In order for the code bellow to work:
▸ Make sure to update the variables ("solrUrl", "collectionName", ...) with your information.

final String[] solrUrl = { "http://localhost:8983/solr" };

final CloudSolrClient cloudSolrClient = new CloudSolrClient.Builder(Arrays.asList(solrUrl)).build();

// list collections
{
    final List<String> collectionsList = CollectionAdminRequest.List.listCollections(cloudSolrClient);

    System.out.println("listCollections: " + collectionsList);
}

cloudSolrClient.close();

final String[] solrUrl = { "http://localhost:8983/solr" };

final CloudSolrClient cloudSolrClient = new CloudSolrClient.Builder(Arrays.asList(solrUrl)).build();

final String collectionName = "collection2";

final int numShards = 2;
final int numReplicas = 2;
final int maxShardsPerNode = 2;

final String solrZKConfigName = "_default";

// create collection
{
    final CollectionAdminRequest.Create adminRequest = CollectionAdminRequest.Create
            .createCollection(collectionName, solrZKConfigName, numShards, numReplicas)
            .setMaxShardsPerNode(maxShardsPerNode);

    CollectionAdminResponse adminResponse = adminRequest.process(cloudSolrClient);

    System.out.println(adminResponse);
}

cloudSolrClient.close();

final String[] solrUrl = { "http://localhost:8983/solr" };

final CloudSolrClient cloudSolrClient = new CloudSolrClient.Builder(Arrays.asList(solrUrl)).build();

final String collectionName = "collection2";

// delete collection
{
    final CollectionAdminRequest.Delete adminRequest = CollectionAdminRequest.Delete.deleteCollection(collectionName);

    CollectionAdminResponse adminResponse = adminRequest.process(cloudSolrClient);

    System.out.println(adminResponse);
}

cloudSolrClient.close();

The "RELOAD" action is used when you have changed a configuration in ZooKeeper.

final String[] solrUrl = { "http://localhost:8983/solr" };

final CloudSolrClient cloudSolrClient = new CloudSolrClient.Builder(Arrays.asList(solrUrl)).build();

final String collectionName = "collection1";

// reload collection
{
    final CollectionAdminRequest.Reload adminRequest = CollectionAdminRequest.Reload.reloadCollection(collectionName);

    CollectionAdminResponse adminResponse = adminRequest.process(cloudSolrClient);

    System.out.println(adminResponse);
}

cloudSolrClient.close();

final String[] solrUrl = { "http://localhost:8983/solr" };

final CloudSolrClient cloudSolrClient = new CloudSolrClient.Builder(Arrays.asList(solrUrl)).build();

final String aliasName = "alias1";
final String aliasedCollections = "collection1,collection2";

// create alias
{
    final CollectionAdminRequest.CreateAlias adminRequest = CollectionAdminRequest.CreateAlias.createAlias(aliasName, aliasedCollections);

    CollectionAdminResponse adminResponse = adminRequest.process(cloudSolrClient);

    System.out.println(adminResponse);
}

cloudSolrClient.close();

final String[] solrUrl = { "http://localhost:8983/solr" };

final CloudSolrClient cloudSolrClient = new CloudSolrClient.Builder(Arrays.asList(solrUrl)).build();

final String aliasName = "alias1";

// delete alias
{
    final CollectionAdminRequest.DeleteAlias adminRequest = CollectionAdminRequest.DeleteAlias.deleteAlias(aliasName);

    CollectionAdminResponse adminResponse = adminRequest.process(cloudSolrClient);

    System.out.println(adminResponse);
}

cloudSolrClient.close();

final String[] solrUrl = { "http://localhost:8983/solr" };

final CloudSolrClient cloudSolrClient = new CloudSolrClient.Builder(Arrays.asList(solrUrl)).build();

// get cluster status
{
    final CollectionAdminRequest.ClusterStatus adminRequest = CollectionAdminRequest.ClusterStatus.getClusterStatus();

    CollectionAdminResponse adminResponse = adminRequest.process(cloudSolrClient);

    System.out.println(adminResponse);
}

cloudSolrClient.close();