Class & Object in Java

Classes and objects are the heart of Java. A class is a blueprint that defines properties (fields) and behaviors (methods). An object is a concrete instance of a class, with its own state. Understanding classes and objects unlocks all of object-oriented programming.

• Declare a class: class ClassName { fields; methods; }
• Create object: ClassName obj = new ClassName();
• Access field: obj.field
• Call method: obj.method()

Let's start with a hands-on example. The program below shows the core idea behind Class and Object in just a few lines of Java. Read it line by line and observe how Java executes each statement in order.

class Student
{
    String name;
    int    age;

    void show()
    {
        System.out.println(name + " : " + age);
    }
}

class Main
{
    public static void main(String args[])
    {
        Student s = new Student();
        s.name = "Aman";
        s.age  = 21;
        s.show();
    }
}

The following examples progress from simple to more practical patterns. Try each in your IDE, change the inputs and observe the output. This is the fastest way to internalize the concept.

class Book
{
    String title;
    double price;

    Book(String t, double p) { title = t; price = p; }

    void print() { System.out.println(title + " - " + price); }
}

class Main
{
    public static void main(String args[])
    {
        Book b = new Book("Java Basics", 299.0);
        b.print();
    }
}

class Car
{
    String model;
    int    year;

    Car(String m, int y) { model = m; year = y; }

    void show() { System.out.println(model + " (" + year + ")"); }
}

class Main
{
    public static void main(String args[])
    {
        Car a = new Car("Swift", 2022);
        Car b = new Car("Civic", 2024);

        a.show();
        b.show();
    }
}

class Person
{
    String name;
    int    age;

    Person(String name, int age)
    {
        this.name = name;
        this.age  = age;
    }

    void show() { System.out.println(name + " " + age); }
}

class Main
{
    public static void main(String args[])
    {
        new Person("Riya", 23).show();
    }
}

class Counter
{
    static int count = 0;

    Counter() { count++; }
}

class Main
{
    public static void main(String args[])
    {
        new Counter(); new Counter(); new Counter();

        System.out.println("Total = " + Counter.count);
    }
}

• Class names follow PascalCase: Student, BankAccount.
• Always initialize fields — Java assigns defaults (0, null, false) if not.
• Use this to disambiguate field vs parameter with same name.
• An object lives on the heap; its reference is on the stack.
• When no references remain, the garbage collector reclaims the object.

Reading is not enough — practice solidifies knowledge. Try every question below in your own editor before peeking at any solution.

• Q1. Create a Rectangle class with length, breadth and area method.
• Q2. Create a Student class with marks of three subjects and an average method.
• Q3. Design a BankAccount class with deposit and withdraw methods.
• Q4. Add a static counter that tracks how many objects of a class have been created.
• Q5. Build a Library class containing an array of Book objects.

These are the most common questions asked in Java interviews on this topic. Memorize the concept, not just the answer — interviewers often follow up with edge cases.

Even experienced developers slip on the same pitfalls. Watch out for these classic mistakes while working with Class and Object in Java:

• Forgetting the semicolon ; at the end of statements while using Class and Object.
• Mixing up similar method names — read Java docs before using new APIs related to Class and Object.
• Ignoring compiler warnings — they often hint at bugs that will appear later in production.
• Hard-coding values that should come from configuration files or environment variables.
• Not handling edge cases: empty inputs, very large inputs, negative numbers and null references.
• Skipping unit tests — small tests prevent big regressions, especially around control flow.