Java Reference
In-Depth Information
Let us use an example to show the differences between overriding and overloading. In (a)
below, the method
p(double i)
in class
A
overrides the same method defined in class
B
. In
(b), however, the class
A
has two overloaded methods:
p(double i)
and
p(int i)
. The
method
p(double i)
is inherited from
B
.
public class
Test {
public static void
main(String[] args) {
A a =
new
A();
a.p(
10
);
a.p(
10.0
);
public class
Test {
public static void
main(String[] args) {
A a =
new
A();
a.p(
10
);
a.p(
10.0
);
}
}
}
}
class
B {
public void
p(
double
i) {
System.out.println(i *
2
);
class
B {
public void
p(
double
i) {
System.out.println(i *
2
);
}
}
}
}
class
A
extends
B {
// This method
class
A
extends
B {
// This method
overrides
overloads
the method in B
the method in B
double
i
int
i
public void
p( ) {
System.out.println(i);
public void
p( ) {
System.out.println(i);
}
}
}
}
(a)
(b)
When you run the
Test
class in (a), both
a.p(10)
and
a.p(10.0)
invoke the
p(double
i)
method defined in class
A
to display
10.0
. When you run the
Test
class in (b),
a.p(10)
invokes the
p(int i)
method defined in class
A
to display
10
, and
a.p(10.0)
invokes the
p(double i)
method defined in class
B
to display
20.0
.
Note the following:
Overridden methods are in different classes related by inheritance; overloaded meth-
ods can be either in the same class or different classes related by inheritance.
■
Overridden methods have the same signature and return type; overloaded methods
have the same name but a different parameter list.
■
To avoid mistakes, you can use a special Java syntax, called
override annotation
, to place
@Override
before the method in the subclass. For example:
override annotation
1
public class
CircleFromSimpleGeometricObject
2
extends
SimpleGeometricObject {
3
// Other methods are omitted
4
5
6
@Override
public
String toString() {
toString
in superclass
7
return super
.toString() +
"\nradius is "
+ radius;
8 }
9 }
This annotation denotes that the annotated method is required to override a method in the
superclass. If a method with this annotation does not override its superclass's method, the
compiler will report an error. For example, if
toString
is mistyped as
tostring
, a compile
error is reported. If the override annotation isn't used, the compile won't report an error.
Using annotation avoids mistakes.
