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PONVEL M
PONVEL M

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Understanding Collections, ArrayList, Wrapper Classes & Memory Storage in Java

#java #ai #programming

Introduction

Java provides powerful tools to store and manage data efficiently. Among them, Collections, ArrayList, and Wrapper Classes play a very important role in real-world applications.

In this blog, we will understand:

  • What is Collection in Java
  • What is ArrayList
  • What are Wrapper Classes
  • How Integer and Wrapper objects are stored in Java memory

What is Collection in Java?

The Collection Framework in Java is a set of classes and interfaces used to store and manipulate groups of objects.

It provides:

  • Dynamic data storage
  • Built-in methods (add, remove, search)
  • Better performance and flexibility

Some common collection types:

  • List (ArrayList, LinkedList)
  • Set (HashSet)
  • Map (HashMap)

What is ArrayList?

ArrayList is a class in Java that implements the List interface.

Key Features:

  • Dynamic size (can grow and shrink)
  • Maintains insertion order
  • Allows duplicate elements

Example: 

import java.util.*;

ArrayList<Integer> list = new ArrayList<>();
list.add(10);
list.add(20);
list.add(30);
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Unlike arrays, ArrayList does not have a fixed size.

What is a Wrapper Class?

Java provides wrapper classes to convert primitive data types into objects.

Example:

  • int → Integer
  • char → Character
  • double → Double

Example: 

int a = 10;
Integer obj = a; // Autoboxing
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Why Wrapper Classes?

  • Collections can store only objects
  • So primitive types must be converted into objects

Autoboxing and Unboxing

  • Autoboxing → primitive to object
  • Unboxing → object to primitive 
Integer obj = 10;  // Autoboxing
int num = obj;     // Unboxing
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How Data is Stored in Java Memory?

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Java mainly uses two types of memory:

1. Stack Memory

  • Stores primitive variables
  • Stores reference variables
  • Fast access

Example:

int a = 10;
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a is stored in stack

🔹 2. Heap Memory

  • Stores objects and instances
  • Managed by JVM

Example:

Integer obj = new Integer(10);
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Object is stored in heap, reference is in stack

Integer & Wrapper Storage in Memory

Case 1: Primitive int 

int a = 10;
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Value 10 stored directly in stack memory

Case 2: Wrapper Integer 

Integer obj = 10;
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What happens:

  • Object created in heap memory
  • Reference stored in stack memory

Case 3: ArrayList Storage 

ArrayList<Integer> list = new ArrayList<>();
list.add(10);
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Internally:

  • ArrayList object → stored in heap
  • Integer objects → stored in heap
  • Reference → stored in stack

Key Difference

Type Stored In
int (primitive) Stack
Integer (object) Heap
ArrayList Heap

Why This is Important?

Understanding memory helps in:

  • Writing efficient code
  • Avoiding memory issues
  • Improving performance

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