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