Overriding and Hiding Methods
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 overridesthe 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 hidesthe 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 tablesummarizes 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 methods—they are new methods, unique to the subclass.
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