code: void seal() {
          if (!this.sealed && !this.contents.isReadOnly()) {
              Preconditions.checkState(writeEntryStartIndex < 0, "An open entry exists. Any open entries must be closed prior to sealing.");
  
              this.header.setContentLength(writePosition);
              this.header.commit();
              this.sealed = true;
          }
      }
docs: Seals the frame for writing. After this method returns, no more modifications are allowed on this DataFrame.
  This method has no effect if the Frame is read-only if it is already sealed.
  
  @throws IllegalStateException If an open entry exists (entries must be closed prior to sealing).