Explore the structure of a package, view its constituent classes, subpackages and dependencies using the UML Class diagram. Right-click a package in the Project view, and from the context menu, select Diagrams | Show Diagram or press &shortcut:ShowUmlDiagram; and then select a diagram type.

You can also use the Class diagram to create a visual model, populate it with node elements and members, and draw links. IntelliJ IDEA will generate the source code, and keep it always in sync with the model.