@Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }

        TimelineGroup<?> that = (TimelineGroup) o;

        return !(id != null ? !id.equals(that.id) : that.id != null);
    }
Assuming the group has nested groups, this will set the initial state of the group - shown or collapsed.
It will default as true.
primefaces/src/main/java/org/primefaces/model/timeline/TimelineGroup.java