8.4.6 Inheritance, Overriding, and Hiding

8.4.6 Inheritance, Overriding, and Hiding

A class inherits from its direct superclass and direct superinterfaces all the methods (whether abstract or not) of the superclass and superinterfaces that are accessible to code in the class and are neither overridden (§8.4.6.1) nor hidden (§8.4.6.2) by a declaration in the class.

8.4.6.1 Overriding (By Instance Methods)

If a class declares an instance method, then the declaration of that method is said to override any and all methods with the same signature in the superclasses and superinterfaces of the class that would otherwise be accessible to code in the class. Moreover, if the method declared in the class is not abstract, then the declaration of that method is said to implement any and all declarations of abstract methods with the same signature in the superclasses and superinterfaces of the class that would otherwise be accessible to code in the class.

A compile-time error occurs if an instance method overrides a static method. In this respect, overriding of methods differs from hiding of fields (§8.3), for it is permissible for an instance variable to hide a static variable.

An overridden method can be accessed by using a method invocation expression (§15.11) that contains the keyword super. Note that a qualified name or a cast to a superclass type is not effective in attempting to access an overridden method; in this respect, overriding of methods differs from hiding of fields. See §15.11.4.10 for discussion and examples of this point.

8.4.6.2 Hiding (By Class Methods)

If a class declares a static method, then the declaration of that method is said to hide any and all methods with the same signature in the superclasses and superinterfaces of the class that would otherwise be accessible to code in the class. A compile-time error occurs if a static method hides an instance method. In this respect, hiding of methods differs from hiding of fields (§8.3), for it is permissible for a static variable to hide an instance variable.

A hidden method can be accessed by using a qualified name or by using a method invocation expression (§15.11) that contains the keyword super or a cast to a superclass type. In this respect, hiding of methods is similar to hiding of fields.

8.4.6.3 Requirements in Overriding and Hiding

If a method declaration overrides or hides the declaration of another method, then a compile-time error occurs if they have different return types or if one has a return type and the other is void. Moreover, a method declaration must not have a throws clause that conflicts (§8.4.4) with that of any method that it overrides or hides; otherwise, a compile-time error occurs. In these respects, overriding of methods differs from hiding of fields (§8.3), for it is permissible for a field to hide a field of another type.

The access modifier (§6.6) of an overriding or hiding method must provide at least as much access as the overridden or hidden method, or a compile-time error occurs. In more detail:

• If the overridden or hidden method is public, then the overriding or hiding method must be public; otherwise, a compile-time error occurs.
• If the overridden or hidden method is protected, then the overriding or hiding method must be protected or public; otherwise, a compile-time error occurs.
• If the overridden or hidden method has default (package) access, then the overriding or hiding method must not be private; otherwise, a compile-time error occurs.

Note that a private method is never accessible to subclasses and so cannot be hidden or overridden in the technical sense of those terms. This means that a subclass can declare a method with the same signature as a private method in one of its superclasses, and there is no requirement that the return type or throws clause of such a method bear any relationship to those of the private method in the superclass.

8.4.6.4 Inheriting Methods with the Same Signature

It is possible for a class to inherit more than one method with the same signature (§8.4.6.4). Such a situation does not in itself cause a compile-time error. There are then two possible cases:

• If one of the inherited methods is not abstract, then there are two subcases:
  • If the method that is not abstract is static, a compile-time error occurs.
  • Otherwise, the method that is not abstract is considered to override, and therefore to implement, all the other methods on behalf of the class that inherits it. A compile-time error occurs if, comparing the method that is not abstract with each of the other of the inherited methods, for any such pair, either they have different return types or one has a return type and the other is void. Moreover, a compile-time error occurs if the inherited method that is not abstract has a throws clause that conflicts (§8.4.4) with that of any other of the inherited methods.
• If none of the inherited methods is not abstract, then the class is necessarily an abstract class and is considered to inherit all the abstract methods. A compile-time error occurs if, for any two such inherited methods, either they have different return types or one has a return type and the other is void. (The throws clauses do not cause errors in this case.)

It is not possible for two or more inherited methods with the same signature not to be abstract, because methods that are not abstract are inherited only from the direct superclass, not from superinterfaces.

There might be several paths by which the same method declaration might be inherited from an interface. This fact causes no difficulty and never, of itself, results in a compile-time error.