Classes and objects are the core of Java’s Object-Oriented Programming (OOP). Understanding them is essential for freshers preparing for Java interviews.
This post covers 14 important interview questions on classes and objects with clear answers to help you succeed. Let’s get started!
1. What is a Class in Java?
A class is a blueprint or template for an object. It serves as a design for creating objects but does not occupy memory until an object is instantiated.
A class contains fields (variables) to store the data of an object, which represent the state of the object.
It also includes methods, which define the behavior of an object—essentially determining how the object will act.
Additionally, a class can include a constructor, which is a special method used to initialize an object.
Example :
// Defining a Class
class Bike{
// Instance Variables (State)
String brand;
int speed;
String color;
// Method (Behavior)
void display() {
System.out.println("Bike Brand: " + brand);
System.out.println("Bike Speed: " + speed);
System.out.println("Bike Color: " + color);
}
}2. What is an Object in Java?
Object is an instance of a class. It is real world entity that has state (attributes) and behavior (methods). Object stores actual value in the state and can call methods.
When we create an object of class then it occupy memory in the heap.
Each object has a unique memory address (identity).
We can create multiple objects from the same class.
Example :
// Defining a Class
class Bike {
// Fields (State)
String brand;
int speed;
// Constructor
Bike(String brand, int speed) {
this.brand = brand;
this.speed = speed;
}
// Method (Behavior)
void displayInfo() {
System.out.println("Brand: " + brand + ", Speed: " + speed + " km/h");
}
// Main Method
public static void main(String[] args) {
// Creating an Object of Bike class
Bike myBike = new Bike("Honda", 60);
// Accessing Object’s Method
myBike.displayInfo();
}
}
Explanation of Object Creation:
Bike myBike = new Bike("Honda", 60);
Bike→ Class namemyBike→ Object reference variablenew→ Allocates memory for the objectBike("Honda", 60)→ Calls the constructor and initializes the object
3. What is the Difference Between a Class and an Object?
| Sno. | Class | Object |
|---|---|---|
| 1 | Class is a template or blueprint or structure of the object | Object is an instance of the class with actual value and perform actions through methods |
| 2 | Class is created only one time | Objects are created multiple times |
| 3 | For define the class structure , class keyword is used | For making object , new keyword is used |
| 4 | Example : class Bike { String brand; int speed; } | Example : Bike myBike = new Bike(“Honda”, 60); |
| 5 | Class : car | Object : BMW, Toyota, Suzuki, Skoda |
4. How do you create a Class and an Object in Java?
Class is created with the help of class keyword. Syntax below :
Syntax of a Class:
class ClassName {
// Variables
// Methods
}
Object is created with the help of new keyword. Syntax below :
Syntax of an Object:
ClassName objectName = new ClassName();
Example of class and object is below:
Example:
class Person {
String firstName;
String lastName;
void display() {
System.out.println("First Name: " + firstName+ ", Last Name: " + lastName);
}
}
public class Main {
public static void main(String[] args) {
Person p1 = new Person(); // Object Creation
p1.firstName= "John";
p1.lastName= "Snow";
p1.display();
}
}
5. Why do we need Classes and Objects in Java?
Without classes and objects, Java would work like a procedural language (similar to C). Managing projects in procedural languages is a harder task because they lack the object-oriented programming (OOP) principles that Java follows.
Here are a few reasons why we need classes and objects in Java:
1. Encapsulation 🔒
✔ With the help of classes and objects, we can secure our data.
✔ We define private variables inside classes and access them using getter and setter methods.
2. Reusability 🔄
✔ Code written in one class can be used multiple times.
✔ A class is just a blueprint, and we can create multiple objects from the same class.
✔ This helps in removing code duplication.
3. Modularity 🛠️
✔ Developers can maintain code easily.
✔ Instead of writing a large block of code, we can divide it into smaller, manageable sections.
✔ During maintenance, it is easier to update and debug the code.
