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Java ArrayList

Master the Java ArrayList. Learn how to create dynamic arrays, add, remove, and access elements, and sort lists.

The ArrayList class is a resizable array, which can be found in the java.util package.

Think of a Magic Backpack 🎒.

  • A normal array is like a box with fixed slots (Size 5). Once full, you can't add more.
  • An ArrayList is like a magic backpack that expands as you put more items in it!
graph LR
    subgraph Memory
    A[Index 0: Volvo] --- B[Index 1: BMW]
    B --- C[Index 2: Ford]
    C -.-> D[Index 3: Empty]
    D -.-> E[Index 4: Empty]
    end

    style A fill:#e1f5fe,stroke:#01579b,stroke-width:2px
    style B fill:#e1f5fe,stroke:#01579b,stroke-width:2px
    style C fill:#e1f5fe,stroke:#01579b,stroke-width:2px
    style D fill:#f5f5f5,stroke:#9e9e9e,stroke-dasharray: 5 5
    style E fill:#f5f5f5,stroke:#9e9e9e,stroke-dasharray: 5 5

Visualizing an ArrayList in Memory

Creating an ArrayList

import java.util.ArrayList;

ArrayList<String> cars = new ArrayList<String>();

Common Operations

1. Add Items

cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");

2. Access Items

System.out.println(cars.get(0)); // Output: Volvo

3. Change Items

cars.set(0, "Opel"); // Changes Volvo to Opel

4. Remove Items

cars.remove(0); // Removes the first item
cars.clear();   // Removes ALL items

5. List Size

Note: Size is the number of elements currently in the list. Capacity is how many elements the list can hold before it needs to resize (grow).

Loop Through an ArrayList

for (int i = 0; i < cars.size(); i++) {
  System.out.println(cars.get(i));
}

// Or using Enhanced For Loop
for (String i : cars) {
  System.out.println(i);
}

Tip 💡: Unlike arrays, ArrayList cannot store primitive types (int, char, etc.). You must use Wrapper Classes like Integer, Character, etc. ArrayList<int> list = new ArrayList<>(); ❌ (Error) ArrayList<Integer> list = new ArrayList<>(); ✅ (Correct)

Sorting an ArrayList

Use the Collections.sort() method.

import java.util.Collections;

Collections.sort(cars); // Sorts alphabetically or numerically

Which method is used to find the number of elements in an ArrayList?

Note : Java is a statically-typed language. It means that all variables must be declared before they can be used.

Challenge

Complete this chapter to unlock the next one.

Challenge

Task:

Create an ArrayList of Integers named 'nums'. Add 10 and 20. Print the size.

Java Collections Framework

Overview of the Java Collections Framework. Understand the hierarchy of List, Set, and Map interfaces and when to use which data structure.

Java LinkedList

Learn about Java LinkedList. Understand how it differs from ArrayList, when to use it, and its special methods for manipulating the beginning and end of the list.