8001667: Comparator combinators and extension methods

Reviewed-by: mduigou, briangoetz
Contributed-by: henry.jen@oracle.com

make/java/java/FILES_java.gmk

@@ -257,6 +257,7 @@ JAVA_JAVA_java = \
         sun/util/calendar/ZoneInfoFile.java \
     java/util/TooManyListenersException.java \
     java/util/Comparator.java \
+    java/util/Comparators.java \
     java/util/Collections.java \
     java/util/Iterator.java \
     java/util/ListIterator.java \

src/share/classes/java/util/Collections.java

@@ -3759,7 +3759,7 @@ public class Collections {
             return c2.compareTo(c1);
         }
 
-        private Object readResolve() { return reverseOrder(); }
+        private Object readResolve() { return Collections.reverseOrder(); }
     }
 
     /**

src/share/classes/java/util/Comparator.java

@@ -25,6 +25,11 @@
 
 package java.util;
 
+import java.util.function.Function;
+import java.util.function.ToIntFunction;
+import java.util.function.ToLongFunction;
+import java.util.function.ToDoubleFunction;
+
 /**
  * A comparison function, which imposes a <i>total ordering</i> on some
  * collection of objects.  Comparators can be passed to a sort method (such
@@ -165,4 +170,93 @@ public interface Comparator<T> {
      * @see Object#hashCode()
      */
     boolean equals(Object obj);
+
+    /**
+     * Returns a comparator that imposes the reverse ordering of this
+     * comparator.
+     *
+     * @return A comparator that imposes the reverse ordering of this
+     *         comparator.
+     * @since 1.8
+     */
+    default Comparator<T> reverseOrder() {
+        return Collections.reverseOrder(this);
+    }
+
+    /**
+     * Constructs a lexicographic order comparator with another comparator.
+     * For example, a {@code Comparator<Person> byLastName} can be composed
+     * with another {@code Comparator<Person> byFirstName}, then {@code
+     * byLastName.thenComparing(byFirstName)} creates a {@code
+     * Comparator<Person>} which sorts by last name, and for equal last names
+     * sorts by first name.
+     *
+     * @param other the other comparator used when equals on this.
+     * @throws NullPointerException if the argument is null.
+     * @since 1.8
+     */
+    default Comparator<T> thenComparing(Comparator<? super T> other) {
+        return Comparators.compose(this, other);
+    }
+
+    /**
+     * Constructs a lexicographic order comparator with a function that
+     * extracts a {@code Comparable} key.  This default implementation calls
+     * {@code thenComparing(this, Comparators.comparing(keyExtractor))}.
+     *
+     * @param <U> the {@link Comparable} type for comparison
+     * @param keyExtractor the function used to extract the {@link Comparable} sort key
+     * @throws NullPointerException if the argument is null.
+     * @see Comparators#comparing(Function)
+     * @see #thenComparing(Comparator)
+     * @since 1.8
+     */
+    default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor) {
+        return thenComparing(Comparators.comparing(keyExtractor));
+    }
+
+    /**
+     * Constructs a lexicographic order comparator with a function that
+     * extracts a {@code int} value.  This default implementation calls {@code
+     * thenComparing(this, Comparators.comparing(keyExtractor))}.
+     *
+     * @param keyExtractor the function used to extract the integer value
+     * @throws NullPointerException if the argument is null.
+     * @see Comparators#comparing(ToIntFunction)
+     * @see #thenComparing(Comparator)
+     * @since 1.8
+     */
+    default Comparator<T> thenComparing(ToIntFunction<? super T> keyExtractor) {
+        return thenComparing(Comparators.comparing(keyExtractor));
+    }
+
+    /**
+     * Constructs a lexicographic order comparator with a function that
+     * extracts a {@code long} value.  This default implementation calls
+     * {@code thenComparing(this, Comparators.comparing(keyExtractor))}.
+     *
+     * @param keyExtractor the function used to extract the long value
+     * @throws NullPointerException if the argument is null.
+     * @see Comparators#comparing(ToLongFunction)
+     * @see #thenComparing(Comparator)
+     * @since 1.8
+     */
+    default Comparator<T> thenComparing(ToLongFunction<? super T> keyExtractor) {
+        return thenComparing(Comparators.comparing(keyExtractor));
+    }
+
+    /**
+     * Constructs a lexicographic order comparator with a function that
+     * extracts a {@code double} value.  This default implementation calls
+     * {@code thenComparing(this, Comparators.comparing(keyExtractor))}.
+     *
+     * @param keyExtractor the function used to extract the double value
+     * @throws NullPointerException if the argument is null.
+     * @see Comparators#comparing(ToDoubleFunction)
+     * @see #thenComparing(Comparator)
+     * @since 1.8
+     */
+    default Comparator<T> thenComparing(ToDoubleFunction<? super T> keyExtractor) {
+        return thenComparing(Comparators.comparing(keyExtractor));
+    }
 }

