5.1.4 Widening Reference Conversions

The following conversions are called the widening reference conversions:

From any class type S to any class type T, provided that S is a subclass of T. (An important special case is that there is a widening conversion to the class type Object from any other class type.)
From any class type S to any interface type K, provided that S implements K.
From the null type to any class type, interface type, or array type.
From any interface type J to any interface type K, provided that J is a subinterface of K.
From any interface type to type Object.
From any array type to type Object.
From any array type to type Cloneable.
From any array type SC[] to any array type TC[], provided that SC and TC are reference types and there is a widening conversion from SC to TC.

Such conversions never require a special action at run time and therefore never throw an exception at run time. They consist simply in regarding a reference as having some other type in a manner that can be proved correct at compile time.

See §8 for the detailed specifications for classes, §9 for interfaces, and §10 for arrays.