The HLU library has a hierarchical class structure. It starts with the Base class as the highest node. All classes are subclassed from the Base class.
Some of the more important nodes on this hierarchical tree are the View class and the Workstation class, plus the App class, DataItem class, Error class, PlotManager class, and AnnoManager class. Later, we will learn how to set object variables (resources) and create specific objects. First, let's examine a bit more about how classes can interact in the HLU library.
Subclasses are classes that are derived from another class. This means that they "inherit" the functionality of the class they are derived from, then they add something to this functionality. Superclasses are classes from which other classes (subclasses) have been derived. For example, TextItem is an immediate subclass of the View class. This means that TextItem is directly below View in the class hierarchy. Therefore, View is also an immediate superclass of TextItem.
Objects of a subclass are also valid objects of classes higher in the hierarchy. Therefore, in the hierarchy of:
Base class (The highest level) | | View class | | Transform class | | DataComm class | | XyPlot class
an XyPlot object is considered to have the characteristics of a DataComm object, a Transform object, a View object, and a Base object. Any XyPlot object inherits the resources of all of these superclasses.
The hierarchical structure and the composite class members are shown in the Class hierarchy diagram. Please study this important diagram if you have not already done so.
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