Introduction

• Relationship between classes describes that how multiple classes interact with or depend on each other.
• These relationships help structure and organize code in a logical and maintainable way.
• Types of relationships between classes in Java:
  1. Association (HAS-A relationship)
  2. Dependency (USES-A relationship)
  3. Inheritance (IS-A relationship)

1. Association (HAS-A relationship)

• Definition : Association is a relationship where one class interacts with another class.
• Example :
  • Student HAS-A Address
  • Car HAS-A Engine.
• How to achieve association ?
  • Association is achieved by declaring object references as instance variables inside a class.
• Program :

  class Address
  {
      String city = "Delhi";
      String country = "India";
  
      void displayAddress()
      {
          System.out.println("City: " + city + ", Country: " + country);
      }
  }
  
  class Student
  {
      String name = "Deepak";
      int rollno = 101;
  
      // Direct reference to another class
      Address address = new Address();  // Object created directly inside the class
  
      void displayInfo()
      {
          System.out.println("Name: " + name + ", Roll No: " + rollno);
          address.displayAddress();
      }
  }
  
  public class MainApp
  {
      public static void main(String[] args)
      {
          Student student = new Student();  // No need to pass Address
          student.displayInfo();            // Displays student info along with address
      }
  }

  Output:

  Name: Deepak, Roll No: 101
  City: Delhi, Country: India

• Click here to read more about Association, Composition & Aggregation in Java.

2. Dependency (USES-A relationship)

• Definition :
  • Dependency is a relationship where one class uses another class to perform a specific task.
  • It typically exists when one class depends on another temporarily, usually within a method.
• Example : Office Worker USES-A Printer.
• How to achieve dependency ?
  • Dependency can be achieved through:
    • Local variables inside methods
    • Method parameters
• Program :

  class Printer
  {
      void printDocument(String doc)
      {
          System.out.println("Printing document: " + doc);
      }
  }
  
  class OfficeWorker
  {
      void doWork()
      {
          Printer printer = new Printer(); // Dependency via local variable
          printer.printDocument("ProjectReport.pdf");
          System.out.println("Work completed.");
      }
  }
  
  public class MainApp
  {
      public static void main(String[] args)
      {
          OfficeWorker worker = new OfficeWorker();
          worker.doWork(); // OfficeWorker depends on Printer to print
      }
  }

  Output:

  Printing document: ProjectReport.pdf
  Work completed.

• Click here to read more about Dependency in Java.

3. Inheritance (IS-A relationship)

• Definition : IS-A (Inheritance) is the process by which a child class (subclass) inherits fields and methods from a parent class (superclass).
• Example : A Car IS-A Vehicle.
• How to achieve inheritance ?
  • Inheritance is achieved using:
    • extends keyword in case of classes.
    • implements keyword in case of interfaces.
• Program :

  class Vehicle
  {
      void start()
      {
          System.out.println("Vehicle starts.");
      }
  }
  
  class Car extends Vehicle
  {
      void drive()
      {
          System.out.println("Car drives.");
      }
  }
  
  public class MainApp
  {
      public static void main(String[] args)
      {
          Car myCar = new Car();
  
          myCar.start(); // inherited from Vehicle
          myCar.drive(); // specific to Car
      }
  }

  Output:

  Vehicle starts.
  Car drives.

• Click here to read more about Inheritance in Java.