code: public static void initialize(KeenClient client) {
          if (client == null) {
              throw new IllegalArgumentException("Client must not be null");
          }
  
          if (ClientSingleton.INSTANCE.client != null) {
              // Do nothing.
              return;
          }
  
          ClientSingleton.INSTANCE.client = client;
      }
docs: Initializes the static Keen client. Only the first call to this method has any effect. All
  subsequent calls are ignored.
  
  @param client The {@link io.keen.client.java.KeenClient} implementation to use as the
  singleton client for the library.