Discovering Classes and Objects - Beginning Java 7 - page 159

{
double X = 2.0;
}
interface C extends A, B
{
}
Listing 2-47 will compile even though C inherits two same-named constants X with
differentreturntypesandinitializers.However,ifyouimplement C andthentrytoac-
cess X , as in Listing 2-48 , you will run into a name collision.
Listing 2-48. Discovering a name collision
class Collision implements C
{
public void output()
{
System.out.println( X ); // Which X is accessed?
}
}
Supposeyouintroducea void foo(); methodheaderdeclarationintointerface A ,
andan int foo(); methodheaderdeclarationintointerface B .Thistime,thecom-
piler will report an error when you attempt to compile the modified Listing 2-47 .
Why Use Interfaces?
Now that the mechanics of declaring, implementing, and extending interfaces are out
of the way, we can focus on the rationale for using them. Unfortunately, newcomers
toJava'sinterfacesfeatureareoftentoldthatthisfeaturewascreatedasaworkaround
toJava'slack ofsupportformultiple implementation inheritance. While interfaces are
useful in this capacity, this is not their reason for existence. Instead, Java's i nterfaces
feature was created to give developers the utmost flexibility in designing their applic-
ations, by decoupling interface from implementation. You should always code to the
interface.
Those who are adherents to agile software development (a group of software devel-
opment methodologies based on iterative development that emphasizes keeping code
simple,testingfrequently,anddeliveringfunctionalpiecesoftheapplicationassoonas
theyaredeliverable)knowtheimportanceofflexiblecoding.Theycannotaffordtotie