Let composite/filtered collections accept null elements

Closes gh-36923

Signed-off-by: Yanming Zhou <zhouyanming@gmail.com>

spring-core/src/main/java/org/springframework/util/CompositeCollection.java

@@ -28,10 +28,11 @@ import org.jspecify.annotations.Nullable;
  * exposed through {@link CompositeMap#values()}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.2
  * @param <E> the type of elements maintained by this collection
  */
-class CompositeCollection<E> implements Collection<E> {
+class CompositeCollection<E extends @Nullable Object> implements Collection<E> {
 
 	private final Collection<E> first;
 

spring-core/src/main/java/org/springframework/util/CompositeIterator.java

@@ -21,6 +21,8 @@ import java.util.LinkedHashSet;
 import java.util.NoSuchElementException;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Composite iterator that combines multiple other iterators,
  * as registered via {@link #add(Iterator)}.
@@ -30,10 +32,11 @@ import java.util.Set;
  *
  * @author Erwin Vervaet
  * @author Juergen Hoeller
+ * @author Yanming Zhou
  * @since 3.0
  * @param <E> the element type
  */
-public class CompositeIterator<E> implements Iterator<E> {
+public class CompositeIterator<E extends @Nullable Object> implements Iterator<E> {
 
 	private final Set<Iterator<E>> iterators = new LinkedHashSet<>();
 

spring-core/src/main/java/org/springframework/util/CompositeMap.java

@@ -32,17 +32,18 @@ import org.jspecify.annotations.Nullable;
  * {@link CollectionUtils#compositeMap(Map, Map, BiFunction, Consumer)}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.2
  * @param <K> the type of keys maintained by this map
  * @param <V> the type of mapped values
  */
-final class CompositeMap<K, V> implements Map<K, V> {
+final class CompositeMap<K, V extends @Nullable Object> implements Map<K, V> {
 
-	private final Map<K,V> first;
+	private final Map<K, V> first;
 
-	private final Map<K,V> second;
+	private final Map<K, V> second;
 
-	private final @Nullable BiFunction<K,V,V> putFunction;
+	private final @Nullable BiFunction<K, V, V> putFunction;
 
 	private final @Nullable Consumer<Map<K, V>> putAllFunction;
 
@@ -53,7 +54,7 @@ final class CompositeMap<K, V> implements Map<K, V> {
 
 	CompositeMap(Map<K, V> first, Map<K, V> second,
 			@Nullable BiFunction<K, V, V> putFunction,
-			@Nullable Consumer<Map<K,V>> putAllFunction) {
+			@Nullable Consumer<Map<K, V>> putAllFunction) {
 
 		Assert.notNull(first, "First must not be null");
 		Assert.notNull(second, "Second must not be null");
@@ -106,7 +107,7 @@ final class CompositeMap<K, V> implements Map<K, V> {
 	}
 
 	@Override
-	public @Nullable V put(K key, V value) {
+	public V put(K key, V value) {
 		if (this.putFunction == null) {
 			throw new UnsupportedOperationException();
 		}
@@ -116,7 +117,7 @@ final class CompositeMap<K, V> implements Map<K, V> {
 	}
 
 	@Override
-	public @Nullable V remove(Object key) {
+	public V remove(Object key) {
 		V firstResult = this.first.remove(key);
 		V secondResult = this.second.remove(key);
 		if (firstResult != null) {

spring-core/src/main/java/org/springframework/util/CompositeSet.java

@@ -25,10 +25,11 @@ import org.jspecify.annotations.Nullable;
  * {@link CompositeMap#keySet()} and {@link CompositeMap#entrySet()}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.2
  * @param <E> the type of elements maintained by this set
  */
-final class CompositeSet<E> extends CompositeCollection<E> implements Set<E> {
+final class CompositeSet<E extends @Nullable Object> extends CompositeCollection<E> implements Set<E> {
 
