Objects you already know

You've been using objects since module 2. A Scanner is an object, every String is an object, and so is every ArrayList. An object holds some data, and it has methods, which you call with a dot after it: scanner.nextLine(), word.length(), list.add("tea").

Two objects of the same kind each have their own data:

String city = "Lima";
String river = "Mississippi";
System.out.println(city.length());
System.out.println(river.length());

ArrayList<String> groceries = new ArrayList<>();
ArrayList<String> chores = new ArrayList<>();
groceries.add("milk");
groceries.add("eggs");
chores.add("dishes");
System.out.println(groceries.size());
System.out.println(chores.size());

4
11
2
1

city.length() and river.length() run the same method, but each answers for its own text. groceries and chores are two separate lists: adding to one doesn't touch the other.

Classes and new

Every object is made from a class. The class is the blueprint: it says what data each object holds and which methods it has. One class can make as many objects as you like, just as one blueprint can be used for a whole street of houses. Each house is built to the same plan, and painting one front door doesn't paint the others.

• ArrayList is a class. new ArrayList<>() builds a new, empty list object from it.
• Scanner is a class too, and new Scanner(System.in) builds a scanner that reads what the user types. The value in the parentheses is given to the new object as it's built.
• String is a class. Strings are usually made by writing text in quotes, without new, but each one is still an object.

The type of a variable that holds an object is the name of its class: Scanner scanner is a variable that holds a Scanner.

A class in its own file

Java comes with a large library of ready-made classes, and a program can also have classes of its own. Each one is written in a file of its own, named after the class: the class Lamp is in the file Lamp.java. Here's a program of two files. You don't need to read Lamp.java closely yet: the next steps show how a class like it is written, line by line.

public class Main {
    public static void main(String[] args) {
        Lamp desk = new Lamp("desk");
        Lamp porch = new Lamp("porch");
        desk.turnOn();
        desk.printState();
        porch.printState();
    }
}

public class Lamp {
    private String place;
    private boolean on;

    public Lamp(String place) {
        this.place = place;
        this.on = false;
    }

    public void turnOn() {
        this.on = true;
    }

    public void turnOff() {
        this.on = false;
    }

    public void printState() {
        if (this.on) {
            System.out.println("The " + this.place + " lamp is on");
        } else {
            System.out.println("The " + this.place + " lamp is off");
        }
    }
}

The desk lamp is on
The porch lamp is off

• new Lamp("desk") builds a new Lamp object and gives it the text "desk". Lamp desk is a variable that holds a Lamp.
• desk.turnOn() calls turnOn on the desk lamp only. The porch lamp is a separate object, so it's still off.
• To use a class, you only need to know its methods and what they do, the same way you use an ArrayList without ever seeing its code.

In a challenge with several files, the editor shows a tab for each file, and Check compiles them all together.