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<H2><A NAME="SECTION02461000000000000000"></A><A NAME="sec:basics:objects:babelom"></A>
<BR>
Babel's Object Model
</H2>
<P>
SIDL defines three types of objects: interfaces, classes,
and abstract classes. A SIDL
<TT><I CLASS="slanted">interface</I></TT><A NAME="5085"></A><A NAME="5086"></A>
is akin to
a Java interface or a C++ pure abstract base class.
It is an object that defines methods (aka member functions),
but carries no implementation of those methods.
A <TT><I CLASS="slanted">class</I></TT><A NAME="5088"></A><A NAME="5089"></A>
by comparison is always concrete; meaning
that there is an implementation for each of its methods
and it can be instantiated. An
<TT><I CLASS="slanted">abstract class</I></TT><A NAME="5091"></A><A NAME="5092"></A>
falls somewhere
between an <TT><I CLASS="slanted">interface</I></TT> and a <TT><I CLASS="slanted">class</I></TT>. It has at least
one method unimplemented, so it cannot be instantiated, but it
also may have several methods that are implemented and these
implementations can be inherited.
<P>
SIDL supports multiple inheritance<A NAME="5095"></A> of interfaces and single
inheritance of implementation.<A NAME="5096"></A> This is a strategy found in
other OO languages such as Java and ObjectiveC. The
words to distinguish these two forms of inheritance are
<TT><I CLASS="slanted">extends</I></TT> and <TT><I CLASS="slanted">implements</I></TT>. Interfaces can
extend multiple interfaces, but they cannot implement anything.
Classes can extend at most one other class (abstract or not),
but can implement multiple interfaces.
<P>
Furthermore, any inherited abstract methods (inherited from either and
abstract parent class or and implemented interface) will default to
abstract unless they are re-declared in the current class. If a
concrete class
implements many large interfaces, this can result in a fairly large
list of redeclared functions in the class definition. As a shortcut,
we included the <TT>implements-all</TT> directive, a short hand
that states explicitly that we intend to implement every
method in the named interface concretely. That's why, in the
following example, class B must be declared abstract, but class D is
concrete. Class B does not redeclare the <TT>printMe</TT> function,
but class D <TT>implements-all</TT>. There is no similar directive for
inheritance from abstract classes.
<P>
We display a small SIDL file below and finish this
Subsection with a discussion of its details.
<A NAME="5102"></A>
<BR>
<PRE CLASS="verbatim">package object version 1.0 {
interface A {
void display();
void printMe();
}
abstract class B implements A {
void display();
}
class C extends B {
void printMe();
}
class D implements-all A {
}
}
</I></PRE></td></tr></table></blockquote>
<P>
<TT><I CLASS="slanted">object.A</I></TT> is
an interface that has two methods <TT><I CLASS="slanted">display()</I></TT> and <TT><I CLASS="slanted">print()</I></TT>.
Both of these methods take no arguments and return no value.
(We will discuss arguments and return values in the next section.)
Since <TT><I CLASS="slanted">object.A</I></TT> is an interface, there is no implementation
associated with it, and Babel will not generate any implementation
code associated with it.
<P>
<TT><I CLASS="slanted">object.B</I></TT> is an abstract class that <A NAME="5109"></A><A NAME="5110"></A>
inherits from <TT><I CLASS="slanted">object.A</I></TT>. Since it redeclares the
<TT><I CLASS="slanted">display()</I></TT> method, Babel will generate the appropriate
code for an implementation of this method only. It will not
generate code for the other inherited method <TT><I CLASS="slanted">print()</I></TT>
(since it wasn't declared in the SIDL file) and it will not
generate constructors/destructors since the class is abstract.
<P>
<TT><I CLASS="slanted">object.C</I></TT> is a concrete class that extends
the abstract class <A NAME="5115"></A><A NAME="5116"></A>
<TT><I CLASS="slanted">object.B</I></TT> it then lists only the unimplemented method
<TT><I CLASS="slanted">print()</I></TT>, implying that it will use the implementation
of <TT><I CLASS="slanted">display()</I></TT> it inherited from its parent.
<P>
<TT><I CLASS="slanted">object.D</I></TT> is also a concrete class that uses the
<TT><I CLASS="slanted">implements-all</I></TT><A NAME="5122"></A><A NAME="5123"></A>
<P>
directive. This is identical to using <TT><I CLASS="slanted">implements</I></TT> and then
listing all the methods declared in the interface.
The <TT><I CLASS="slanted">implements-all</I></TT> directive was added to SIDL
as a convenience construct and to save excessive typing
in the SIDL file. By virtue of the <TT><I CLASS="slanted">implements-all</I></TT>
directive, <TT><I CLASS="slanted">object.D</I></TT> will provide its own implementation of
all of <TT><I CLASS="slanted">object.A</I></TT>'s methods, namely
<TT><I CLASS="slanted">display()</I></TT> and <TT><I CLASS="slanted">print()</I></TT>.
<P>
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