Introduction

• A Marker Interface is an interface that has no methods and no fields.
• It is used to "mark" a class, giving a special meaning or behavior to the class which implements it.
• The Java compiler and JVM use this marker to decide how to treat objects of that class.
• Examples of marker interfaces in Java:
  • java.io.Serializable
  • java.lang.Cloneable
  • java.util.RandomAccess

• Key Points:
  • There are no abstract methods in a marker interface.
  • They serve as a metadata tag that can be checked at runtime using instanceof operator or reflection.
  • The real work is done by JVM or API classes that check whether the object is marked with that interface.

Uses of Marker Interfaces

• To give permission to a class for some special operation.
  • Example: A class must implement Serializable to be eligible for object serialization.
• To request special behavior from JVM or libraries.
  • Example: If a class implements Cloneable, then Object.clone() method will perform cloning, otherwise it throws CloneNotSupportedException.
• To group classes logically for a specific purpose.

Program:

• Program 1: Example of a Marker Interface:

  // Marker Interface
  interface Marker { }
  
  // Class implementing Marker
  class MyClass implements Marker
  {
      public void display()
      {
          System.out.println("Display method called");
      }
  }
  
  public class MainApp
  {
      public static void main(String[] args)
      {
          MyClass obj = new MyClass();
  
          if (obj instanceof Marker) {
              obj.display();
          } else {
              System.out.println("Object not allowed");
          }
      }
  }

  Output (sample):

  Display method called