Why there are some null interface in JAVA? What does it mean? Give some null interface in JAVA?

Question

In Java
I am using statement a += b where a is int and b is double, when I use a += b, it won't give error but in case of a = a + b it gives error, why ?

Answer 1

Null interfaces are markable interfaces and it act as markers.it tells the compiler that the objects of this class need to be treated differently. some examples of null interfaces in java are:
1.Cloneable
2.Runnable
3.Serializable
4.Externalizable.
5.SingleThreadModel,
6.Event listener