Preserve spliterator characteristics in collection views.

Fix CollectSpliterators.filter() to preserve IMMUTABLE and CONCURRENT
characteristics from the source spliterator, matching JDK Stream.filter()
behavior which only clears SIZED.

Add spliterator() overrides to Lists inner classes (OnePlusArrayList,
TwoPlusArrayList, CharSequenceAsList, AbstractListWrapper) and Maps
inner classes (KeySet, Values) so that collection views properly report
their characteristics rather than falling back to AbstractList defaults.

Fixes #8165

Author: nickita-khylkouski

guava-tests/test/com/google/common/collect/CollectSpliteratorsTest.java

@@ -25,6 +25,7 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Spliterator;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.stream.DoubleStream;
 import java.util.stream.IntStream;
 import java.util.stream.LongStream;
@@ -101,6 +102,32 @@ public void testFlatMapToDouble_nullStream() {
         .expect(1.0, 1.0, 2.0, 3.0);
   }
 
+  @GwtIncompatible // CopyOnWriteArrayList
+  public void testFilterPreservesImmutableCharacteristic() {
+    CopyOnWriteArrayList<String> cow = new CopyOnWriteArrayList<>(Arrays.asList("a", "b", "c"));
+    Spliterator<String> source = cow.spliterator();
+    assertTrue((source.characteristics() & Spliterator.IMMUTABLE) != 0);
+
+    Spliterator<String> filtered =
+        CollectSpliterators.filter(cow.spliterator(), s -> !s.equals("b"));
+    assertTrue(
+        "filter() should preserve IMMUTABLE from source",
+        (filtered.characteristics() & Spliterator.IMMUTABLE) != 0);
+  }
+
+  @GwtIncompatible
+  public void testFilterDropsSized() {
+    Spliterator<String> source =
+        Arrays.spliterator(new String[] {"a", "b", "c"});
+    assertTrue((source.characteristics() & Spliterator.SIZED) != 0);
+
+    Spliterator<String> filtered =
+        CollectSpliterators.filter(source, s -> !s.equals("b"));
+    assertFalse(
+        "filter() should not preserve SIZED",
+        (filtered.characteristics() & Spliterator.SIZED) != 0);
+  }
+
   public void testMultisetsSpliterator() {
     Multiset<String> multiset = TreeMultiset.create();
     multiset.add("a", 3);

