MATH-1525: Make "EmptyClusterStrategy" and related logic in "KMeansPlusPlusClusterer" reusable.

src/main/java/org/apache/commons/math4/ml/clustering/KMeansPlusPlusClusterer.java

@@ -188,6 +188,14 @@ public class KMeansPlusPlusClusterer<T extends Clusterable> extends Clusterer<T>
         return emptyStrategy;
     }
 
+    /**
+     * Return the CentroidInitializer used by this instance.
+     * @return the CentroidInitializer
+     */
+    CentroidInitializer getCentroidInitializer() {
+        return centroidInitializer;
+    }
+
     /**
      * Runs the K-means++ clustering algorithm.
      *
@@ -219,7 +227,26 @@ public class KMeansPlusPlusClusterer<T extends Clusterable> extends Clusterer<T>
         // iterate through updating the centers until we're done
         final int max = (maxIterations < 0) ? Integer.MAX_VALUE : maxIterations;
         for (int count = 0; count < max; count++) {
-            boolean emptyCluster = false;
+            boolean hasEmptyCluster = clusters.stream().anyMatch(cluster->cluster.getPoints().isEmpty());
+            List<CentroidCluster<T>> newClusters = adjustClustersCenters(clusters);
+            int changes = assignPointsToClusters(newClusters, points, assignments);
+            clusters = newClusters;
+
+            // if there were no more changes in the point-to-cluster assignment
+            // and there are no empty clusters left, return the current clusters
+            if (changes == 0 && !hasEmptyCluster) {
+                return clusters;
+            }
+        }
+        return clusters;
+    }
+
+    /**
+     * Adjust the clusters's centers with means of points
+     * @param clusters the origin clusters
+     * @return adjusted clusters with center points
+     */
+    List<CentroidCluster<T>> adjustClustersCenters(List<CentroidCluster<T>> clusters) {
         List<CentroidCluster<T>> newClusters = new ArrayList<>();
         for (final CentroidCluster<T> cluster : clusters) {
             final Clusterable newCenter;
@@ -237,22 +264,12 @@ public class KMeansPlusPlusClusterer<T extends Clusterable> extends Clusterer<T>
                     default :
                         throw new ConvergenceException(LocalizedFormats.EMPTY_CLUSTER_IN_K_MEANS);
                 }
-                    emptyCluster = true;
             } else {
                 newCenter = cluster.centroid();
             }
-                newClusters.add(new CentroidCluster<T>(newCenter));
+            newClusters.add(new CentroidCluster<>(newCenter));
         }
-            int changes = assignPointsToClusters(newClusters, points, assignments);
+        return newClusters;
-            clusters = newClusters;
-
-            // if there were no more changes in the point-to-cluster assignment
-            // and there are no empty clusters left, return the current clusters
-            if (changes == 0 && !emptyCluster) {
-                return clusters;
-            }
-        }
-        return clusters;
     }
 
     /**