@Override
    public GroupAssignment assign(
        GroupSpec groupSpec,
        SubscribedTopicDescriber subscribedTopicDescriber
    ) throws PartitionAssignorException {
        if (groupSpec.memberIds().isEmpty())
            return new GroupAssignment(Collections.emptyMap());

        if (groupSpec.subscriptionType().equals(HOMOGENEOUS)) {
            return assignHomogenous(groupSpec, subscribedTopicDescriber);
        } else {
            return assignHeterogeneous(groupSpec, subscribedTopicDescriber);
        }
    }
A simple partition assignor that assigns each member all partitions of the subscribed topics.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/SimpleAssignor.java