add tests; fix ClassCastException

utility/src/main/java/com/dua3/utility/lang/BatchCollector.java

@@ -61,7 +61,7 @@ public BiConsumer<Deque<Pair<K, List<T>>>, T> accumulator() {
             K key = keyMapper.apply(item);
 
             List<T> bucket;
-            if (accu.isEmpty() || (key != null && !Objects.equals(key, accu.peekLast().first()))) {
+            if (accu.isEmpty() || (!Objects.equals(key, accu.peekLast().first()))) {
                 bucket = new ArrayList<>();
                 accu.addLast(Pair.of(key == null ? defaultKey : key, bucket));
             } else {
@@ -87,6 +87,6 @@ public Function<Deque<Pair<K, List<T>>>, List<Pair<K, List<T>>>> finisher() {
 
     @Override
     public Set<Characteristics> characteristics() {
-        return EnumSet.of(Characteristics.IDENTITY_FINISH);
+        return EnumSet.noneOf(Characteristics.class);
     }
 }

utility/src/test/java/com/dua3/utility/lang/BatchCollectorTest.java

@@ -0,0 +1,60 @@
+package com.dua3.utility.lang;
+
+import com.dua3.utility.data.Pair;
+import org.junit.jupiter.api.Test;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Collector;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * This is a test class for the BatchCollector class.
+ * It aims to tests the functionality of the methods within the BatchCollector class.
+ */
+public class BatchCollectorTest {
+
+    /**
+     * This test is for testing the functionality of BatchCollector when generating grouping.
+     */
+    @Test
+    public void batchCollector_groupingGenerationTest() {
+        BatchCollector<String, Integer> bc = new BatchCollector<>(s -> s != null ? s.length() : null);
+        var result = Stream.of("one", "two", "three", "four", "five", "six").collect(bc);
+
+        assertEquals(4, result.size());
+        assertEquals(3, result.get(0).first().intValue());
+        assertEquals(List.of("one","two"), result.get(0).second());
+        assertEquals(5, result.get(1).first().intValue());
+        assertEquals(List.of("three"), result.get(1).second());
+        assertEquals(4, result.get(2).first().intValue());
+        assertEquals(List.of("four","five"), result.get(2).second());
+        assertEquals(3, result.get(3).first().intValue());
+        assertEquals(List.of("six"), result.get(3).second());
+    }
+
+    /**
+     * This test is for testing the functionality of BatchCollector when generating grouping with default key.
+     */
+    @Test
+    public void batchCollector_defaultKeyGroupingGenerationTest() {
+        BatchCollector<String, Integer> bc = new BatchCollector<>(s -> s != null ? s.length() : null, 0);
+        var result = Stream.of("one", "two", "three", "four", "five", "six", null).collect(bc);
+
+        assertEquals(5, result.size());
+        assertEquals(3, result.get(0).first().intValue());
+        assertEquals(List.of("one","two"), result.get(0).second());
+        assertEquals(5, result.get(1).first().intValue());
+        assertEquals(List.of("three"), result.get(1).second());
+        assertEquals(4, result.get(2).first().intValue());
+        assertEquals(List.of("four","five"), result.get(2).second());
+        assertEquals(3, result.get(3).first().intValue());
+        assertEquals(List.of("six"), result.get(3).second());
+        assertEquals(0, result.get(4).first().intValue());
+        assertEquals(Collections.singletonList(null), result.get(4).second());
+    }
+
+}