Class Definition in Java

In object-oriented programming, a class is a basic building block. It can be defined as template that describes the data and behaviour associated with the class instantiation. Instantiating is a class is to create an object (variable) of that class that can be used to access the member variables and methods of the class.

A class can also be called a logical template to create the objects that share common properties and methods.

For example, an Employee class may contain all the employee details in the form of variables and methods. If the class is instantiated i.e. if an object of the class is created (say e1), we can access all the methods or properties of the class.

Defining a Class in Java

Java provides a reserved keyword class to define a class. The keyword must be followed by the class name. Inside the class, we declare methods and variables.

In general, class declaration includes the following in the order as it appears:

Modifiers: A class can be public or has default access.
class keyword: The class keyword is used to create a class.
Class name: The name must begin with an initial letter (capitalized by convention).
Superclass (if any): The name of the class's parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent.
Interfaces (if any): A comma-separated list of interfaces implemented by the class, if any, preceded by the keyword implements. A class can implement more than one interface.
Body: The class body surrounded by braces, { }.

Syntax:

<access specifier> class class_name 
{
// member variables 
// class methods 
}

Java Class Example

Example 1:

Let's consider the following example to understand how to define a class in Java and implement it with the object of class.

Calculate.java

// class definition
public class Calculate {
   
   // instance variables
   int a;
   int b;

   // constructor to instantiate
   public Calculate (int x, int y) {
      this.a = x;
      this.b = y;
   }

   // method to add numbers
   public int add () {
      int res = a + b;
      return res;
   }

   // method to subtract numbers
   public int subtract () {
      int res = a - b;
      return res;
   }

   // method to multiply numbers
   public int multiply () {
      int res = a * b;
      return res;
   }

   // method to divide numbers
   public int divide () {
      int res = a / b;
      return res;
   }


   // main method
   public static void main(String[] args) {
      // creating object of Class
      Calculate c1 = new Calculate(45, 4);
      
      // calling the methods of Calculate class
      System.out.println("Addition is :" + c1.add());
      System.out.println("Subtraction is :" + c1.subtract());
      System.out.println("Multiplication is :" + c1.multiply());
      System.out.println("Division is :" + c1.divide());


   }

Output:

Example 2:

In the following example, we are creating two classes Employee and EmployeeClass. The Employee class fetches and displays the employee details. In the EmployeeClass, we create the objects of Employee class and use its methods. Here, we are initializing the objects using the class constructor.

EmployeeClass.java

// class to get the employee details
class Employee {
	// declaring variables
	int emp_id;
	String name;
	String dept;
	float salary;

	// method to initialize the variables
	void add_info (int id, String n, String d, float sal) {
		this.emp_id = id;
		this.name = n;
		this.dept = d;
		this.salary = sal;
	}

	// method to display the employee details
	void display() {
		System.out.println("Employee id: " + emp_id );
		System.out.println("Employee name: " + name );
		System.out.println("Employee department: " + dept );
		System.out.println("Employee salary: " + salary );
	}
}

public class EmployeeClass {
	public static void main(String[] args) {
		// creating objects of class Employee
	Employee e1 = new Employee();  
    Employee e2 = new Employee();  
    Employee e3 = new Employee();  

    // calling the methods
    e1.add_info (101, "Naman", "Salesforce", 45000);  
    e2.add_info (102, "Riya", "Tax", 25000);  
    e3.add_info (103, "Anu", "Development", 55000);  
    
    e1.display();  
    e2.display();  
    e3.display();  
	}
}

Output: