Classes and Objects - Java

Java is built around classes. A class is a blueprint that describes what data an object holds (its fields) and what it can do (its methods). An object is one instance created from that blueprint with new.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        Book b1 = new Book("Godaan", "Premchand", 312);
        Book b2 = new Book("Gitanjali", "Tagore", 103);
        System.out.println(b1.describe());
        System.out.println(b2.title);
    }
}

class Book {
    String title;
    String author;
    int pages;

    Book(String title, String author, int pages) {   // constructor
        this.title = title;
        this.author = author;
        this.pages = pages;
    }

    String describe() {
        return title + " by " + author + " (" + pages + " pages)";
    }
}

Output

Godaan by Premchand (312 pages)
Gitanjali

The constructor has the same name as the class and no return type. It runs when you call new.
this.title means "the field of this object", which separates it from the parameter with the same name.

Note: One file can hold several classes, but only one can be public, and its name must match the file name. Keep Main first in the file: java Main.java runs the first class it finds.

One class, many objects

Encapsulation

Good practice is to make fields private and expose them through methods. That way the class controls how its data may change.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount("Ravi");
        acc.deposit(1000);
        acc.deposit(-50);
        System.out.println(acc.withdraw(300));
        System.out.println(acc.withdraw(5000));
        System.out.println(acc.getOwner() + ": " + acc.getBalance());
    }
}

class BankAccount {
    private final String owner;
    private double balance;

    BankAccount(String owner) {
        this.owner = owner;
    }

    void deposit(double amount) {
        if (amount <= 0) {
            System.out.println("Deposit must be positive");
            return;
        }
        balance += amount;
    }

    boolean withdraw(double amount) {
        if (amount > balance) return false;
        balance -= amount;
        return true;
    }

    double getBalance() {
        return balance;
    }

    String getOwner() {
        return owner;
    }
}

Output

Deposit must be positive
true
false
Ravi: 700.0

toString and static members

Override toString() to control how an object prints. A static field belongs to the class itself, shared by all objects.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        System.out.println(new Ticket("Lagaan"));
        System.out.println(new Ticket("Swades"));
        System.out.println(new Ticket("Lagaan"));
    }
}

class Ticket {
    private static int nextId = 1;
    private final int id;
    private final String movie;

    Ticket(String movie) {
        this.movie = movie;
        this.id = nextId++;
    }

    @Override
    public String toString() {
        return "Ticket #" + id + " for " + movie;
    }
}

Output

Ticket #1 for Lagaan
Ticket #2 for Swades
Ticket #3 for Lagaan