ArrayList

Arrays are great, but they have one big limitation: their size is fixed. Once you create an array of 5 elements, you can't add a 6th. In real programs, you often don't know upfront how many items you'll need. This is where ArrayList comes in

An ArrayList is a dynamic array: it grows and shrinks as needed. You need to import it from java.util

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> crew = new ArrayList<String>();
        crew.add("Tommy Vercetti");
        crew.add("Lance Vance");
        crew.add("Phil Cassidy");

        System.out.println(crew);
    }
}

Output

[Tommy Vercetti, Lance Vance, Phil Cassidy]

The <String> part is called a generic type: it tells Java what type of elements the list holds. ArrayList<String> means "a list that holds Strings." For numbers you use ArrayList<Integer> or ArrayList<Double> (not int/double directly, Java converts automatically)

Main methods

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> guests = new ArrayList<String>();

        // add: adds to the end
        guests.add("Tommy");
        guests.add("Lance");
        guests.add("Mercedes");
        guests.add("Sonny");

        // size: how many elements
        System.out.println("Total: " + guests.size());

        // get: element at an index (starting from 0)
        System.out.println("First: " + guests.get(0));

        // remove: removes at an index
        guests.remove(3);
        System.out.println("After removal: " + guests);
    }
}

Output

Total: 4
First: Tommy
After removal: [Tommy, Lance, Mercedes]

.size() with parentheses, not .length like arrays, one of the differences to remember

Iterating with a classic for: when you need the index

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> crew = new ArrayList<String>();
        crew.add("Tommy Vercetti");
        crew.add("Lance Vance");
        crew.add("Phil Cassidy");

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

Output

0: Tommy Vercetti
1: Lance Vance
2: Phil Cassidy

Iterating with for-each: cleaner when you don't need the index

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> crew = new ArrayList<String>();
        crew.add("Tommy Vercetti");
        crew.add("Lance Vance");
        crew.add("Phil Cassidy");

        for (String name : crew) {
            System.out.println("Member: " + name);
        }
    }
}

Output

Member: Tommy Vercetti
Member: Lance Vance
Member: Phil Cassidy

Read for (String name : crew) as: "for each String called name in crew"

ArrayList becomes really powerful when you hold objects in it, not just Strings or numbers, but instances of your own classes

import java.util.ArrayList;

class Car {
    String name;
    int speed;

    Car(String name, int speed) {
        this.name = name;
        this.speed = speed;
    }
}

public class Main {
    public static void main(String[] args) {
        ArrayList<Car> garage = new ArrayList<Car>();
        garage.add(new Car("Infernus", 240));
        garage.add(new Car("Cheetah", 230));
        garage.add(new Car("Banshee", 200));

        for (Car c : garage) {
            System.out.println(c.name + " - " + c.speed + " km/h");
        }
    }
}

Output

Infernus - 240 km/h
Cheetah - 230 km/h
Banshee - 200 km/h

ArrayList<Car> holds objects of type Car. You can access each object's fields directly in the loop

Quick comparison

Mission: The Crew List

Tommy keeps a list of his Vice City crew members. Each member has a name and a role. Tommy starts with Lance Vance (driver), Phil Cassidy (weapons), Umberto Robina (ally), and Hilary King (driver). At some point, Lance betrays him and needs to be removed from the list

Build a CrewMember class (with name and role). In main, store each member's data in variables: name1/role1 through name4/role4 (start with Lance/driver, Phil/weapons, Umberto/ally, Hilary/driver). Add all four to an ArrayList<CrewMember>, remove Lance (index 0), then iterate through the list and print each remaining member as "name - role"

Example

Phil Cassidy - weapons
Umberto Robina - ally
Hilary King - driver