private OffsetAndMetadata get(
            String groupId,
            String topic,
            int partition
        ) {
            TimelineHashMap<String, TimelineHashMap<Integer, OffsetAndMetadata>> topicOffsets = offsetsByGroup.get(groupId);
            if (topicOffsets == null) {
                return null;
            } else {
                TimelineHashMap<Integer, OffsetAndMetadata> partitionOffsets = topicOffsets.get(topic);
                if (partitionOffsets == null) {
                    return null;
                } else {
                    return partitionOffsets.get(partition);
                }
            }
        }
The offsets keyed by group id, topic name and partition id.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/OffsetMetadataManager.java