question: public boolean addCallback ( Uri uri , ItemLoadedCallback callback ) { if ( Log . isLoggable ( TAG , Log . DEBUG ) ) { Log . d ( TAG , "Adding image callback " + callback ) ; } if ( uri == null ) { throw new NullPointerException ( "uri is null" ) ; } if ( callback == null ) { throw new NullPointerException ( "callback is null" ) ; } Set < ItemLoadedCallback > callbacks = mCallbacks . get ( uri ) ; if ( callbacks == null ) { callbacks = new HashSet < ItemLoadedCallback > ( 4 ) ; mCallbacks . put ( uri , callbacks ) ; } callbacks . add ( callback ) ; return true ; }
target:  Attempts to add a callback for a resource.