In object-oriented terms, overriding means to override the functionality of an existing method.
Example:
Let us look at an example.class Animal{
public void move(){
System.out.println("Animals can move");
}
}
class Dog extends Animal{
public void move(){
System.out.println("Dogs can walk and run");
}
}
public class TestDog{
public static void main(String args[]){
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object
a.move();// runs the method in Animal class
b.move();//Runs the method in Dog class
}
}This would produce the following result:
Animals can move
Dogs can walk and run
In the above example, you can see that the even though b is a type of
Animal it runs the move method in the Dog class. The reason for this is: In
compile time, the check is made on the reference type. However, in the runtime,
JVM figures out the object type and would run the method that belongs to that
particular object.Therefore, in the above example, the program will compile properly since Animal class has the method move. Then, at the runtime, it runs the method specific for that object.
Consider the following example :
class Animal{
public void move(){
System.out.println("Animals can move");
}
}
class Dog extends Animal{
public void move(){
System.out.println("Dogs can walk and run");
}
public void bark(){
System.out.println("Dogs can bark");
}
}
public class TestDog{
public static void main(String args[]){
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object
a.move();// runs the method in Animal class
b.move();//Runs the method in Dog class
b.bark();
}
}This would produce the following result:
TestDog.java:30: cannot find symbol
symbol : method bark()
location: class Animal
b.bark();
^
This program will throw a compile time error since b's reference type Animal
doesn't have a method by the name of bark.
Rules for method overriding:
·
The argument list should be exactly the same as
that of the overridden method.
·
The return type should be the same or a subtype
of the return type declared in the original overridden method in the
superclass.
·
The access level cannot be more restrictive than
the overridden method's access level. For example: if the superclass method is
declared public then the overridding method in the sub class cannot be either
private or protected.
·
Instance methods can be overridden only if they
are inherited by the subclass.
·
A method declared final cannot be overridden.
·
A method declared static cannot be overridden
but can be re-declared.
·
If a method cannot be inherited, then it cannot
be overridden.
·
A subclass within the same package as the
instance's superclass can override any superclass method that is not declared
private or final.
·
A subclass in a different package can only
override the non-final methods declared public or protected.
·
An overriding method can throw any uncheck
exceptions, regardless of whether the overridden method throws exceptions or
not. However the overriding method should not throw checked exceptions that are
new or broader than the ones declared by the overridden method. The overriding
method can throw narrower or fewer exceptions than the overridden method.
·
Constructors cannot be overridden.
Using the super keyword:
When invoking a superclass version of an overridden method the super
keyword is used.class Animal{
public void move(){
System.out.println("Animals can move");
}
}
class Dog extends Animal{
public void move(){
super.move(); // invokes the super class method
System.out.println("Dogs can walk and run");
}
}
public class TestDog{
public static void main(String args[]){
Animal b = new Dog(); // Animal reference but Dog object
b.move(); //Runs the method in Dog class
}
}This would produce the following result:
Animals can move
Dogs can walk and run
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