Grouping by object type and database user (tree structure) is the most convenient and accustomed way of database objects representation. Alt SQL Developer uses a tree structure to display the database objects. The database objects could contain other objects and links.
Object Window allows to tune an objects list display according to selected username in the window. User list and their order could be tune using appropriate settings windows. Also it is possible to specify a default user.
If you don't use any of database objects group or if you want to create your own object group, you can use settings for this (Tools → Preferences → Objects).