Java for Programmers

Collections 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.

Java Collections

**ArrayList** is the most widely used implementation of the **List** interface. Some of the salient features of an **ArrayList** are:
1. Elements are stored in the order of insertion.
2. It allows the storage of duplicate elements.
3. ArrayList also supports null elements.

## Internal implementation of ArrayList

An **ArrayList** stores data in a resizable array. Before Java 8, when an **ArrayList** was created, an array of default size ten was created internally. Now, when an **ArrayList** is created, an array of size zero is created. Only when the first element is inserted does the array size  change to ten. This is called lazy initialization, and it saves a lot of memory.

Before adding an element in **ArrayList**, its capacity is checked. If the internal array is full, then a new array of size **$\lgroup n+\frac{n}{2}+1\rgroup$** is created (e.g., if the capacity is ten, then a new array of size 16 will be created). The elements from the old array will be copied to the new array. This increases the capacity of an **ArrayList**, which is a time-consuming process.

**ArrayList** is the most widely used implementation of the **List** interface. Some of the salient features of an **ArrayList** are:
1. Elements are stored in the order of insertion.
2. It allows the storage of duplicate elements.
3. ArrayList also supports null elements.

# Internal implementation of ArrayList

An **ArrayList** stores data in a resizable array. Before Java 8, when an **ArrayList** was created, an array of default size ten was created internally. Now, when an **ArrayList** is created, an array of size zero is created. Only when the first element is inserted does the array size  change to ten. This is called lazy initialization, and it saves a lot of memory.

Before adding an element in **ArrayList**, its capacity is checked. If the internal array is full, then a new array of size **$\lgroup n+\frac{n}{2}+1\rgroup$** is created (e.g., if the capacity is ten, then a new array of size 16 will be created). The elements from the old array will be copied to the new array. This increases the capacity of an **ArrayList**, which is a time-consuming process.

Let's discuss how ArrayList works in Java.

Introduction

ArrayList

LinkedList

CopyOnWriteArrayList

HashSet

TreeSet

HashMap

TreeMap

LinkedHashMap

Additional Map Implmentations

Arrays

Collections class

ArrayList: Introduction

Internal implementation of ArrayList