacity of HashMap is 16; whereas, for IdentityHashMap, it is 32.

These are the major differences between a **HashMap** and **IdentityHashMap**. All the operations such as create, add, fetch, remove, etc in an **IdentityHashMap** are the same as these operations in hashMap. We will be looking at an example to understand the difference in how HashMap and **IdentityHashMap** run.

First, let’s create an `Employee` class that overrides both the `equals()` and the `hashCode()` method.

**IdentityHashMap** is another type of Map that implements Map, Serializable, and Cloneable interfaces and extends the AbstractMap class. The main feature of this map is that while storing elements, the equality of the keys is checked on the basis of reference instead of the equals method. What this means is that if we have two keys, **key1** and **key2**, then **key1** will be considered equal to **key2** if key1 == key2. In other words, if both the keys reference the same object.

This means that **IdentityHashMap** intentionally violates Map’s general contract which mandates the use of the equals method when comparing objects. This class is designed for use only in rare cases wherein reference-equality semantics are required.

Some of the features of **IdentityHashMap** are:

1. The IdentityHashMap stores the elements in random order. 
2. The **IdentityHashMap** allows a single null key.
3. The **IdentityHashMap** is not thread-safe.

# Difference between HashMap and IdentityHashMap

Let's discuss some of the differences between a HashMap and IdentityHashMap.

1. IdentityHashMap uses reference equality to compare keys and values while HashMap uses object equality to compare keys and values.
2.  IdentityHashMap does not use the `hashCode()` method. Instead it uses `System.identityHashCode()` to find the bucket location.
3. IdentityHashMap does not require keys to be immutable as it does not rely on the `equals()` and `hashCode()` methods.
To safely store the object in HashMap, keys must be immutable.
4. The default initial capacity of HashMap is 16; whereas, for IdentityHashMap, it is 32.

These are the major differences between a **HashMap** and **IdentityHashMap**. All the operations such as create, add, fetch, remove, etc in an **IdentityHashMap** are the same as these operations in hashMap. We will be looking at an example to understand the difference in how HashMap and **IdentityHashMap** run.

First, let’s create an `Employee` class that overrides both the `equals()` and the `hashCode()` method.

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

IdentityHashMap