code: @Override
    public boolean setResult(T value, boolean isLast) {
      return super.setResult(Preconditions.checkNotNull(value), isLast);
    }
docs: Sets the result to {@code value}.
  
  <p> This method will return {@code true} if the value was successfully set, or
  {@code false} if the data source has already been set, failed or closed.
  
  <p> If the value was successfully set and {@code isLast} is {@code true}, state of the
  data source will be set to {@link AbstractDataSource.DataSourceStatus#SUCCESS}.
  
  <p> This will also notify the subscribers if the value was successfully set.
  
  @param value the value to be set
  @param isLast whether or not the value is last.
  @return true if the value was successfully set.