You can open an editor on any type in the workbench.
Note: The Open Type dialog maintains a history of recently opened types. These are shown when the dialog is opened and stay above a separator line when you start to type a filter expression.
Note: If you open a type from a CLASS or JAR file, you will see a special editor showing only method signatures unless you have attached source to it.
Attaching source to a JAR file
Opening an editor for a selected element
Using the Java editor