Java Collections, ArrayList, Wrapper Class & Memory

When you start learning Java, these topics come up everywhere — collections, ArrayList, wrapper classes, and memory. Instead of memorizing definitions, it’s better to understand how they actually work together.

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Java Collections – The Big Picture

In Java, the Collection framework is used to store and manage groups of data.

Think of it like a toolbox. Inside it, you have different tools depending on your need.

Here’s a simple structure:

Collection (Interface)
   |
   |--- List
   |      |--- ArrayList
   |      |--- LinkedList
   |      |--- Vector
   |
   |--- Set
          |--- HashSet
          |--- LinkedHashSet
          |--- TreeSet

• List → ordered, allows duplicates
• Set → no duplicates

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What is ArrayList?

ArrayList is one of the most commonly used classes in Java.

It works like a dynamic array — meaning:

• You don’t need to fix the size
• It grows automatically when you add elements

Why developers like ArrayList:

• Maintains insertion order
• Allows duplicates
• Fast access using index

Example:

import java.util.*;

class Demo {
    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<>();

        list.add(10);
        list.add(20);
        list.add(10);

        System.out.println(list);
    }
}

Output:

[10, 20, 10]

So yes — duplicates are allowed, and order is preserved.

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Wrapper Class – Why Do We Need It?

Java has primitive types like:

int, char, double, boolean

But collections (like ArrayList) only store objects, not primitives.

That’s where wrapper classes come in.

Primitive	Wrapper Class
int	Integer
char	Character
double	Double
boolean	Boolean

Example:

int a = 10;
Integer obj = a;  // Autoboxing

Java automatically converts int → Integer. This is called autoboxing.

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Printing Wrapper Objects

Integer obj = 100;
System.out.println(obj);

Output:

100

You might expect a memory address, but you get the value.

Why?

Because wrapper classes override the toString() method — so they print the actual value.

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Stack vs Heap – Simple Understanding

This is where many beginners get confused. Keep it simple:

Stack

• Stores primitive values
• Stores references (addresses) of objects
• Fast memory

Heap

• Stores actual objects
• Shared memory

Quick Table:

Type	Stack	Heap
int (primitive)	Stored directly	❌
Integer	Reference stored	Object stored
ArrayList	Reference stored	Object stored

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Example to understand:

int a = 10;
Integer obj = new Integer(20);

• a → stored in stack
• obj → reference in stack, actual object in heap

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