src/share/classes/java/util/Comparators.java

+/*
+ * Copyright (c) 2012, 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.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * 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.
+ */
+package java.util;
+
+import java.io.Serializable;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.ToDoubleFunction;
+import java.util.function.ToIntFunction;
+import java.util.function.ToLongFunction;
+
+/**
+ * This class consists of {@code static} utility methods for comparators. Mostly
+ * factory method that returns a {@link Comparator}.
+ *
+ * <p> Unless otherwise noted, passing a {@code null} argument to a method in
+ * this class will cause a {@link NullPointerException} to be thrown.
+ *
+ * @see Comparator
+ * @since 1.8
+ */
+public class Comparators {
+    private Comparators() {
+        throw new AssertionError("no instances");
+    }
+
+    /**
+     * Compares {@link Comparable} objects in natural order.
+     *
+     * @see Comparable
+     */
+    private enum NaturalOrderComparator implements Comparator<Comparable<Object>> {
+        INSTANCE;
+
+        @Override
+        public int compare(Comparable<Object> c1, Comparable<Object> c2) {
+            return c1.compareTo(c2);
+        }
+    }
+
+    /**
+     * Returns a comparator that imposes the reverse of the <em>natural
+     * ordering</em>.
+     *
+     * <p>The returned comparator is serializable.
+     *
+     * @param <T> {@link Comparable} type
+     *
+     * @return A comparator that imposes the reverse of the <i>natural
+     *         ordering</i> on a collection of objects that implement
+     *         the {@link Comparable} interface.
+     * @see Comparable
+     */
+    public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
+        return Collections.reverseOrder();
+    }
+
+    /**
+     * Returns a comparator that imposes the reverse ordering of the specified
+     * {@link Comparator}.
+     *
+     * <p>The returned comparator is serializable (assuming the specified
+     * comparator is also serializable).
+     *
+     * @param <T> the element type to be compared
+     * @param cmp a comparator whose ordering is to be reversed by the returned
+     *            comparator
+     * @return A comparator that imposes the reverse ordering of the
+     *         specified comparator.
+     */
+    public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
+        Objects.requireNonNull(cmp);
+        return Collections.reverseOrder(cmp);
+    }
+
+    /**
+     * Gets a comparator compares {@link Comparable} type in natural order.
+     *
+     * @param <T> {@link Comparable} type
+     */
+    public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
+        return (Comparator<T>) NaturalOrderComparator.INSTANCE;
+    }
+
+    /**
+     * Gets a comparator compares {@link Map.Entry} in natural order on key.
+     *
+     * @param <K> {@link Comparable} key type
+     * @param <V> value type
+     */
+    public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> naturalOrderKeys() {
+        return (Comparator<Map.Entry<K, V>> & Serializable)
+            (c1, c2) -> c1.getKey().compareTo(c2.getKey());
+    }
+
+    /**
+     * Gets a comparator compares {@link Map.Entry} in natural order on value.
+     *
+     * @param <K> key type
+     * @param <V> {@link Comparable} value type
+     */
+    public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> naturalOrderValues() {
+        return (Comparator<Map.Entry<K, V>> & Serializable)
+            (c1, c2) -> c1.getValue().compareTo(c2.getValue());
+    }
+
+    /**
+     * Gets a comparator compares {@link Map.Entry} by key using the given
+     * {@link Comparator}.
+     *
+     * <p>The returned comparator is serializable assuming the specified
+     * comparators are also serializable.
+     *
+     * @param <K> key type
+     * @param <V> value type
+     * @param cmp the key {@link Comparator}
+     */
+    public static <K, V> Comparator<Map.Entry<K, V>> byKey(Comparator<? super K> cmp) {
+        Objects.requireNonNull(cmp);
+        return (Comparator<Map.Entry<K, V>> & Serializable)
+            (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
+    }
+
+    /**
+     * Gets a comparator compares {@link Map.Entry} by value using the given
+     * {@link Comparator}.
+     *
+     * @param <K> key type
+     * @param <V> value type
+     * @param cmp the value {@link Comparator}
+     */
+    public static <K, V> Comparator<Map.Entry<K, V>> byValue(Comparator<? super V> cmp) {
+        Objects.requireNonNull(cmp);
+        return (Comparator<Map.Entry<K, V>> & Serializable)
+            (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
+    }
+
+    /**
+     * Accepts a function that extracts a {@link java.lang.Comparable
+     * Comparable} sort key from a type {@code T}, and returns a {@code
+     * Comparator<T>} that compares by that sort key.  For example, if a class
+     * {@code Person} has a {@code String}-valued getter {@code getLastName},
