Classes and objects

Why classes?

So far you have used separate variables and static methods. Real programs model things: an account has an owner and a balance and can deposit and withdraw; a product has a name, a price and stock. A class groups the data of a thing with the code that works on that data. This style is object-oriented programming (OOP).

public class BankAccount {
    String owner;          // fields: the data each object has
    double balance;

    void deposit(double amount) {   // instance method: works on THIS object's fields
        balance += amount;
    }

    void show() {
        System.out.println(owner + " has Rs " + balance);
    }
}

Creating objects

BankAccount a = new BankAccount();   // new builds an object on the heap
a.owner = "Asha";
a.deposit(5000);

BankAccount b = new BankAccount();   // a second, independent object
b.owner = "Ravi";
b.deposit(1200);

a.show();   // Asha has Rs 5000.0
b.show();   // Ravi has Rs 1200.0

Each object has its own copy of the fields. a.deposit changes only a's balance. Fields get default values (0, 0.0, false, null) if you do not set them.

References and the heap

Objects live in a memory area called the heap. A variable like a does not contain the object; it holds a reference (like an address) to it. Assigning BankAccount c = a; copies the reference, so c and a refer to the same account. A reference that points to nothing is null; calling a method on null throws NullPointerException.

BankAccount c = a;     // same object as a
c.deposit(100);
a.show();              // Asha has Rs 5100.0

BankAccount nobody = null;
// nobody.show();      // NullPointerException

Instance members versus static members

Fields and methods without static belong to each object (instance members): every account has its own balance. Members marked static belong to the class as a whole (topic 4). Inside an instance method you can use fields directly; the full form is this.balance, where this means "the object this method was called on".