Dependency (USES-A relationship)

• Definition :
  • Dependency is a relationship where one class uses another class temporarily to perform a specific task. This means the dependent object is often used within a method, rather than being stored as an instance variable.
  • It represents a USES-A relationship.
  • The dependency is typically short-lived, existing only during the execution of a method.
• Example :
  • Teacher USES-A Whiteboard
  • Office Worker USES-A Printer
• How to achieve dependency ?
  • Dependency is achieved by creating or using objects of another class inside a method, instead of holding them as instance variables.
  • We can inject dependent objects using:
    1. Local Variable inside Method
       • The dependent object is created directly in a method
    2. Method Parameter
       • The dependent object is passed as a method argument
       • It promotes more flexibility and testability
• Program 1 (Dependency using Local Variable inside Method) :

  // Dependent class
  class Whiteboard
  {
      void writeOnBoard()
      {
          System.out.println("Writing on the whiteboard...");
      }
  }
  
  // Main class that uses Whiteboard
  class Teacher
  {
      void teachLesson()
      {
          // Local variable: Dependency created inside the method
          Whiteboard board = new Whiteboard();
          board.writeOnBoard();  // Temporary usage
          System.out.println("Teacher is explaining the topic.");
      }
  }
  
  // Entry point
  public class MainApp
  {
      public static void main(String[] args)
      {
          Teacher teacher = new Teacher();
          teacher.teachLesson();  // Trigger method that shows dependency
      }
  }

  Output:

  Writing on the whiteboard...
  Teacher is explaining the topic.

• Program 2 (Dependency using Method Parameter) :

  // Dependent class
  class Printer
  {
      void printDocument()
      {
          System.out.println("Printing document...");
      }
  }
  
  // Main class that depends on Printer
  class OfficeWorker
  {
      // Dependency injected via method parameter
      void performTask(Printer printer)
      {
          printer.printDocument();  // Temporary usage
          System.out.println("OfficeWorker has completed printing task.");
      }
  }
  
  // Entry point
  public class MainApp
  {
      public static void main(String[] args)
      {
          Printer printer = new Printer();            // Create dependency
          OfficeWorker worker = new OfficeWorker();   // Create dependent
  
          // Inject dependency via method parameter
          worker.performTask(printer);
      }
  }

  Output:

  Printing document...
  OfficeWorker has completed printing task.

Dependency vs Association: