code: public void setWeakLearner(Classifier weakL)
      {
          if(weakL == null)
              throw new NullPointerException();
          this.weakL = weakL;
          if(weakL instanceof Regressor)
              this.weakR = (Regressor) weakL;
      }
docs: Sets the weak learner used for classification. If it also supports
  regressions that will be set as well.
  @param weakL the weak learner to use