question: @ Override public void addStatement ( final Resource s , final URI p , final Value o , final Resource ... contexts ) throws SailException { if ( log . isDebugEnabled ( ) ) log . debug ( "s=" + s + ", p=" + p + ", o=" + o + ", contexts=" + Arrays . toString ( contexts ) ) ; OpenRDFUtil . verifyContextNotNull ( contexts ) ; if ( contexts . length == 0 ) { addStatement ( s , p , o , ( Resource ) null ) ; } if ( contexts . length == 1 && contexts [ 0 ] == null ) { addStatement ( s , p , o , ( Resource ) null ) ; } for ( Resource c : contexts ) { addStatement ( s , p , o , c ) ; } }
target:  Sesame has a concept of a "null" graph. Any statement inserted whose context position is NOT bound will be inserted into the "null" graph. Statements inserted into the "null" graph are visible from the SPARQL default graph, when no data set is specified (in this case all statements in all contexts are visible).