question: public void addVertex ( Object id ) throws IllegalArgumentException { if ( initialized ) { throw new IllegalArgumentException ( ) ; } Vertex vertex = new Vertex ( id ) ; Object existing = vertexMap . put ( id , vertex ) ; if ( existing != null ) { throw new IllegalArgumentException ( ) ; } vertexList . add ( vertex ) ; }
target:  Defines a new vertex with the given id. The depth-first search is performed in the relative order in which vertexes were added to the graph.