DrJava can detect if .java, .drjava and .djapp files have not been associated with DrJava. If this is the case, DrJava may display the "Set File Associations?" dialog at startup. This dialog has four buttons.

Yes. Set file associations so .java, .drjava and .djapp files open in DrJava when double-clicked.

No. Do not set file associations.

Always. Set file associations, and do this automatically from now on without asking me again.

Never. Do not set file associations, and never ask me about it again.

These settings can also be changed within DrJava by opening the Preferences window and selecting the "File Types" pane (see Configuring DrJava).