Every diagram has a diagram type and this type should be provided during diagram creation.
An example shows how to create and add a diagram to a parent element:

 

    Project project = ...;
    Package parent = project.getPrimaryModel();
    // create a new session
    SessionManager.getInstance().createSession(project, "Create a diagram");
    try
    {
        //a class diagram is created and added to a parent model element
        Diagram diagram = ModelElementsManager.getInstance().createDiagram(DiagramTypeConstants.UML_CLASS_DIAGRAM, parent);
        //open a diagram 
        project.getDiagram(diagram).open();
    }
    catch (ReadOnlyElementException e)
    {
    }
    SessionManager.getInstance().closeSession(project);
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