In the previous chapter we talked about super classes and sub
classes. If a class inherits a method from its super class, then there
is a chance to override the method provided that it is not marked final.
The benefit of overriding is: ability to define a behavior that's
specific to the sub class type. Which means a subclass can implement a
parent calss method based on its requirement.
In object oriented terms, overriding means to override the functionality of any 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 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 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 super class.
- The access level cannot be more restrictive than the overridden
method's access level. For example: if the super class method is
declared public then the overridding method in the sub class cannot be
either private or public. However the access level can be less
restrictive than the overridden method's access level.
- 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 following result:
Animals can move
Dogs can walk and run
|
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