Better error message for Collectors.toMap

Author: zbynek

user/super/com/google/gwt/emul/java/util/stream/Collectors.java

@@ -376,12 +376,31 @@ public static <T> Collector<T,?,List<T>> toList() {
   public static <T, K, U> Collector<T, ?, Map<K, U>> toMap(
       final Function<? super T, ? extends K> keyMapper,
       final Function<? super T, ? extends U> valueMapper) {
-    return toMap(
-        keyMapper,
-        valueMapper,
+    return Collector.of(
+        HashMap::new,
+        (map, item) -> {
+          K key = keyMapper.apply(item);
+          U newValue = Objects.requireNonNull(valueMapper.apply(item));
+          if (map.containsKey(key)) {
+            throw getDuplicateKeyException(key);
+          } else {
+            map.put(key, newValue);
+          }
+        },
         (m1, m2) -> {
-          throw new IllegalStateException("Duplicate key " + m1);
-        });
+          for (Map.Entry<K, U> entry : m2.entrySet()) {
+            if (m1.get(entry.getKey()) != null) {
+              throw getDuplicateKeyException(entry.getKey());
+            }
+            m1.put(entry.getKey(), entry.getValue());
+          }
+          return m1;
+        },
+        Collector.Characteristics.IDENTITY_FINISH);
+  }
+
+  private static RuntimeException getDuplicateKeyException(Object key) {
+    return new IllegalStateException("Duplicate key: " + key);
   }
 
   public static <T, K, U> Collector<T, ?, Map<K, U>> toMap(
@@ -423,7 +442,7 @@ private static <T, R> Function<T, R> disallowNulls(Function<T, R> func) {
         mapSupplier,
         (map, item) -> {
           K key = keyMapper.apply(item);
-          U newValue = valueMapper.apply(item);
+          U newValue = Objects.requireNonNull(valueMapper.apply(item));
           if (map.containsKey(key)) {
             map.put(key, mergeFunction.apply(map.get(key), newValue));
           } else {

user/test/com/google/gwt/emultest/java8/util/stream/CollectorsTest.java

@@ -49,6 +49,7 @@
 import java.util.IntSummaryStatistics;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.LongSummaryStatistics;
 import java.util.Map;
 import java.util.Optional;
@@ -406,12 +407,13 @@ public void testList() {
   }
 
   public void testMap() {
-    Collector<String, ?, Map<String, String>> c = toMap(Function.identity(), Function.identity());
+    Function<String, String> toUpper = s -> s.toUpperCase(Locale.ROOT);
+    Collector<String, ?, Map<String, String>> c = toMap(Function.identity(), toUpper);
 
     // two distinct items
     Map<String, String> map = new HashMap<>();
-    map.put("a", "a");
-    map.put("b", "b");
+    map.put("a", "A");
+    map.put("b", "B");
     applyItems(map, c, "a", "b");
 
     // inline applyItems and test each to confirm failure for duplicates
@@ -425,10 +427,11 @@ public void testMap() {
       applyItemsWithSplitting(c, "a", "a");
       fail("expected IllegalStateException");
     } catch (IllegalStateException expected) {
+      assertEquals("Duplicate key a", expected.getMessage());
     }
 
     assertZeroItemsCollectedAs(Collections.emptyMap(), c);
-    assertSingleItemCollectedAs(Collections.singletonMap("a", "a"), c, "a");
+    assertSingleItemCollectedAs(Collections.singletonMap("a", "A"), c, "a");
 
     List<String> seen = new ArrayList<>();
     c = toMap(Function.identity(), Function.identity(), (s, s2) -> {
@@ -442,6 +445,24 @@ public void testMap() {
     assertEquals(Arrays.asList("first: a", "second: a", "first: a", "second: a"), seen);
   }
 
+  public void testMapInvalid() {
+    Collector<String, ?, Map<String, String>> c = toMap(Function.identity(), ignore -> null);
+
+    // inline applyItems and test each to confirm failure for duplicates
+    try {
+      applyItemsWithoutSplitting(c, "a", "b");
+      fail("expected NullPointerException");
+    } catch (NullPointerException expected) {
+      assertEquals("Duplicate key a", expected.getMessage());
+    }
+    try {
+      applyItemsWithSplitting(c, "a", "a");
+      fail("expected NullPointerException");
+    } catch (NullPointerException expected) {
+      assertEquals("Duplicate key a", expected.getMessage());
+    }
+  }
+
   public void testSet() {
     Collector<String, ?, Set<String>> c = toSet();