guava-tests/test/com/google/common/collect/ListsTest.java

@@ -57,6 +57,7 @@
 import java.util.ListIterator;
 import java.util.NoSuchElementException;
 import java.util.RandomAccess;
+import java.util.Spliterator;
 import java.util.concurrent.CopyOnWriteArrayList;
 import junit.framework.Test;
 import junit.framework.TestCase;
@@ -998,4 +999,68 @@ public void testPartitionSize_1() {
   public void testPartitionSize_2() {
     assertEquals(2, partition(nCopies(0x40000001, 1), 0x40000000).size());
   }
+
+  @GwtIncompatible // Spliterator
+  @J2ktIncompatible
+  public void testAsListSpliteratorIsImmutable() {
+    List<String> list = Lists.asList("a", new String[] {"b", "c"});
+    int characteristics = list.spliterator().characteristics();
+    assertTrue("asList should report IMMUTABLE", (characteristics & Spliterator.IMMUTABLE) != 0);
+    assertTrue("asList should report ORDERED", (characteristics & Spliterator.ORDERED) != 0);
+    assertTrue("asList should report SIZED", (characteristics & Spliterator.SIZED) != 0);
+  }
+
+  @GwtIncompatible // Spliterator
+  @J2ktIncompatible
+  public void testAsListSpliteratorIteration() {
+    List<String> list = Lists.asList("a", new String[] {"b", "c"});
+    List<String> collected = new ArrayList<>();
+    list.spliterator().forEachRemaining(collected::add);
+    assertEquals(asList("a", "b", "c"), collected);
+  }
+
+  @GwtIncompatible // Spliterator
+  @J2ktIncompatible
+  public void testAsListTwoSpliteratorIsImmutable() {
+    List<String> list = Lists.asList("a", "b", new String[] {"c"});
+    int characteristics = list.spliterator().characteristics();
+    assertTrue(
+        "asList(2+) should report IMMUTABLE", (characteristics & Spliterator.IMMUTABLE) != 0);
+  }
+
+  @GwtIncompatible // Spliterator
+  @J2ktIncompatible
+  public void testAsListTwoSpliteratorIteration() {
+    List<String> list = Lists.asList("a", "b", new String[] {"c"});
+    List<String> collected = new ArrayList<>();
+    list.spliterator().forEachRemaining(collected::add);
+    assertEquals(asList("a", "b", "c"), collected);
+  }
+
+  @GwtIncompatible // Spliterator
+  @J2ktIncompatible
+  public void testCharactersOfSpliteratorCharacteristics() {
+    List<Character> chars = Lists.charactersOf("hello");
+    int characteristics = chars.spliterator().characteristics();
+    assertTrue("charactersOf should report ORDERED", (characteristics & Spliterator.ORDERED) != 0);
+    assertTrue("charactersOf should report NONNULL", (characteristics & Spliterator.NONNULL) != 0);
+    assertTrue("charactersOf should report SIZED", (characteristics & Spliterator.SIZED) != 0);
+  }
+
+  @GwtIncompatible // Spliterator
+  @J2ktIncompatible
+  public void testCharactersOfSpliteratorIteration() {
+    List<Character> chars = Lists.charactersOf("hello");
+    List<Character> collected = new ArrayList<>();
+    chars.spliterator().forEachRemaining(collected::add);
+    assertEquals(asList('h', 'e', 'l', 'l', 'o'), collected);
+  }
+
+  @GwtIncompatible // CopyOnWriteArrayList
+  @J2ktIncompatible
+  public void testAbstractListWrapperSpliteratorDelegates() {
+    CopyOnWriteArrayList<String> cow = new CopyOnWriteArrayList<>(asList("a", "b", "c"));
+    int cowCharacteristics = cow.spliterator().characteristics();
+    assertTrue((cowCharacteristics & Spliterator.IMMUTABLE) != 0);
+  }
 }

guava-tests/test/com/google/common/collect/MapsTest.java

@@ -64,6 +64,7 @@
 import java.util.Set;
 import java.util.SortedMap;
 import java.util.SortedSet;
+import java.util.Spliterator;
 import java.util.TreeMap;
 import java.util.concurrent.ConcurrentMap;
 import junit.framework.TestCase;
@@ -1624,4 +1625,31 @@ public void testSubMap_unnaturalOrdering() {
         ImmutableSortedMap.of(2, 0, 4, 0, 6, 0, 8, 0, 10, 0),
         Maps.subMap(map, Range.<Integer>all()));
   }
+
+  @GwtIncompatible // Spliterator
+  @J2ktIncompatible
+  public void testKeySetSpliteratorIsDistinct() {
+    HashMap<String, Integer> map = new HashMap<>();
+    map.put("a", 1);
+    map.put("b", 2);
+    int characteristics = map.keySet().spliterator().characteristics();
+    assertTrue(
+        "keySet spliterator should report DISTINCT",
+        (characteristics & Spliterator.DISTINCT) != 0);
+  }
+
+  @GwtIncompatible // Spliterator
+  @J2ktIncompatible
+  public void testValuesSpliteratorCharacteristics() {
+    HashMap<String, Integer> map = new HashMap<>();
+    map.put("a", 1);
+    map.put("b", 2);
+    Spliterator<Integer> valuesSpliterator = map.values().spliterator();
+    assertTrue(
+        "values spliterator should report SIZED",
+        (valuesSpliterator.characteristics() & Spliterator.SIZED) != 0);
+    assertFalse(
+        "values spliterator should NOT report DISTINCT",
+        (valuesSpliterator.characteristics() & Spliterator.DISTINCT) != 0);
+  }
 }