 	CompositeSet(Set<E> first, Set<E> second) {
 		super(first, second);

spring-core/src/main/java/org/springframework/util/FilteredCollection.java

@@ -21,15 +21,18 @@ import java.util.Collection;
 import java.util.Iterator;
 import java.util.function.Predicate;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Collection that filters out values that do not match a predicate.
  * This type is used by {@link CompositeMap}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.2
  * @param <E> the type of elements maintained by this collection
  */
-class FilteredCollection<E> extends AbstractCollection<E> {
+class FilteredCollection<E extends @Nullable Object> extends AbstractCollection<E> {
 
 	private final Collection<E> delegate;
 

spring-core/src/main/java/org/springframework/util/FilteredIterator.java

@@ -28,10 +28,11 @@ import org.jspecify.annotations.Nullable;
  * <p>This type is used by {@link CompositeMap}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.2
  * @param <E> the type of elements returned by this iterator
  */
-final class FilteredIterator<E> implements Iterator<E> {
+final class FilteredIterator<E extends @Nullable Object> implements Iterator<E> {
 
 	private final Iterator<E> delegate;
 
@@ -56,12 +57,11 @@ final class FilteredIterator<E> implements Iterator<E> {
 	}
 
 	@Override
-	public E next() {
+	public @Nullable E next() {
 		if (!this.hasNext && !setNext()) {
 			throw new NoSuchElementException();
 		}
 		this.hasNext = false;
-		Assert.state(this.next != null, "Next should not be null");
 		return this.next;
 	}
 

spring-core/src/main/java/org/springframework/util/FilteredMap.java

@@ -28,11 +28,12 @@ import org.jspecify.annotations.Nullable;
  * This type is used by {@link CompositeMap}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.2
  * @param <K> the type of keys maintained by this map
  * @param <V> the type of mapped values
  */
-final class FilteredMap<K, V> extends AbstractMap<K, V> {
+final class FilteredMap<K, V extends @Nullable Object> extends AbstractMap<K, V> {
 
 	private final Map<K, V> delegate;
 

spring-core/src/main/java/org/springframework/util/FilteredSet.java

@@ -26,10 +26,11 @@ import org.jspecify.annotations.Nullable;
  * This type is used by {@link CompositeMap}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.2
  * @param <E> the type of elements maintained by this set
  */
-final class FilteredSet<E> extends FilteredCollection<E> implements Set<E> {
+final class FilteredSet<E extends @Nullable Object> extends FilteredCollection<E> implements Set<E> {
 
 	public FilteredSet(Set<E> delegate, Predicate<E> filter) {
 		super(delegate, filter);

spring-core/src/test/java/org/springframework/util/CompositeCollectionTests.java

@@ -21,6 +21,7 @@ import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -28,6 +29,7 @@ import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 class CompositeCollectionTests {
 
@@ -192,4 +194,17 @@ class CompositeCollectionTests {
 		assertThat(first).isEmpty();
 		assertThat(second).isEmpty();
 	}
+
+	@Test
+	void nullable() {
+		List<@Nullable String> first = new ArrayList<>();
+		first.add("foo");
+		first.add(null);
+		List<@Nullable String> second = new ArrayList<>();
+		second.add("bar");
+		second.add(null);
+		CompositeCollection<@Nullable String> composite = new CompositeCollection<>(first, second);
+
+		assertThat(composite).containsExactly("foo", null, "bar", null);
+	}
 }

spring-core/src/test/java/org/springframework/util/CompositeIteratorTests.java

@@ -21,6 +21,7 @@ import java.util.Iterator;
 import java.util.List;
 import java.util.NoSuchElementException;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -33,6 +34,7 @@ import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
  *
  * @author Erwin Vervaet
  * @author Juergen Hoeller
+ * @author Yanming Zhou
  */
 class CompositeIteratorTests {
 
@@ -99,4 +101,13 @@ class CompositeIteratorTests {
 				it.add(iterator));
 	}
 
+	@Test
+	void nullable() {
+		List<@Nullable String> first = Arrays.asList("1", null);
+		List<@Nullable String> second = Arrays.asList("2", null);
+		CompositeIterator<String> it = new CompositeIterator<>();
+		it.add(first.iterator());
+		it.add(second.iterator());
+		assertThat(it).toIterable().containsExactly("1", null, "2", null);
+	}
 }

spring-core/src/test/java/org/springframework/util/CompositeMapTests.java

@@ -23,6 +23,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 
@@ -32,6 +33,7 @@ import static org.assertj.core.api.Assertions.entry;
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 class CompositeMapTests {
 
@@ -220,6 +222,17 @@ class CompositeMapTests {
 		assertThat(entries).containsExactly(entry("foo", "bar"), entry("baz", "qux"));
 	}
 
