Horstmann Chapter 7

Subtype Relationship

S is a subtype of T if

S and T are the same type
S and T are both class types, and T is a direct or indirect superclass of S
S is a class type, T is an interface type, and S or one of its superclasses implements T
S and T are both interface types, and T is a direct or indirect superinterface of S
S and T are both array types, and the component type of S is a subtype of the component type of T
S is not a primitive type and T is the type Object
S is an array type and T is Cloneable or Serializable
S is the null type and T is not a primitive type