8317993: Add capturing factories to classes in java.util.function package

src/java.base/share/classes/java/util/function/BiConsumer.java

@@ -101,8 +101,9 @@ default BiConsumer<T, U> andThen(BiConsumer<? super T, ? super U> after) {
      * @param <T> the type of the first argument to the operation
      * @param <U> the type of the second argument to the operation
      */
-    static <T, U> BiConsumer<T, U> of(BiConsumer<T, U> uncaptured) {
-        return Objects.requireNonNull(uncaptured);
+    @SuppressWarnings("unchecked")
+    static <T, U> BiConsumer<T, U> of(BiConsumer<? super T, ? super U> uncaptured) {
+        return (BiConsumer<T, U>) Objects.requireNonNull(uncaptured);
     }
 
 }

src/java.base/share/classes/java/util/function/BiFunction.java

@@ -91,8 +91,9 @@ default <V> BiFunction<T, U, V> andThen(Function<? super R, ? extends V> after)
      * @param <U> the type of the second argument to the function
      * @param <R> the type of the result of the function
      */
-    static <T, U, R> BiFunction<T, U, R> of(BiFunction<T, U, R> uncaptured) {
-        return Objects.requireNonNull(uncaptured);
+    @SuppressWarnings("unchecked")
+    static <T, U, R> BiFunction<T, U, R> of(BiFunction<? super T, ? super U, ? extends R> uncaptured) {
+        return (BiFunction<T, U, R>) Objects.requireNonNull(uncaptured);
     }
 
 }

src/java.base/share/classes/java/util/function/BiPredicate.java

@@ -125,8 +125,9 @@ default BiPredicate<T, U> or(BiPredicate<? super T, ? super U> other) {
      * @param <T> the type of the first argument to the predicate
      * @param <U> the type of the second argument the predicate
      */
-    static <T, U> BiPredicate<T, U> of(BiPredicate<T, U> uncaptured) {
-        return Objects.requireNonNull(uncaptured);
+    @SuppressWarnings("unchecked")
+    static <T, U> BiPredicate<T, U> of(BiPredicate<? super T, ? super U> uncaptured) {
+        return (BiPredicate<T, U>) Objects.requireNonNull(uncaptured);
     }
 
 }

src/java.base/share/classes/java/util/function/Consumer.java

@@ -84,8 +84,9 @@ default Consumer<T> andThen(Consumer<? super T> after) {
      * @param uncaptured to capture
      * @param <T> the type of the input to the operation
      */
-    static <T> Consumer<T> of(Consumer<T> uncaptured) {
-        return Objects.requireNonNull(uncaptured);
+    @SuppressWarnings("unchecked")
+    static <T> Consumer<T> of(Consumer<? super T> uncaptured) {
+        return (Consumer<T>) Objects.requireNonNull(uncaptured);
     }
 
 }

src/java.base/share/classes/java/util/function/Function.java

@@ -118,8 +118,9 @@ static <T> Function<T, T> identity() {
      * @param <T> the type of the input to the function
      * @param <R> the type of the result of the function
      */
-    static <T, R> Function<T, R> of(Function<T, R> uncaptured) {
-        return Objects.requireNonNull(uncaptured);
+    @SuppressWarnings("unchecked")
+    static <T, R> Function<T, R> of(Function<? super T, ? extends R> uncaptured) {
+        return (Function<T, R>) Objects.requireNonNull(uncaptured);
     }
 
 }

src/java.base/share/classes/java/util/function/Predicate.java

@@ -157,8 +157,9 @@ static <T> Predicate<T> not(Predicate<? super T> target) {
      * @param uncaptured to capture
      * @param <T> the type of the input to the predicate
      */
-    static <T> Predicate<T> of(Predicate<T> uncaptured) {
-        return Objects.requireNonNull(uncaptured);
+    @SuppressWarnings("unchecked")
+    static <T> Predicate<T> of(Predicate<? super T> uncaptured) {
+        return (Predicate<T>) Objects.requireNonNull(uncaptured);
     }
 
 }

test/jdk/java/util/function/CapturingFactoriesTest.java

@@ -39,7 +39,6 @@
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.function.UnaryOperator;
-import java.util.logging.Logger;
 
 import static org.junit.jupiter.api.Assertions.*;
 
@@ -165,6 +164,21 @@ void unaryOperator() {
         assertEquals("a", composed.apply(" a "));
     }
 
+
+    @Test
+    void coVariants() {
+        var function = Function.of(Number::toString);                // Function<Number, String>
+        Function<Integer, String> sFunction = Function.of(function); // Function<String, String>
+        var sFunction2 = Function.of(function);                      // Function<Number, String>
+    }
+
+    @Test
+    void contraVariants() {
+        var function = Function.of(Object::toString);                // Function<Object, String>
+        Function<String, String> sFunction = Function.of(function);  // Function<String, String>
+        var sFunction2 = Function.of(function);                      // Function<Object, String>
+    }
+
     // Methods
 
     static boolean not(boolean original) {