@Override
    public String toString() {
        return "GroupedComplexEvent{" +
                "complexEvent=" + complexEvent +
                ", groupKey='" + groupKey + '\'' +
                ", next=" + next +
                '}';
    }
Implementation of {@link ComplexEvent} to hold events belonging to the same GroupBy group.
modules/siddhi-core/src/main/java/io/siddhi/core/event/GroupedComplexEvent.java