Need some correct vocabulary - interfaces

Question

Helo again world.
If an interface cannot be instantiated with the new keyword, what is it about interfaces that allows one to use an interface name as a data type when declaring an object?
Thank you one and all.
Ciao for now.

Answer 1

Hello.
Imagine a class like this one:
class A
implements StartValue
implements EndValue
private:
int start, end;
Then you have the interfaces:
interface StartTime
void setStart(int);
interface EndTime
void setEnd(int);
Theese interfaces define different ways to interact with an object.
Then consider another class:
class B
private:
StartValue start;
public:
void Initialize()
start = (StartValue) new A();
start.setStart(1234);
Now class B declares a variable with the datatype StartValue. The variable is able to hold a reference to any object that can be interacted with using the StartValue interface.
B.Initialize() will instantiate class A, and put a reference to the new object into the 'start' variable.
This way, any code that follows in class B does not need to know what kind of object that is put into the
'start' variable, as long as that object does implement StartValue.
Now, if you try to compile the above code, you get errors saying that you cannot instantiate class A.
This is because the class definition states that class A implements two interfaces, but I have not yet written
the implementation of those interfaces. I need to add the two methods to class A:
class A
implements StartValue
implements EndValue
private:
int start, end;
public:
void setStart(int newValue) { start = newValue);
void setEnd(int newValue) { start = newValue);
I hopes that you can get something out of this 'tour'.
If you have been coded a lot of C++, you might think of this as multiple class inheritance, having StartValue
and EndValue as abstract classes and setStart and setEnd as pure virtual functions.
If you have been coded COM under winodws, then you should really have no problem with the concepts
INTERFACE and OBJECT, as they work the same way as in java.
Best regards,
Ivar Svendsen