question: public void addEdge ( Dependency dependency ) { if ( ! edges . contains ( dependency ) ) { edges . add ( dependency ) ; Set < Dependency > outboundEdges = vertices . get ( dependency . getDepender ( ) ) ; if ( outboundEdges == null ) { outboundEdges = new HashSet ( ) ; vertices . put ( dependency . getDepender ( ) , outboundEdges ) ; } outboundEdges . add ( dependency ) ; if ( vertices . get ( dependency . getDependsOn ( ) ) == null ) { vertices . put ( dependency . getDependsOn ( ) , new HashSet ( ) ) ; } } }
target:  Add an edge to the dependency graph.