Marker interface is main frequently asked question to JAVA interview. Generally interviewer asked this question to check internal knowledge of JAVA developer some times ask on architect level also because it’s follow the Marker Interface Design pattern. Here I have covered everything to crack questions related to Marker Interface.

What is a Marker Interface?

Marker Interfaces are empty interface it does not contains any properties and behaviors to implement. It’s also called as marker or the tag interface.

Why Marker Interface and Who Implement it?

Marker interface functionality is predefined implicitly in JVM. When a class implement a marker interface then class is not expected to implement anything to adhere to the contract defined by the interface. In contract it is a tag to indicate the JVM regarding an expected functionality to perform implicitly.

For Example :

java.io.Serializable : If class implements the Serializable interface then JVM perform some special operation on it and writes the state of the object into object stream then this object stream will available to read by another JVM.

See Also:

• Java: Serialization and Deserialization

java.lang.Cloneable: Same way if class is implementing Cloneable interface then it perform some special operation to clone object by copy all fields.

See Also:

• Java : Shallow and Deep Object Cloning
• Java : Shallow Cloning Vs Deep Cloning

What are available Marker Interfaces in JAVA?

Mainly used built-in marker interfaces are as below :

• java.lang.Cloneable
• java.io.Serializable
• java.rmi.Remote
• java.util.EventListener (its officially know as ‘tagging interface’)

There are some more marker interface present in Java.

• java.util.concurrent.CompletableFuture.AsynchronousCompletionTask
• java.sql.ParameterMetaData
• javax.xml.stream.events.EndDocument
• javax.management.loading.PrivateClassLoader
• java.security.KeyStore.Entry
• java.security.KeyStore.LoadStoreParameter
• java.security.KeyStore.ProtectionParameter
• java.security.Policy.Parameters
• javax.security.auth.callback.Callback
• javax.security.auth.login.Configuration.Parameter

Can we create custom marker interfaces ?

Yes

How to Create Custom marker Interfaces?

public interface MyMarker {
//no properties and method
}

public class MyClass implements MyMarker{
//define properties and method
}

public class TestMarker {

	public static void main(String[] args) {
		MyClass myClass=new MyClass();

		if(myClass instanceof MyMarker)
		{
			System.out.println("I am special treat me VIP");
		}

	}

}

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