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<H2><A NAME="SECTION00043000000000000000"></A><A NAME="ss:objecthierarchy"></A>
<BR>
The Object Hierarchy
</H2>

<P>
Now that we have created our first ZoomMapZoomMap, let us examine how it
relates to other kinds of ObjectObject before investigating what we can do
with it.

<P>
We have so far indicated that a ZoomMap is a kind of Object and have
also mentioned that it is a kind of MappingMapping as well. These statements
can be represented very simply using the following hierarchy:

<P>
<SMALL CLASS="SMALL">
<BR>
<IMG
 WIDTH="108" HEIGHT="53" ALIGN="BOTTOM" BORDER="0"
 SRC="img35.png"
 ALT="\begin{terminalv}
Object
Mapping
ZoomMap
\end{terminalv}">
<BR>
</SMALL>
<P>
which is a way of stating that a ZoomMap is a special class of
Mapping, while a Mapping, in turn, is a special class of Object.  This
is exactly like saying that an Oak is a special form of Tree, while a
Tree, in turn, is a special form of Plant. This may seem almost
trivial, but before you turn to read something less dull, be assured
that it is a very important idea to keep in mind in what follows.

<P>
If we look at some of the other Objects used by the AST library, we
can see how these are all related in a similar way (don't worry about
what they do at this stage):
<A NAME="ss:mappinghierarchy"></A>
<P>
<SMALL CLASS="SMALL">
<BR>
<IMG
 WIDTH="141" HEIGHT="183" ALIGN="BOTTOM" BORDER="0"
 SRC="img36.png"
 ALT="\begin{terminalv}
Object
Mapping
Frame
FrameSet
Plot
UnitMap
ZoomMap
Channel
FitsChan
XmlChan
\end{terminalv}">
<BR>
</SMALL>
<P>
Notice that there are several different types of Mapping available
(<SPAN  CLASS="textit">i.e.</SPAN> there are classes of Object indented beneath the
``Mapping'' heading) and, in addition, other types of Object which are
not Mappings--Channels for instance (which are at the same
hierarchical level as Mappings).

<P>
The most specialised Object we have shown here is the PlotPlot (which we
will not discuss in detail until &#167;<A HREF="node194.html#ss:plots">21</A>). As you can see, a
Plot is a FrameSetFrameSet... and a FrameFrame... and a Mapping... and,
like everything else, ultimately an Object.

<P>
What this means is that you can use a Plot not only for its own
specialised behaviour, but also whenever any of these other
less-specialised classes of Object is called for. The general rule is
that an Object of a particular class may substitute for any of the
classes appearing above it in this hierarchy. The Object is then said
to <SPAN  CLASS="textit">inherit</SPAN> the behaviour of these higher classes. We can
therefore use our ZoomMap whenever a ZoomMap, a Mapping or an Object
is called for.

<P>
Sometimes, this can lead to some spectacular short-cuts by avoiding
the need to break large Objects down in order to access their
components. With some practice and a little lateral thinking you
should soon be able to spot opportunities for this.

<P>
You can find the full <SPAN  CLASS="textit">class hierarchy</SPAN>, as this is called, for
the AST library in Appendix&nbsp;<A HREF="node214.html#ss:classhierarchy">A</A> and you may need to
refer to it occasionally until you are familiar with the classes you
need to use.

<P>

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