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Trail: Learning the Java Language
Lesson: Interfaces and Inheritance
Section: Inheritance
Interfaces and Inheritance
Interfaces
Defining an Interface
Implementing an Interface
Using an Interface as a Type
Rewriting Interfaces
Summary of Interfaces
Questions and Exercises
Inheritance
Overriding and Hiding Methods
Hiding Fields
Using the Keyword super
Object as a Superclass
Writing Final Classes and Methods
Abstract Methods and Classes
Summary of Inheritance
Questions and Exercises
Home Page > Learning the Java Language > Interfaces and Inheritance
Overriding and Hiding Methods

> ### Instance Methods

> An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method.

> The ability of a subclass to override a method allows a class to inherit from a superclass whose behavior is "close enough" and then to modify behavior as needed. The overriding method has the same name, number and type of parameters, and return type as the method it overrides. An overriding method can also return a subtype of the type returned by the overridden method. This is called a covariant return type.

> When overriding a method, you might want to use the `@Override` annotation that instructs the compiler that you intend to override a method in the superclass. If, for some reason, the compiler detects that the method does not exist in one of the superclasses, it will generate an error. For more information on `@Override`, see `Annotations`.

> ### Class Methods

> If a subclass defines a class method with the same signature as a class method in the superclass, the method in the subclass hides the one in the superclass.

> The distinction between hiding and overriding has important implications. The version of the overridden method that gets invoked is the one in the subclass. The version of the hidden method that gets invoked depends on whether it is invoked from the superclass or the subclass. Let's look at an example that contains two classes. The first is `Animal`, which contains one instance method and one class method:

> >     public class Animal {
> >         public static void testClassMethod() {
> >             System.out.println("The class method in Animal.");
> >         }
> >         public void testInstanceMethod() {
> >             System.out.println("The instance method in Animal.");
> >         }
> >     }

> The second class, a subclass of `Animal`, is called `Cat`:

> >     public class Cat extends Animal {
> >         public static void testClassMethod() {
> >             System.out.println("The class method in Cat.");
> >         }
> >         public void testInstanceMethod() {
> >             System.out.println("The instance method in Cat.");
> >         }

> >         public static void main(String[] args) {
> >             Cat myCat = new Cat();
> >             Animal myAnimal = myCat;
> >             Animal.testClassMethod();
> >             myAnimal.testInstanceMethod();
> >         }
> >     }

> The `Cat` class overrides the instance method in `Animal` and hides the class method in `Animal`. The `main` method in this class creates an instance of `Cat` and calls `testClassMethod()` on the class and `testInstanceMethod()` on the instance.

> The output from this program is as follows:

> >     The class method in Animal.
> >     The instance method in Cat.

> As promised, the version of the hidden method that gets invoked is the one in the superclass, and the version of the overridden method that gets invoked is the one in the subclass.

> ### Modifiers

> The access specifier for an overriding method can allow more, but not less, access than the overridden method. For example, a protected instance method in the superclass can be made public, but not private, in the subclass.

> You will get a compile-time error if you attempt to change an instance method in the superclass to a class method in the subclass, and vice versa.

> ### Summary

> The following table summarizes what happens when you define a method with the same signature as a method in a superclass.

> Defining a Method with the Same Signature as a Superclass's Method
> | Superclass Instance Method | Superclass Static Method
> --- | --- | ---
> Subclass Instance Method | Overrides | Generates a compile-time error
> Subclass Static Method | Generates a compile-time error | Hides

> > Note: In a subclass, you can overload the methods inherited from the superclass. Such overloaded methods neither hide nor override the superclass methods—they are new methods, unique to the subclass.

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