private OffsetAndMetadata put(
            String groupId,
            String topic,
            int partition,
            OffsetAndMetadata offsetAndMetadata
        ) {
            TimelineHashMap<String, TimelineHashMap<Integer, OffsetAndMetadata>> topicOffsets = offsetsByGroup
                .computeIfAbsent(groupId, __ -> new TimelineHashMap<>(snapshotRegistry, 0));
            TimelineHashMap<Integer, OffsetAndMetadata> partitionOffsets = topicOffsets
                .computeIfAbsent(topic, __ -> new TimelineHashMap<>(snapshotRegistry, 0));
            return partitionOffsets.put(partition, offsetAndMetadata);
        }
The offsets keyed by group id, topic name and partition id.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/OffsetMetadataManager.java