Find the diagram object in uml/diagram/XXXX/ui/UMLYYYYDiagram.java, where XXXX is the diagram type (lower case) and YYYY the diagram type (bumpy caps). For example uml/diagram/use_case/ui/UMLUseCaseDiagram.java. This will be a subclass of UMLDiagram (in uml/diagram/ui/UMLDiagram.java).

Each tool-bar action is declared as a private static field of class Action, initiated as a new CmdCreateNode (for nodal UML elements) or a new CmdSetMode (for behavior, or creation of line UML elements). These classes are part of the GEF library.

The common ones (select, broom, graphic annotations) are inherited from UMLDiagram, the diagram specific ones in the class itself. For example in UMLUseCaseDiagram.java we have the following for creating Use Case nodes.

private static Action actionUseCase; 

which is then created as follows:

    protected Action getActionUseCase() {
        if (actionUseCase == null) {
            actionUseCase = new RadioAction(new CmdCreateNode(
                    Model.getMetaTypes().getUseCase(), "button.new-usecase"));
        }
        return actionUseCase;
    }

The first argument of the CmdCreateNode() function is the class of the node to create from the Model subsystem Metatypes interface, the second a textual tool tip.

The tool-bar is actually created by defining a method, UMLDiagram.initToolBar() which adds the tools in turn to the tool-bar (a private member named toolBar).

The default constructor for the diagram is declared public, despite that it must not be called directly - it is needed for loading projects from files. The desired constructor takes a name-space as an argument, and sets up a graph model (UseCaseDiagramGraphModel), layer perspective and renderer (UseCaseDigramRenderer) for nodes and edges.

The graph model is the bridge between the UML meta-model representation of the design and the graph model of GEF. They are found in the parent directory of the corresponding diagram class, and have the general name YYYYDiagramGraphModel.java, where YYYY is the diagram name in bumpy caps. For example the use case diagram graph model is in uml/diagram/use_case/UseCaseDiagramGraphModel.java

The graph model is defined as UMLMutableGraphSupport, a child of the GEF class MutableGraphSupport, and should implement MutableGraphModel (GEF).

The renderer is responsible for creating graphic figs as required on the diagram. It is found in the same directory of the corresponding diagram class, and has the general name YYYYDiagramRenderer.java, where YYYY is the diagram name in bumpy caps. For example the use case diagram graph model is in uml/diagram/use_case/ui/UseCaseDiagramRenderer.java

This provides two routines, getFigNodeFor(), which provides a fig object to represent a given UML node object and getFigEdgeFor(), which provides a fig object to represent a given UML edge object.

In our example, we must extend getFigEdgeFor() so it can handle UML Extend objects (producing a FigExtend).

New objects that are to appear on a diagram will require new Fig classes to represent them. In our example we have created FigExtend. They are placed in the same directory as the diagram that uses them.

The implementation of a Fig must provide constructors for both a generic fig, (i.e. without setting the owner - which is used for loading from files) and one representing a specific Model subsystem object. It should provide a setFig() method to set a particular figure as the representation. It should provide a method canEdit() to indicate whether the Fig can be edited. It should provide an event handler modelChanged() to cope with advice that the UML model has changed.

Assuming you have your model element already defined in the model and your PropPanel for that model element you should make the Fig class.