Java Reference
In-Depth Information
Consider the following snippet of code that shows a definition of a
square()
method, which accepts a parameter
of
double
data type:
double square(double num) {
return num * num;
}
The
square()
method can be called with actual parameter of
double
data type as
double d1 = 20.23;
double result = square(d1);
The same
square()
method may also be called with actual parameter of
int
data type as
int k = 20;
double result = square(k);
You have just seen that the
square()
method works on
double
data type as well as
int
data type although you
have defined it only once in terms of a formal parameter of
double
data type. This is exactly what polymorphism
means. In this case, the
square()
method is called a polymorphic method with respect to
double
and
int
data type.
Thus the
square()
method is exhibiting polymorphic behavior even though the programmer who wrote the code
did not intend it. The
square()
method is polymorphic because of the implicit type conversion (coercion from
int
to
double
) provided by Java language. Here is a more formal definition of a polymorphic method:
Suppose m is a method that declares a formal parameter of type T. If S is a type that can be implicitly
converted to T, the method m is said to be polymorphic with respect to S and T.
Inclusion Polymorphism
Inclusion is a universal polymorphism. It is also known as subtype (or subclass) polymorphism because it is achieved
using subtyping or subclassing. This is the most common type of polymorphism supported by object-oriented
programming languages. Java supports it. Inclusion polymorphism occurs when a piece of code that is written using
a type works for all its subtypes. This type of polymorphism is possible based on the subtyping rule that a value that
belongs to a subtype also belongs to the supertype. Suppose
T
is a type and
S1
,
S2
,
S3...
are subtypes of
T
. A value that
belongs to
S1
,
S2
,
S3...
also belongs to
T
. This subtyping rule makes us write code as follows:
T t;
S1 s1;
S2 s2;
...
t = s1; // A value of type s1 can be assigned to variable of type T
t = s2; // A value of type s2 can be assigned to variable of type T
