question: public synchronized void updateEntityPositionLookup ( Entity e , HashSet < Coords > oldPositions ) { HashSet < Coords > newPositions = e . getOccupiedCoords ( ) ; if ( newPositions . equals ( oldPositions ) ) { return ; } if ( oldPositions != null ) { for ( Coords pos : oldPositions ) { HashSet < Integer > posEntities = entityPosLookup . get ( pos ) ; if ( posEntities != null ) { posEntities . remove ( e . getId ( ) ) ; } } } for ( Coords pos : newPositions ) { HashSet < Integer > posEntities = entityPosLookup . get ( pos ) ; if ( posEntities == null ) { posEntities = new HashSet < Integer > ( ) ; posEntities . add ( e . getId ( ) ) ; entityPosLookup . put ( pos , posEntities ) ; } else { posEntities . add ( e . getId ( ) ) ; } } }
target:  Updates the map that maps a position to the list of Entity's in that position.