List

`List` Interface¶

Think of List as a well-behaved queue with memory 🎭 It keeps order, allows duplicates, and lets you poke elements by index like a playlist you can rearrange mid-song.

🧩 What is `List`?¶

List is part of the Java Collections Framework

List<String> names = new ArrayList<>();

👉 It represents an ordered collection (sequence)

🔑 Key Characteristics¶

Maintains insertion order 📌
Allows duplicate elements 🔁
Supports index-based access (0,1,2...)
Can contain null values

🏗️ Common Implementations¶

1. ArrayList¶

List<String> list = new ArrayList<>();

Backed by dynamic array
Fast for read (O(1))
Slow for insert/delete in middle

👉 Best when: frequent reads

2. LinkedList¶

List<String> list = new LinkedList<>();

Uses doubly linked list
Fast insert/delete
Slow random access

👉 Best when: frequent insert/delete

3. Vector (Legacy)¶

List<String> list = new Vector<>();

Thread-safe (synchronized)
Slower performance
Rarely used now

4. Stack (Legacy)¶

Stack<Integer> stack = new Stack<>();

Follows LIFO (Last In First Out)

⚙️ Important Methods¶

List<String> list = new ArrayList<>();

list.add("A");          // add element
list.add(1, "B");       // add at index
list.get(0);            // get element
list.set(0, "Z");       // update
list.remove(1);         // remove
list.contains("Z");     // check
list.size();            // size

🧠 Example with Output¶

List<String> list = new ArrayList<>();

list.add("Java");
list.add("Python");
list.add("Java");

System.out.println(list);

👉 Output:

id="6b9y7y" [Java, Python, Java]

✔ Order maintained ✔ Duplicate allowed

🔄 ArrayList vs LinkedList (Quick View)¶

Feature	ArrayList 🧱	LinkedList 🔗
Structure	Dynamic Array	Doubly Linked List
Access	Fast ⚡	Slow 🐢
Insert/Delete	Slow	Fast
Memory	Less	More

🎯 Interview Nuggets¶

List is interface, not class
Default choice → ArrayList
Use LinkedList for frequent modifications
Vector = legacy (avoid unless needed)

🧠 Memory Trick¶

List = Ordered + Duplicate + Index access