Subtype Relationship

• 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

You can substitute a subtype whenever a supertype value is expected.