@Override
    public String toString() {

        return "IsNull{" +
                "streamId='" + streamId + '\'' +
                ", streamIndex=" + streamIndex +
                ", isInnerStream=" + isInnerStream +
                ", isFaultStream=" + isFaultStream +
                ", expression=" + expression +
                '}';
    }
Condition {@link Expression} checking whether the event is null
modules/siddhi-query-api/src/main/java/io/siddhi/query/api/expression/condition/IsNull.java