code: public RandomForest merge(RandomForest other) {
          if (this.importance.length != other.importance.length) {
              throw new IllegalArgumentException("RandomForest have different sizes of feature vectors");
          }
  
          ArrayList<RegressionTree> mergedTrees = new ArrayList<>();
          mergedTrees.addAll(this.trees);
          mergedTrees.addAll(other.trees);
  
          double weightedMergedError = ((this.error * this.trees.size()) + (other.error * other.trees.size())) / (this.trees.size() + other.trees.size());
          double[] mergedImportance = calculateImportance(mergedTrees, this.importance.length);
  
          return new RandomForest(mergedTrees, weightedMergedError, mergedImportance);
      }
docs: Merges together two random forests and returns a new forest consisting of trees from both input forests.