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Marker Interface in Java with Examples

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Get ready to explore the fascinating world of marker interfaces in Java! In the Java language, an interface is a reference type similar to a class that can contain only constants, method signatures, default methods, static methods, and nested types. It is essentially a contract that defines a set of methods without providing an implementation for those methods. In this article, we will explore the concept of marker Interfaces and understand their significance in Java programming.

What is a Marker Interface?

Marker interfaces are interfaces that do not have fields, methods, or constants. In other words, a marker or tag interface is an empty interface. The marker interface provides information about an object’s run-time type. Marker interfaces are also known as tag interfaces. They don’t provide any methods or constants to implement, but instead, they serve as a marker for a class.

The interface for a marker must be empty, but the declaration is the same as for an interface in Java.

Syntax of Marker Interface in Java

Here is the syntax of the Marker Interface in Java



public class MyClass implements MarkerInterfaceName {

// Class Implementation

}

Also Read: Hierarchical Inheritance in Java

JDK Marker Interfaces

In Java language, There are many built-in marker interfaces such as Serializable, Cloneable, and Remote

Cloneable Interface: This interface is present in java.lang.package. There is method clone() in the Object() class. This class implements a Cloneable interface, indicating that it is legal for the clone() method to make a field for the field

copy of instances of that class. The ‘Cloneable’ interface does not contain any methods. It’s simply a marker interface to denote that a class is eligible for cloning

The following program demonstrates the Cloneable Interface.

Program



    class MyClass implements Cloneable {

        private int value;
    
     
    
        public MyClass(int value) {
    
            this.value = value;
    
        }
    
     
    
        public void setValue(int value) {
    
            this.value = value;
    
        }
    
     
    
        public int getValue() {
    
            return value;
    
        }
    
     
    
        @Override
    
        public Object clone() throws CloneNotSupportedException {
    
            return super.clone();
    
        }
    
    }
    
     
    
    public class InterfaceExample {
    
        public static void main(String[] args) {
    
            MyClass obj1 = new MyClass(343);
    
     
    
            try {
    
                MyClass obj2 = (MyClass) obj1.clone();
    
                System.out.println(“Original value: “ + obj1.getValue());
    
                System.out.println(“Cloned value: “ + obj2.getValue());
    
            } catch (CloneNotSupportedException e) {
    
                e.printStackTrace();
    
            }
    
        }
    
    }

Output



Original value: 343

Cloned value: 343

Serializable interface: This interface is present in the java.io package and is used to make an object eligible for saving its state into a file. It means its instance can be converted into a stream of bytes that can be saved to a file or sent over a network. This process is called serialization.

The following program Demonstrates the Serializable. This example creates two files xyz.txt file in your current working directory.

Program



    // Java Serializable Example in Programming

 

    import java.io.*; 
    
     
    
    class Box implements Serializable
    
    {
    
         String name ;
    
         int number ;
    
     
    
        // A class constructor
    
        public Box(String name,int number)
    
        {
    
            this.name = name ;
    
            this.number = number ;
    
        }
    
    } 
    
    public class SerializableExample{
    
        public static void main(String[] args)
    
          throws IOException, ClassNotFoundException  { 
    
     
    
            Box ob = new Box(“Harry Potter”,20); 
    
          
    
            FileOutputStream fos = new FileOutputStream(“xyz.txt”);
    
            ObjectOutputStream oos = new ObjectOutputStream(fos);
    
            oos.writeObject(ob);
    
     
    
            FileInputStream fis = new FileInputStream(“xyz.txt”);
    
            ObjectInputStream ois = new ObjectInputStream(fis);
    
            Box ob2 = (Box)ois.readObject();
    
     
    
            System.out.println(ob2.name+” “+ob2.number);
    
     
    
            oos.close();
    
            ois.close();
    
        }
    
    }

Output



Harry Potter 20

Remote Interface: This interface is present in the Java.rmi package. A ‘remote interface’ typically refers to an interface intended for use in remote method invocation(RMI). This method of an object running in another JVM (Java Virtual Machine). It is possibly a different physical machine.

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Custom Marker Interface

In this section, we will create our own custom marker interface in Java using the example below. In this example, we have 2 classes: teachers and Students. We classify the objects of these 2 classes in 2 different lists using the marker interface.

