Java Method Overriding
In the last tutorial, we learned about inheritance. Inheritance is an OOP property that allows us to derive a new class (subclass) from an existing class (superclass). The subclass inherits the attributes and methods of the superclass.
Now, if the same method is defined in both the superclass and the subclass, then the method of the subclass class overrides the method of the superclass. This is known as method overriding.
Example 1: Method Overriding
class Animal < public void displayInfo() < System.out.println("I am an animal."); >> class Dog extends Animal < @Override public void displayInfo() < System.out.println("I am a dog."); >> class Main < public static void main(String[] args) < Dog d1 = new Dog(); d1.displayInfo(); >> In the above program, the displayInfo() method is present in both the Animal superclass and the Dog subclass.
When we call displayInfo() using the d1 object (object of the subclass), the method inside the subclass Dog is called. The displayInfo() method of the subclass overrides the same method of the superclass.
Notice the use of the @Override annotation in our example. In Java, annotations are the metadata that we used to provide information to the compiler. Here, the @Override annotation specifies the compiler that the method after this annotation overrides the method of the superclass.
It is not mandatory to use @Override . However, when we use this, the method should follow all the rules of overriding. Otherwise, the compiler will generate an error.
Java Overriding Rules
- Both the superclass and the subclass must have the same method name, the same return type and the same parameter list.
- We cannot override the method declared as final and static .
- We should always override abstract methods of the superclass (will be discussed in later tutorials).
super Keyword in Java Overriding
A common question that arises while performing overriding in Java is:
Can we access the method of the superclass after overriding?
Well, the answer is Yes. To access the method of the superclass from the subclass, we use the super keyword.
Example 2: Use of super Keyword
class Animal < public void displayInfo() < System.out.println("I am an animal."); >> class Dog extends Animal < public void displayInfo() < super.displayInfo(); System.out.println("I am a dog."); >> class Main < public static void main(String[] args) < Dog d1 = new Dog(); d1.displayInfo(); >> Overriding and Hiding 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 that it overrides. An overriding method can also return a subtype of the type returned by the overridden method. This subtype 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, then it will generate an error. For more information on @Override , see Annotations .
Static Methods
If a subclass defines a static method with the same signature as a static method in the superclass, then the method in the subclass hides the one in the superclass.
The distinction between hiding a static method and overriding an instance method has important implications:
- The version of the overridden instance method that gets invoked is the one in the subclass.
- The version of the hidden static method that gets invoked depends on whether it is invoked from the superclass or the subclass.
Consider an example that contains two classes. The first is Animal , which contains one instance method and one static method:
public class Animal < public static void testClassMethod() < System.out.println("The static 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 static 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 static method in Animal . The main method in this class creates an instance of Cat and invokes testClassMethod() on the class and testInstanceMethod() on the instance.
The output from this program is as follows:
The static method in Animal The instance method in Cat
As promised, the version of the hidden static method that gets invoked is the one in the superclass, and the version of the overridden instance method that gets invoked is the one in the subclass.
Interface Methods
Default methods and abstract methods in interfaces are inherited like instance methods. However, when the supertypes of a class or interface provide multiple default methods with the same signature, the Java compiler follows inheritance rules to resolve the name conflict. These rules are driven by the following two principles:
- Instance methods are preferred over interface default methods. Consider the following classes and interfaces:
public interface Mythical < default public String identifyMyself() < return "I am a mythical creature."; >>
public class Pegasus extends Horse implements Flyer, Mythical < public static void main(String. args) < Pegasus myApp = new Pegasus(); System.out.println(myApp.identifyMyself()); >>
public interface EggLayer extends Animal < default public String identifyMyself() < return "I am able to lay eggs."; >>
public interface FireBreather extends Animal
public class Dragon implements EggLayer, FireBreather < public static void main (String. args) < Dragon myApp = new Dragon(); System.out.println(myApp.identifyMyself()); >>
If two or more independently defined default methods conflict, or a default method conflicts with an abstract method, then the Java compiler produces a compiler error. You must explicitly override the supertype methods.
Consider the example about computer-controlled cars that can now fly. You have two interfaces ( OperateCar and FlyCar ) that provide default implementations for the same method, ( startEngine ):
public interface OperateCar < // . default public int startEngine(EncryptedKey key) < // Implementation >>
A class that implements both OperateCar and FlyCar must override the method startEngine . You could invoke any of the of the default implementations with the super keyword.
public class FlyingCar implements OperateCar, FlyCar < // . public int startEngine(EncryptedKey key) < FlyCar.super.startEngine(key); OperateCar.super.startEngine(key); >>
The name preceding super (in this example, FlyCar or OperateCar ) must refer to a direct superinterface that defines or inherits a default for the invoked method. This form of method invocation is not restricted to differentiating between multiple implemented interfaces that contain default methods with the same signature. You can use the super keyword to invoke a default method in both classes and interfaces.
Inherited instance methods from classes can override abstract interface methods. Consider the following interfaces and classes:
public class Mustang extends Horse implements Mammal < public static void main(String. args) < Mustang myApp = new Mustang(); System.out.println(myApp.identifyMyself()); >>
The method Mustang.identifyMyself returns the string I am a horse. The class Mustang inherits the method identifyMyself from the class Horse , which overrides the abstract method of the same name in the interface Mammal .
Note: Static methods in interfaces are never inherited.
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 static 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.
| 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 instance methodsthey are new methods, unique to the subclass.