A number of classes providing the same functionality in different contexts can implement same interface and a class will compose a reference of interface and hence makes changes in any of the classes. This provides single accessor for classes providing same functionality in different context.

e.g.

interface address{
getaddress()
setaddress()
}

class usaddress implements address{
getaddress()
setaddress()
}

class indiaaddress implements address{
getaddress()
setaddress()
}

class CreateAddress {
address addr;
static returnAddress()
} //a factory in fact.


class client {
address add = CreateAddress.returnAddress()
}