Let composite/filtered collections accept null elements

1. Keep `FilteredIterator` consistent with `CompositeIterator` to accept `null`
2. Fix warning `Non-null type argument is expected` and `Assigning a class with nullable type arguments when a class with not-null type arguments is expected`

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

Author: quaff

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

@@ -28,10 +28,11 @@
  * 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

@@ -16,6 +16,8 @@
 
 package org.springframework.util;
 
+import org.jspecify.annotations.Nullable;
+
 import java.util.Iterator;
 import java.util.LinkedHashSet;
 import java.util.NoSuchElementException;
@@ -30,10 +32,11 @@
  *
  * @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 @@
  * {@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 @@ public boolean containsValue(Object value) {
 	}
 
 	@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 @@ public boolean containsValue(Object value) {
 	}
 
 	@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 @@
  * {@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

@@ -16,6 +16,8 @@
 
 package org.springframework.util;
 
+import org.jspecify.annotations.Nullable;
+
 import java.util.AbstractCollection;
 import java.util.Collection;
 import java.util.Iterator;
@@ -26,10 +28,11 @@
  * 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 @@
  * <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 @@ public boolean hasNext() {
 	}
 
 	@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 @@
  * 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 @@
  * 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

@@ -21,13 +21,15 @@
 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;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 class CompositeCollectionTests {
 
@@ -192,4 +194,17 @@ void clear() {
 		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.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 @@
  *
  * @author Erwin Vervaet
  * @author Juergen Hoeller
+ * @author Yanming Zhou
  */
 class CompositeIteratorTests {
 
@@ -99,4 +101,13 @@ void duplicateIterators() {
 				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);
+	}
 }