8050818: Predicate::not - provide an easier way to negate a predicate

Reviewed-by: chegar, dl, psandoz, forax, smarks, redestad

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

@@ -116,4 +116,20 @@ public interface Predicate<T> {
                 ? Objects::isNull
                 : object -> targetRef.equals(object);
     }
+
+    /**
+     * Returns a predicate that is the negation of the supplied predicate.
+     *
+     * @param <T>     the type of arguments to the specified predicate
+     * @param target  predicate to negate
+     *
+     * @return a predicate that negates the results of the supplied
+     *         predicate
+     *
+     * @since 11
+     */
+    @SuppressWarnings("unchecked")
+    static <T> Predicate<T> not(Predicate<? super T> target) {
+        return (Predicate<T>)target.negate();
+    }
 }

test/jdk/java/util/function/PredicateNotTest.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8050818
+ * @run testng PredicateNotTest
+ */
+
+import java.util.List;
+import java.util.function.Predicate;
+import org.testng.annotations.Test;
+import static java.util.function.Predicate.not;
+import static java.util.stream.Collectors.joining;
+import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.fail;
+
+@Test(groups = "unit")
+public class PredicateNotTest {
+    static class IsEmptyPredicate implements Predicate<String> {
+        @Override
+        public boolean test(String s) {
+            return s.isEmpty();
+        }
+    }
+
+    public void test() {
+        List<String> test = List.of(
+           "A non-empty line",
+           "",
+           "A non-empty line",
+           "",
+           "A non-empty line",
+           "",
+           "A non-empty line",
+           ""
+        );
+        String expected = "A non-empty line\nA non-empty line\nA non-empty line\nA non-empty line";
+
+        assertEquals(test.stream().filter(not(String::isEmpty)).collect(joining("\n")), expected);
+        assertEquals(test.stream().filter(not(s -> s.isEmpty())).collect(joining("\n")), expected);
+        assertEquals(test.stream().filter(not(new IsEmptyPredicate())).collect(joining("\n")), expected);
+        assertEquals(test.stream().filter(not(not(not(String::isEmpty)))).collect(joining("\n")), expected);
+        assertEquals(test.stream().filter(not(not(not(s -> s.isEmpty())))).collect(joining("\n")), expected);
+        assertEquals(test.stream().filter(not(not(not(new IsEmptyPredicate())))).collect(joining("\n")), expected);
+    }
+}
+