code: public void trim(int ntrees) {
          if (ntrees > trees.size()) {
              throw new IllegalArgumentException("The new model size is larger than the current size.");
          }
          
          if (ntrees <= 0) {
              throw new IllegalArgumentException("Invalid new model size: " + ntrees);
          }
  
          List<Tree> model = new ArrayList<>(ntrees);
          for (int i = 0; i < ntrees; i++) {
              model.add(trees.get(i));
          }
          
          trees = model;
      }
docs: Trims the tree model set to a smaller size in case of over-fitting.
  Or if extra decision trees in the model don't improve the performance,
  we may remove them to reduce the model size and also improve the speed of
  prediction.
  
  @param ntrees the new (smaller) size of tree model set.