public Builder id(String id) {
      if (id == null) throw new NullPointerException("id == null");
      int length = id.length();
      if (length == 0) throw new IllegalArgumentException("id is empty");
      if (length > 16) throw new IllegalArgumentException("id.length > 16");
      if (validateHexAndReturnZeroPrefix(id) == 16) {
        throw new IllegalArgumentException("id is all zeros");
      }
      this.id = length < 16 ? padLeft(id, 16) : id;
      return this;
    }
Sets {@link Span#id()} or throws {@link IllegalArgumentException} if not lower-hex format.
zipkin/src/main/java/zipkin2/Span.java