... A null interface is an interface with no method declaration. I don't really like this term since "null" has a special meaning in Java. I would prefer calling such an interface an "empty interface" or better a "tag interface" but not a "null interface". Maybe someone else has an idea as to why such interfaces have been called "null interfaces". I don't know who invented that but "null interface" does not exist in the official Java terminology. [ December 10, 2002: Message edited by: Valentin Crettaz ]
Marker interface, also known as Tag interface, are like directives to the compiler without any method declaration. It is implemented in a class so that other java codes and JVM can test an object of it with "instanceof interface". java.io.Serializable, java.lang.Cloneable, java.util.EventListener, java.util.RandomAccess are marker interfaces from J2SE API that I can recall quickly.
To support with an example from the J2SE API Documentation, you'll see that Serializable is a marker interface, in its comment its written that All subtypes of a serializable class are themselves serializable.
BTW shewta, ArrayList is a concrete final class, its not an interface rather an implementation of RandomAccess marker interface.
Ashik Uzzaman Lead Member of Technical Staff, Salesforce.com, San Francisco, CA, USA.