I was being taught that Marker interface in Java is an empty interface and is used to signal to compiler or JVM that the objects of the class implementing this interface must be treated in a special way, like serializing, cloning, etc.
But lately I have learned that it actually has nothing to do with the compiler or the JVM. For example, in case of Serializable
interface the method writeObject(Object)
of ObjectOutputStream
does something like instanceOf Serializable
to detect whether the class implements Serializable
& throws NotSerializableException
accordingly.
Everything is handled in the code and this seems to be a design-pattern so I think we can define our own marker interfaces.
Now my doubts:
Is the definition of a marker interface mentioned above in 1st point wrong? How can we define a Marker interface then?
And instead of using the instanceOf
operator why can't the method be something like writeObject(Serializable)
so that there is a compile-time type checking rather than runtime?
How are Annotations better than Marker Interfaces?
writeObject(Serializable)
so that there is a compile-time type checking - This lets you avoid polluting your code with the name of the marker interface when a "plain Object
" is needed. For example, if you make a class that needs to be serializable, and has object members, you would be forced to either do casting or make your objects Serializable
at compile time. This is inconvenient, because the interface is devoid of any functionality.