Classes and Objects

Declaring a Class

A class declaration consists of an optional access modifier, the class keyword, a name following PascalCase convention, and a body enclosed in braces containing fields, constructors, and methods. Fields declared inside a class but outside any method are called instance variables, and each object created from the class gets its own independent copy of them.

Creating Objects

An object is created using the new keyword followed by a constructor call: ClassName obj = new ClassName();. This statement does three things in order: it allocates memory on the heap for the object, calls the constructor to initialize its fields, and assigns the resulting memory reference to the variable obj. The variable itself does not hold the object — it holds a reference to where the object lives in memory.

Accessing Members

Once you have an object reference, you access its fields and methods using the dot (.) operator: obj.fieldName or obj.methodName(). Each object maintains its own copy of instance variables, so changing a field on one object never affects another object created from the same class.

• A class is compiled once; objects are created as many times as needed at runtime.
• Each object has its own instance variables but shares the same method code defined in the class.
• An uninitialized object reference holds null until assigned with new.

State, Behavior and Identity

Every object has three characteristics. Its state is the current value of its fields, such as a car's model and speed. Its behavior is what its methods can do, such as accelerate(). Its identity is the fact that it is a distinct object in memory: two cars can have exactly the same model and speed and still be two different objects.

References: Two Variables, One Object

Because a variable holds a reference rather than the object itself, assigning one object variable to another copies the reference, not the object. After Car b = a; there is still only one Car on the heap, and a and b both point to it, so a change made through b is visible through a. To get a second, independent object you must call new again.

The == operator on object variables compares these references. It is true only when both variables point to the same object, even if two separate objects hold identical field values. The second example below shows both cases.

Examples

public class Student {
    String name;
    int marks;

    void showDetails() {
        System.out.println(name + " scored " + marks);
    }

    public static void main(String[] args) {
        Student s1 = new Student();
        s1.name = "Riya";
        s1.marks = 88;

        Student s2 = new Student();
        s2.name = "Kabir";
        s2.marks = 92;

        s1.showDetails();
        s2.showDetails();
    }
}

Riya scored 88
Kabir scored 92

public class Car {
    String model;
    int speed;

    void accelerate(int amount) {
        speed += amount;
    }

    public static void main(String[] args) {
        Car a = new Car();
        a.model = "Swift";

        Car b = a; // no new object: b refers to the same Car as a
        b.accelerate(40);
        System.out.println(a.model + " speed read through a: " + a.speed);

        Car c = new Car(); // a separate object with the same field values
        c.model = "Swift";
        c.speed = 40;
        System.out.println("a == b: " + (a == b));
        System.out.println("a == c: " + (a == c));
    }
}

Swift speed read through a: 40
a == b: true
a == c: false

Exercise

Create a Book class with a title and a page count, and a method isLong() that returns true when the book has more than 300 pages. Create two books and print the result for each.

Expected output: Clean Code: true then Java Basics: false

class Book {
    String title;
    int pages;

    // TODO: isLong() returns true when pages is greater than 300
}

public class BookDemo {
    public static void main(String[] args) {
        // TODO: create a Book titled "Clean Code" with 464 pages
        // TODO: create a Book titled "Java Basics" with 180 pages
        // TODO: print "<title>: <isLong()>" for each
    }
}

class Book {
    String title;
    int pages;

    boolean isLong() {
        return pages > 300;
    }
}

public class BookDemo {
    public static void main(String[] args) {
        Book first = new Book();
        first.title = "Clean Code";
        first.pages = 464;

        Book second = new Book();
        second.title = "Java Basics";
        second.pages = 180;

        System.out.println(first.title + ": " + first.isLong());
        System.out.println(second.title + ": " + second.isLong());
    }
}

Quick quiz

Interview questions