How to write our own marker interface in Java?

Question

I know marker interface in java. It is used to define a specific behaviour about a class. For example, Serializable interface has the specific ability to store an object into byte stream and its reverse process. But I don't know where this specific behaviour is implemented, because it doesn't have any method in it.

1. How JVM invoke this specific behaviour?
2. How to write our own marker interface? Can you give me a simple user defined marker interface for my understanding?
3. Is it possible to have methods in marker interface?

Please guide me to resolve this issue.

Answer 1

• How JVM invoke this specific behavior

ObjectOutputStream and ObjectInputStream will check your class whether or not it implementes Serializable, Externalizable. If yes it will continue or else will thrown NonSerializableException.

• How to write our own marker interface

Create an interface without any method and that is your marker interface.

Sample

public interface IMarkerEntity {


}

If any class which implement this interface will be taken as database entity by your application.

Sample Code:

public boolean save(Object object) throws InvalidEntityException {
   if(!(object instanceof IMarkerEntity)) {
       throw new InvalidEntityException("Invalid Entity Found, cannot proceed");
   } 
   database.save(object);
}

• Is this possible to have methods in marker interface?

The whole idea of Marker Interface Pattern is to provide a mean to say "yes I am something" and then system will proceed with the default process, like when you mark your class as Serialzable it just tells that this class can be converted to bytes.