### Question:
ConcatenatedDataset extends AbstractDataset { public ConcatenatedDataset(Dataset... datasets) { this(Arrays.asList(datasets)); } ConcatenatedDataset(Dataset... datasets);  ConcatenatedDataset(Collection<Dataset> datasets); @Override CloseableIterator<Instance> iterator(); @Override FeatureInformation getFeatureInformation(); @Override long size();  }### Answer:
@Test public void testConcatenatedDataset() { Dataset d1 = createTestDataset(10, "dataset_1"); Dataset d2 = createTestDataset(5, "dataset_2"); Dataset d3 = createTestDataset(15, "dataset_3"); ConcatenatedDataset concatenatedDataset = new ConcatenatedDataset(d1, d2, d3); assertEquals(30, concatenatedDataset.size()); assertEquals(30, CollectionHelper.count(concatenatedDataset.iterator())); }