✔ Example: In a car simulation, we can have separate classes for Car, Engine, and Wheels.
4. Inheritance 🔄
✔ With inheritance, one class can extend another class.
✔ No need to rewrite code—we can reuse the features of one class in another using the extends keyword.
Classes and objects make Java more powerful, maintainable, and modular!
6. Can we create a Class without an Object?
Yes! We can create a class without an object. The code inside a class can be accessed in two ways:
- By creating an object of the class.
- By accessing the class directly (using
staticmethods and variables).
If a class contains only static methods or static variables, then you don’t need to create an object of that class. This is because static methods belong to the class itself, not to an object.
Example:
class Greeting {
// Static variable
static String message = "Hello, World!";
// Static method
static void sayHello() {
System.out.println(message);
}
}
public class Main {
public static void main(String[] args) {
// Accessing static variable and method without creating an object
System.out.println(Greeting.message); // Access static variable
Greeting.sayHello(); // Call static method
}
}
In this example we have not create any object of class Greeting.
message is a static variable, which is why we can access it using the class name Greeting.
sayHello() is a static method, which is why we access it using the class name Greeting.
7. How Many Objects Can Be Created from One Class?
There is no limit to the number of objects that can be created from a single class. You can create as many objects as you want from the same class.
A class is just a blueprint, while an object is a real entity or an actual instance of that class.
Example:
Bike bike1 = new Bike();
Bike bike2 = new Bike();
Bike bike3 = new Bike();
You can create any number of objects from a single class.
8. What are Real-Life Examples of Classes and Objects?
Example 1: Animal (Class) and Different Animals(Objects)
Class : Animal
Objects : Dog, Elephant, Fox, Lion
Example 2: Bike (Class) and Different Bikes (Objects)
Class : Bike
Objects : Honda, Pulsar, Hero
Example 3: Student (Class) and Different Students(Objects)
Class : Student
Objects : Arya, John, Nancy
9. How is Memory Allocated for an Object in Java?
When an object is created using the new keyword, memory is allocated in the heap. All objects are stored in the heap memory, and each object gets a unique memory address.
Bike bike1 = new Bike(); // Memory allocated in heap
10. What Happens if an Object is Not Referenced in Java?
If an object is not referenced in Java, it becomes eligible for Garbage Collection. This means the Garbage Collector (GC) will automatically remove the object from memory and free up space.
We can make an object unreferenced by setting its reference to null.
Bike b1 = new Bike();
b1 = null; // Now the object is unreferenced
11. How Many Types of Objects Are There in Java?
Many types of Objects in Java.
1. Mutable Objects : Mutable objects are those whose state can be changed after creation.
Example : String Builder
StringBuilder sb = new StringBuilder("Hello");
sb.append(" World"); // Modifies the existing object
System.out.println(sb); // Output: Hello World
2. Immutable Objects : Immutable objects cannot be modified after creation. Any modification creates a new object instead of changing the existing one.
Example : String
String str = "Java";
str.concat(" Programming"); // Creates a new object, original remains unchanged
System.out.println(str); // Output: Java
3. Anonymous Objects : Objects created without using reference variable and used immediately.
class Bike{
void display() {
System.out.println("Bike is running");
}
}
new Bike().display(); // Output: Bike is running
4. Singleton Objects : A Singleton Object is an object that can have only one instance in the entire program. This means no matter how many times you try to create it, you’ll always get the same object.
Runtime instance1 = Runtime.getRuntime();
Runtime instance2 = Runtime.getRuntime();
System.out.println(instance1 == instance2); // Output: true
12. What is an Instance in Java?
When we create an object of a class, we say that an instance of the class is created. An instance is simply another term for an object of a class.
13. What is the Syntax of an Object in Java?
Syntax :
Syntax: ClassName objectName = new ClassName();Example :
Bike b1 = new Bike();
14. How Do You Create Multiple Objects from a Class?
Bike bike1= new Bike();
Bike bike2= new Bike();
Bike bike3= new Bike();
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