A
subclass has the same instance method signature like the super class
instance method is called overriding.
Example
class Animal{
void print(){
System.out.println("I am animal");
}
}
class Tiger extends Animal{
void print(){
System.out.println("I am Tiger");
}
}
As
you observe, the classes Animal and Tiger has the same method
signature print(). So print method in the class Tiger overrides the
method in the class Animal.
Dynamic
Method Dispatch
It
is a mechanism, where a call to the overridden method is resolved at
run time. It is also called as run time polymorphism.
Take
the same examples Animal and Tiger.
class DynamicMethodDispatchEx{
public static void main(String args[]){
Animal anim1 = new Tiger();
anim1.print();
}
}
As
you see the statement
Animal
anim1 = new Tiger();
An
object of the Tiger class is assigned to the Animal reference
variable. This is completely correct in Java. We can assign a
subclass object to super class reference variable.
When
the program calls the print(), then at run time JVM checks, the
actual object pointed by the reference variable and calls the
respective method of the actual object. So it calls the print method
in the Tiger class.
Modifiers
The
access specifier for an overriding method can allow more, but not
less, access than the overridden method. For example, a default
instance method in the superclass can be made public, protected,
default, but not private, in the subclass.
Some
points to Remember
No,
static methods can't be overridden, they just hide the static method
of the super class.
2.
Is the instance variables overridden ?
No
class Animal{
String str = "Animal";
void print(){
System.out.println("I am animal");
}
}
class Tiger extends Animal{
String str = "Tiger";
void print(){
System.out.println("I am Tiger");
}
}
class DynamicMethodDispatchEx{
public static void main(String args[]){
Animal anim1 = new Tiger();
anim1.print();
System.out.println(anim1.str);
}
}
Output
I
am Tiger
Animal
If
the instance variables are overridden, then the output should print
the message “Tiger”. I.e, instance variables are just hide the
instance variables of super class, these can't be overridden.
3.
Can I apply static modifier to the subclass overriding method?
No,
compiler throws an error.
Example
class Animal{
String str = "Animal";
void print(){
System.out.println("I am animal");
}
}
class Tiger extends Animal{
String str = "Tiger";
static void print(){
System.out.println("I am Tiger");
}
}
When
tries to compile the Tiger class, below error thrown.
Tiger.java:3:
error: print() in Tiger cannot override print() in Animal
static
void print(){
^
overriding
method is static
1
error
4.
What is wrong with the below program ?
class Animal{
String str = "Animal";
void print(){
System.out.println("I am animal");
}
}
class Tiger extends Animal{
String str = "Tiger";
private void print(){
System.out.println("I am Tiger");
}
}
When
you tries to compile the above program, compiler throws the “Weaker
access specifier error”
Tiger.java:3:
error: print() in Tiger cannot override print() in Animal
private
void print(){
^
attempting
to assign weaker access privileges; was package
1
error
The
access specifier for an overriding method can allow more, but not
less, access than the overridden method. For example, a default
instance method in the superclass can be made public, protected,
default, but not private, in the subclass.
5.
What is the difference between static binding and Dynamic binding ?
Static
binding occurs at compile time, where as Dynamic binding at run time.
Dynamic method dispatch is an example for Dynamic binding.
6.
What is wrong with the below program ?
class Animal{
String str = "Animal";
void print(){
System.out.println("I am animal");
}
}
class Tiger extends Animal{
String str = "Tiger";
void print()throws Exception{
System.out.println("I am Tiger");
}
}
The
overriding method cannot throw any exceptions that are not thrown by
the overridden method.
When
you tries to compile the above program compiler throws the below
error.
Tiger.java:3:
error: print() in Tiger cannot override print() in Animal
void
print()throws Exception{
^
overridden
method does not throw Exception
1
error
7.
Is the constructors inherited ?
A
subclass inherits all the members (fields, methods, and nested
classes) from its superclass. Constructors are not members, so they
are not inherited by subclasses.
8.
When a subclass object assigned to super class reference variable,
then the reference variable can call the methods which are defined in
the super class, trying to call the methods which are not defined in
super class cause the compile time error.
Example
class Animal{
void print()throws Exception{
System.out.println("I am animal");
}
}
class Tiger extends Animal{
void print(){
System.out.println("I am Tiger");
}
void show(){
System.out.println("I am show method");
}
}
class DynamicMethodDispatchEx{
public static void main(String args[]){
Animal anim1 = new Tiger();
anim1.show();
}
}
When
you are trying to compile the program, below error thrown, since the
reference variable of class Animal, “anim1” trying to call
the method “show: which is not defined in the class “Animal”.
DynamicMethodDispatchEx.java:5:
error: cannot find symbol
anim1.show();
^
symbol:
method show()
location:
variable anim1 of type Animal
1
error
9.
Can I specify weaker access specifier to the overriding method in the
sub class ?
No
Multi path inheritance
Covariant return type
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