guava/src/com/google/common/collect/CollectSpliterators.java

@@ -199,11 +199,15 @@ public long estimateSize() {
 
       @Override
       public int characteristics() {
+        // IMMUTABLE and CONCURRENT describe the source, not the elements, so filtering
+        // does not invalidate them.  This matches JDK Stream.filter(), which only clears SIZED.
         return fromSpliterator.characteristics()
             & (Spliterator.DISTINCT
                 | Spliterator.NONNULL
                 | Spliterator.ORDERED
-                | Spliterator.SORTED);
+                | Spliterator.SORTED
+                | Spliterator.IMMUTABLE
+                | Spliterator.CONCURRENT);
       }
     }
     return new Splitr();

guava/src/com/google/common/collect/Lists.java

@@ -50,6 +50,7 @@
 import java.util.NoSuchElementException;
 import java.util.Objects;
 import java.util.RandomAccess;
+import java.util.Spliterator;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.function.Predicate;
 import org.jspecify.annotations.Nullable;
@@ -356,6 +357,14 @@ public E get(int index) {
       return (index == 0) ? first : rest[index - 1];
     }
 
+    @Override
+    @GwtIncompatible
+    @J2ktIncompatible
+    public Spliterator<E> spliterator() {
+      return CollectSpliterators.indexed(
+          size(), Spliterator.ORDERED | Spliterator.IMMUTABLE | Spliterator.SIZED, this::get);
+    }
+
     @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
   }
 
@@ -394,6 +403,14 @@ public E get(int index) {
       }
     }
 
+    @Override
+    @GwtIncompatible
+    @J2ktIncompatible
+    public Spliterator<E> spliterator() {
+      return CollectSpliterators.indexed(
+          size(), Spliterator.ORDERED | Spliterator.IMMUTABLE | Spliterator.SIZED, this::get);
+    }
+
     @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
   }
 
@@ -829,6 +846,14 @@ public Character get(int index) {
     public int size() {
       return sequence.length();
     }
+
+    @Override
+    @GwtIncompatible
+    @J2ktIncompatible
+    public Spliterator<Character> spliterator() {
+      return CollectSpliterators.indexed(
+          size(), Spliterator.ORDERED | Spliterator.NONNULL | Spliterator.SIZED, this::get);
+    }
   }
 
   /**
@@ -1201,6 +1226,13 @@ public boolean contains(@Nullable Object o) {
     public int size() {
       return backingList.size();
     }
+
+    @Override
+    @GwtIncompatible
+    @J2ktIncompatible
+    public Spliterator<E> spliterator() {
+      return backingList.spliterator();
+    }
   }
 
   private static class RandomAccessListWrapper<E extends @Nullable Object>

guava/src/com/google/common/collect/Maps.java

@@ -3942,6 +3942,14 @@ public boolean remove(@Nullable Object o) {
     public void clear() {
       map().clear();
     }
+
+    @Override
+    @GwtIncompatible
+    @J2ktIncompatible
+    public Spliterator<K> spliterator() {
+      return CollectSpliterators.map(
+          map().entrySet().spliterator(), Spliterator.DISTINCT, Entry::getKey);
+    }
   }
 
   static <K extends @Nullable Object> @Nullable K keyOrNull(@Nullable Entry<K, ?> entry) {
@@ -4171,6 +4179,13 @@ public boolean contains(@Nullable Object o) {
     public void clear() {
       map().clear();
     }
+
+    @Override
+    @GwtIncompatible
+    @J2ktIncompatible
+    public Spliterator<V> spliterator() {
+      return CollectSpliterators.map(map().entrySet().spliterator(), 0, Entry::getValue);
+    }
   }
 
   abstract static class EntrySet<K extends @Nullable Object, V extends @Nullable Object>