+	@Test
+	void nullable() {
+		Map<String, @Nullable String> first = new HashMap<>();
+		first.put("foo", "bar");
+		Map<String, @Nullable String> second = new HashMap<>();
+		second.put("baz", null);
+		CompositeMap<String, @Nullable String> composite = new CompositeMap<>(first, second);
+
+		assertThat(composite).containsExactly(entry("foo", "bar"), entry("baz", null));
+	}
+
 	@Nested
 	class CollisionTests {
 

spring-core/src/test/java/org/springframework/util/CompositeSetTests.java

@@ -20,12 +20,14 @@ import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 class CompositeSetTests {
 
@@ -44,4 +46,16 @@ class CompositeSetTests {
 		assertThat(composite).isNotEqualTo(Collections.emptySet());
 	}
 
+	@Test
+	void nullable() {
+		Set<@Nullable String> first = new HashSet<>();
+		first.add("foo");
+		first.add(null);
+		Set<@Nullable String> second = new HashSet<>();
+		second.add("bar");
+		first.add(null);
+		CompositeSet<@Nullable String> composite = new CompositeSet<>(first, second);
+
+		assertThat(composite).containsExactlyInAnyOrder("foo", null, "bar");
+	}
 }

spring-core/src/test/java/org/springframework/util/FilteredCollectionTests.java

@@ -17,14 +17,17 @@
 package org.springframework.util;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 class FilteredCollectionTests {
 
@@ -74,4 +77,11 @@ class FilteredCollectionTests {
 		assertThat(contained).isFalse();
 	}
 
+	@Test
+	void nullable() {
+		List<@Nullable String> list = Arrays.asList("foo", "bar", null);
+		FilteredCollection<@Nullable String> filtered = new FilteredCollection<>(list, s -> !"bar".equals(s));
+
+		assertThat(filtered).containsExactlyInAnyOrder("foo", null);
+	}
 }

spring-core/src/test/java/org/springframework/util/FilteredIteratorTests.java

@@ -16,14 +16,17 @@
 
 package org.springframework.util;
 
+import java.util.Arrays;
 import java.util.List;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 final class FilteredIteratorTests {
 
@@ -35,4 +38,12 @@ final class FilteredIteratorTests {
 		assertThat(filtered).toIterable().containsExactly("foo", "baz");
 	}
 
+	@Test
+	void nullable() {
+		List<@Nullable String> list = Arrays.asList("foo", "bar", null);
+		FilteredIterator<@Nullable String> filtered = new FilteredIterator<>(list.iterator(), s -> !"bar".equals(s));
+
+		assertThat(filtered).toIterable().containsExactly("foo", null);
+	}
+
 }

spring-core/src/test/java/org/springframework/util/FilteredMapTests.java

@@ -20,6 +20,7 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static java.util.Map.entry;
@@ -27,6 +28,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 class FilteredMapTests {
 
@@ -100,4 +102,15 @@ class FilteredMapTests {
 		Set<String> keySet = filtered.keySet();
 		assertThat(keySet).containsExactlyInAnyOrder("foo", "quux");
 	}
+
+	@Test
+	void nullable() {
+		Map<String, @Nullable String> map = new HashMap<>();
+		map.put("foo", null);
+		map.put("bar", "bar");
+		map.put("baz", "baz");
+		FilteredMap<String, @Nullable String> filtered = new FilteredMap<>(map, s -> !s.equals("baz"));
+
+		assertThat(filtered).containsEntry("foo", null).containsEntry("bar", "bar").doesNotContainKeys("baz");
+	}
 }

spring-core/src/test/java/org/springframework/util/FilteredSetTests.java

@@ -17,14 +17,17 @@
 package org.springframework.util;
 
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 class FilteredSetTests {
 
@@ -39,4 +42,15 @@ class FilteredSetTests {
 		assertThat(filtered).isNotEqualTo(set);
 		assertThat(filtered).isNotEqualTo(Collections.emptySet());
 	}
+
+	@Test
+	void nullable() {
+		Set<@Nullable String> set = new HashSet<>();
+		set.add("foo");
+		set.add("bar");
+		set.add(null);
+		FilteredSet<@Nullable String> filtered = new FilteredSet<>(set, s -> !"bar".equals(s));
+
+		assertThat(filtered).containsExactlyInAnyOrder("foo", null);
+	}
 }