Java Reference
In-Depth Information
A method can be implemented in several classes along the inheritance chain. The JVM
decides which method is invoked at runtime.
Key
Point
A method can be defined in a superclass and overridden in its subclass. For example, the
toString()
method is defined in the
Object
class and overridden in
GeometricObject
.
Consider the following code:
Object o =
new
GeometricObject();
System.out.println(o.toString());
Which
toString()
method is invoked by
o
? To answer this question, we first introduce
two terms: declared type and actual type. A variable must be declared a type. The type that
declares a variable is called the variable's
declared type
. Here
o
's declared type is
Object
. A
variable of a reference type can hold a
null
value or a reference to an instance of the declared
type. The instance may be created using the constructor of the declared type or its subtype.
The
actual type
of the variable is the actual class for the object referenced by the variable.
Here
o
's actual type is
GeometricObject
, because
o
references an object created using
new
GeometricObject()
. Which
toString()
method is invoked by
o
is determined by
o
's
actual type. This is known as
dynamic binding
.
Dynamic binding works as follows: Suppose an object
o
is an instance of classes
C
1
,
C
2
, . . . ,
C
n-1
, and
C
n
, where
C
1
is a subclass of
C
2
,
C
2
is a subclass of
C
3
, . . . , and
C
n-1
is a subclass of
C
n
,
as shown in Figure 11.2. That is,
C
n
is the most general class, and
C
1
is the most specific class. In
Java,
C
n
is the
Object
class. If
o
invokes a method
p
, the JVM searches for the implementation
of the method
p
in
C
1
,
C
2
, . . . ,
C
n-1
, and
C
n
, in this order, until it is found. Once an implementa-
tion is found, the search stops and the first-found implementation is invoked.
declared type
actual type
dynamic binding
C
n
C
n-1
C
2
C
1
.....
If
o
is an instance of
C
1
,
o
is also an
instance of
C
2
,
C
3
,…,
C
n-1
, and
C
n
java.lang.Object
F
IGURE
11.2
The method to be invoked is dynamically bound at runtime.
VideoNote
Listing 11.6 gives an example to demonstrate dynamic binding.
Polymorphism and dynamic
binding demo
L
ISTING
11.6
DynamicBindingDemo.java
1
public class
DynamicBindingDemo {
2
public static void
main(String[] args) {
3 m(
new
GraduateStudent());
4 m(
new
Student());
5 m(
new
Person());
6 m(
new
Object());
7 }
8
9
public static void
m(Object x) {
10 System.out.println(x.toString());
11 }
12 }
13
14
class
GraduateStudent
extends
Student {
15 }
16
17
class
Student
extends
Person {
18 @Override
19
polymorphic call
dynamic binding
override
toString()
public
String toString() {
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