A list of objects

The lists in part 3 held Strings and Integers. The type in the angle brackets can be any class, including the ones you write yourself. Here's a Pet class, with a name, an age, two getters and a toString, and a list of pets:

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Pet> pets = new ArrayList<>();
        pets.add(new Pet("Rex", 4));
        pets.add(new Pet("Misty", 2));
        Pet kiwi = new Pet("Kiwi", 11);
        pets.add(kiwi);

        System.out.println(pets.size() + " pets");
        for (Pet pet : pets) {
            System.out.println(pet);
        }
        System.out.println("The first one is " + pets.get(0).getName());
    }
}

public class Pet {
    private String name;
    private int age;

    public Pet(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return this.name;
    }

    public int getAge() {
        return this.age;
    }

    public String toString() {
        return this.name + ", " + this.age + " years old";
    }
}

3 pets
Rex, 4 years old
Misty, 2 years old
Kiwi, 11 years old
The first one is Rex

• ArrayList<Pet> pets = new ArrayList<>(); creates a list for Pet objects. Like every new list, it starts empty.
• pets.add(new Pet("Rex", 4)); creates a pet and adds it in one go. The object needs no variable of its own: the list refers to it. Adding a pet that has a variable, like kiwi, works the same way.
• In the for-each loop, pet is a Pet: in each round, the next object in the list. System.out.println(pet) prints what the object's toString returns.
• pets.get(0) is a Pet too, so you can call its methods straight away: pets.get(0).getName().

size(), get(index) and remove(index) work just as they did with Strings. The methods that look for a value, such as contains and indexOf, are different: pets.contains(new Pet("Rex", 4)) is false, because the new pet is another object than the Rex in the list, even though its name and age are the same. (A later module shows how a class can change that.) To find a pet by its name or age, go through the list and ask each pet with a getter, as step 5 does.

The list takes only pets: pets.add("Rex") doesn't compile, because a String isn't a Pet.

Printing an object whose class has no toString shows only the class name, an @ and a code that can change from run to run. That's why the classes in this module have a toString.