public String toString() {
        if (this.key instanceof MapReduce.NullObject && this.value instanceof MapReduce.NullObject) {
            return EMPTY_STRING;
        } else if (this.key instanceof MapReduce.NullObject) {
            return makeString(this.value);
        } else if (this.value instanceof MapReduce.NullObject) {
            return makeString(this.key);
        } else {
            return makeString(this.key) + TAB + makeString(this.value);
        }
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/KeyValue.java