### Question:
PartitionedFeatureVectors implements FeatureVectors { @Override public String toString() { int maxSize = 16; Collection<String> partitionSizes = new ArrayList<>(maxSize); for (int i = 0; i < partitions.length; i++) { int size = partitions[i].size(); if (size > 0) { partitionSizes.add(i + ":" + size); if (partitionSizes.size() == maxSize) { partitionSizes.add("..."); break; } } } return partitionSizes.toString(); } PartitionedFeatureVectors(int numPartitions,
                                   ExecutorService executor);  PartitionedFeatureVectors(int numPartitions,
                                   ExecutorService executor,
                                   ToIntBiFunction<String,float[]> partitioner); @Override int size(); Stream<T> mapPartitionsParallel(Function<FeatureVectorsPartition,Stream<T>> fn,
                                             boolean background); Stream<T> mapPartitionsParallel(Function<FeatureVectorsPartition,Stream<T>> fn,
                                             int[] candidateIndices,
                                             boolean background); @Override float[] getVector(String item); @Override void setVector(String item, float[] vector); @Override void addAllIDsTo(Collection<String> allIDs); @Override void removeAllIDsFrom(Collection<String> allIDs); @Override void addAllRecentTo(Collection<String> allRecent); @Override void retainRecentAndIDs(Collection<String> newModelIDs); @Override double[] getVTV(boolean background); @Override String toString();  }### Answer:
@Test public void testToString() { PartitionedFeatureVectors partitioned = new PartitionedFeatureVectors(2, getExecutor()); partitioned.setVector("A", new float[] { 1.0f, 3.0f, 6.0f }); assertEquals("[1:1]", partitioned.toString()); }