code: public ThreadFactoryBuilder setNameFormat(String nameFormat) {
      String unused = format(nameFormat, 0); // fail fast if the format is bad or null
      this.nameFormat = nameFormat;
      return this;
    }
docs: Sets the naming format to use when naming threads ({@link Thread#setName}) which are created
  with this ThreadFactory.
  
  @param nameFormat a {@link String#format(String, Object...)}-compatible format String, to which
  a unique integer (0, 1, etc.) will be supplied as the single parameter. This integer will
  be unique to the built instance of the ThreadFactory and will be assigned sequentially. For
  example, {@code "rpc-pool-%d"} will generate thread names like {@code "rpc-pool-0"}, {@code
  "rpc-pool-1"}, {@code "rpc-pool-2"}, etc.
  @return this for the builder pattern