Classes & Objects

Class vs object

• A **class** is a type (blueprint): fields + methods. An **object** is an instance created at runtime with new.
• The variable holds a *reference*. The object lives on the heap until nothing references it, then GC may reclaim it.
• Every class (except Object) has a superclass. If you omit extends, you extend java.lang.Object.

public class Student {
    String name;
    int roll;

    void display() {
        System.out.println(roll + " " + name);
    }

    public static void main(String[] args) {
        Student s = new Student();
        s.name = "Neha";
        s.roll = 21;
        s.display();
    }
}

Creating objects

• new Student() allocates heap memory, runs the constructor, and returns a reference.
• Several references can point to one object (Student t = s;). Assigning s = null does not delete the object if t still refers to it.
• Field defaults: 0 / false / null. Locals still need assignment.

Members

Identity, equals, and allocation

• == on objects is identity. Override equals and hashCode together when value equality matters (Objects.hash).
• new always creates a new object (except interned strings / cached wrappers you did not new).
• A class can have many objects; objects do not exist without a class (or an anonymous/local class, which still has a class file).
• Records (Java 16+) are compact immutable data classes with generated equals/hashCode/toString.

Two objects, independent state

public class Account {
    String owner;
    double balance;

    void deposit(double amount) {
        balance += amount;
    }

    public static void main(String[] args) {
        Account a = new Account();
        a.owner = "Ravi";
        a.deposit(500);
        Account b = new Account();
        b.owner = "Mia";
        b.deposit(120);
        System.out.println(a.owner + " " + a.balance);
        System.out.println(b.owner + " " + b.balance);
    }
}