The following program demonstrates the Custom Marker interface

Program



    import java.util.*; 

    interface StudentMarkerInterface { 
    
    } 
    
    class Teacher implements StudentMarkerInterface {
    
        int teacher_id;
    
        String name; 
    
        Teacher(int teacher_id, String name) {
    
            this.teacher_id = teacher_id;
    
            this.name = name;
    
        }
    
     
    
        public String toString() {
    
            return “[ID: “ + teacher_id + “, Name: “ + name + “] “;
    
        }
    
     
    
    }
    
     
    
    class Student {
    
        int student_id;
    
        String name;
    
     
    
       public Student(int student_id, String name) {
    
            this.student_id = student_id;
    
            this.name = name;
    
        }
    
     
    
        public String toString(){
    
            return “[ID: “ + student_id + “, Name: “ + name + “] “;
    
        }
    
    }
    
     
    
    class CustomMarkerInterface {
    
     
    
        static List<Teacher> teacherlist = new ArrayList<>();
    
        static List<Student> studentlist = new ArrayList<>();
    
     
    
        public static boolean save(Object object) {
    
            if (object instanceof StudentMarkerInterface) {
    
                teacherlist.add((Teacher)object) ;
    
            } else {
    
                studentlist.add((Student) object);
    
            }
    
     
    
            return true;
    
        }
    
     
    
        public static void main(String args[]) {
    
            Teacher ob = new Teacher(1, “Katrina Kaif”);
    
            save(ob) ;
    
            Teacher ob2 = new Teacher(2, “Sonam”);
    
            save(ob2) ;
    
            Student ob3 = new Student(4, “Sonali”);
    
            save(ob3) ;
    
            Student ob4 = new Student(5, “Aditi”);
    
            save(ob4);
    
            System.out.println(“Teacher List” + teacherlist);
    
            System.out.println(“Student List” + studentlist);
    
     
    
        }
    
    }

Output



Teacher List[[ID: 1, Name: Katrina Kaif] , [ID: 2, Name: Sonam] ]

Student List[[ID: 4, Name: Sonali]] , [ID: 5, Name: Aditi] ]<strong> </strong>

Also Read: What is Palindrome Number in Java?

Marker Interface vs Annotations

The following table differentiates the Marker Interface and Annotation in Java language:

Feature	Marker Interface	Annotations
Definition	Interfaces without methods serving as markers	Declarations that can be applied to Java elements
Usage	Denote participation in certain functionalities.	Carry metadata, specify attributes, and retention
Examples	Serializable’, ‘Cloneable’, ‘Remote’ etc	‘@Override’, ‘@Deprected’, @SuppressingWarnings, etc.
Flexibility	Limited, cannot carry additional features	Highly flexible, support for attributes.
Retention	Not applicable, always available at runtime	Can specify retention policies (source, class, runtime)
Reflection	Not required for basic usage	It is often used for reflection for runtime introspection.
Complexity	Simple to define and use in Java	It is more complex due to attributes and policies

Marker Interfaces vs Typical Interfaces

In this section, we will discuss the Marker Interfaces and Typical Interfaces differences. The following table demonstrates the Marker Interface and Typical Interface:

Feature	Marker Interface	Typical Interfaces
Definition	Interfaces without methods	Interface with one or more abstract methods
Purpose	It indicates certain characteristics or behaviors	It defines a contract for classes to implement methods
Usage	Provide metadata about classes at compile or runtime	It specifies a set of methods that classes must implement
Examples	‘Serializable’, ‘Cloneable’ etc	‘Comparable’, ‘Runnable’, and ‘Iterable’ etc.
Methods	No methods only used for marking	Contains one or more abstract methods to implement
Implementation	Implementing classes inherit marker characteristics	Implementing classes must provide a method of implementation
Flexibility	Limited, cannot carry additional behavior	Highly flexible, can define any required behavior
Extensibility	Cannot add methods or modify behavior	Highly extensible methods or modified behavior

Conclusion

In this article, we have learned about the Marker interfaces in Java that serve as mechanisms to provide metadata about classes, indicating certain characteristics or behaviors without requiring the implementation of any methods. We also learned about some examples of Marker Interfaces, such as “Serializable,” “Cloneable,” and “Remote.” By implementing these interfaces, classes denote their ability to be serialized, cloned, or used in remote method invocation, respectively.

FAQs

What is a marker interface in Java language?

A marker interface in Java does not declare any methods. Its sole purpose is to mark or identify a class for some special treatment or behavior.

What is the difference between a marker interface and a runnable interface?

The runnable interface is not a maker interface because it has the public void run() method declared inside it. A marker interface example is Serializable, where the class implements can be used with ObjectOutputStream and ObjectInputStream.

Is serializable a marker interface?

Yes, ‘Serializable’ is a marker interface in Java language. It doesn’t declare any methods. It serves as a marker to indicate that the objects of the class implementing it can be serialized. Serialization is the process of converting an object into a stream.

What can we use instead of a marker interface in Java?

There is an alternative available in Java language. We can use annotation instead of maker interface, which is metadata that can be attached to classes, methods, fields, or parameters.

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