+     * then {@code comparing(Person::getLastName)} would return a {@code
+     * Comparator<Person>} that compares {@code Person} objects by their last
+     * name.
+     *
+     * @param <T> the original element type
+     * @param <U> the {@link Comparable} type for comparison
+     * @param keyExtractor the function used to extract the {@link Comparable} sort key
+     */
+    public static <T, U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor);
+        return (Comparator<T> & Serializable)
+            (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
+    }
+
+    /**
+     * Accepts a function that extracts an {@code int} value from a type {@code
+     * T}, and returns a {@code Comparator<T>} that compares by that value.
+     *
+     * <p>The returned comparator is serializable assuming the specified
+     * function is also serializable.
+     *
+     * @see #comparing(Function)
+     * @param <T> the original element type
+     * @param keyExtractor the function used to extract the integer value
+     */
+    public static <T> Comparator<T> comparing(ToIntFunction<? super T> keyExtractor) {
+        Objects.requireNonNull(keyExtractor);
+        return (Comparator<T> & Serializable)
+            (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
+    }
+
+    /**
+     * Accepts a function that extracts a {@code long} value from a type {@code
+     * T}, and returns a {@code Comparator<T>} that compares by that value.
+     *
+     * <p>The returned comparator is serializable assuming the specified
+     * function is also serializable.
+     *
+     * @see #comparing(Function)
+     * @param <T> the original element type
+     * @param keyExtractor the function used to extract the long value
+     */
+    public static <T> Comparator<T> comparing(ToLongFunction<? super T> keyExtractor) {
+        Objects.requireNonNull(keyExtractor);
+        return (Comparator<T> & Serializable)
+            (c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
+    }
+
+    /**
+     * Accepts a function that extracts a {@code double} value from a type
+     * {@code T}, and returns a {@code Comparator<T>} that compares by that
+     * value.
+     *
+     * <p>The returned comparator is serializable assuming the specified
+     * function is also serializable.
+     *
+     * @see #comparing(Function)
+     * @param <T> the original element type
+     * @param keyExtractor the function used to extract the double value
+     */
+    public static<T> Comparator<T> comparing(ToDoubleFunction<? super T> keyExtractor) {
+        Objects.requireNonNull(keyExtractor);
+        return (Comparator<T> & Serializable)
+            (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
+    }
+
+    /**
+     * Constructs a lexicographic order from two {@link Comparator}s.  For
+     * example, if you have comparators {@code byLastName} and {@code
+     * byFirstName}, each of type {@code Comparator<Person>}, then {@code
+     * compose(byLastName, byFirstName)} creates a {@code Comparator<Person>}
+     * which sorts by last name, and for equal last names sorts by first name.
+     *
+     * <p>The returned comparator is serializable assuming the specified
+     * comparators are also serializable.
+     *
+     * @param <T> the element type to be compared
+     * @param first the first comparator
+     * @param second the secondary comparator used when equals on the first
+     */
+    public static<T> Comparator<T> compose(Comparator<? super T> first, Comparator<? super T> second) {
+        Objects.requireNonNull(first);
+        Objects.requireNonNull(second);
+        return (Comparator<T> & Serializable) (c1, c2) -> {
+            int res = first.compare(c1, c2);
+            return (res != 0) ? res : second.compare(c1, c2);
+        };
+    }
+
+    /**
+     * Constructs a {@link BinaryOperator} which returns the lesser of two elements
+     * according to the specified {@code Comparator}
+     *
+     * @param comparator A {@code Comparator} for comparing the two values
+     * @param <T> the type of the elements to be compared
+     * @return a {@code BinaryOperator} which returns the lesser of its operands,
+     * according to the supplied {@code Comparator}
+     */
+    public static<T> BinaryOperator<T> lesserOf(Comparator<? super T> comparator) {
+        return (a, b) -> comparator.compare(a, b) <= 0 ? a : b;
+    }
+
+    /**
+     * Constructs a {@link BinaryOperator} which returns the greater of two elements
+     * according to the specified {@code Comparator}
+     *
+     * @param comparator A {@code Comparator} for comparing the two values
+     * @param <T> the type of the elements to be compared
+     * @return a {@code BinaryOperator} which returns the greater of its operands,
+     * according to the supplied {@code Comparator}
+     */
+    public static<T> BinaryOperator<T> greaterOf(Comparator<? super T> comparator) {
+        return (a, b) -> comparator.compare(a, b) >= 0 ? a : b;
+    }
+}

test/java/util/Collections/ReverseOrder.java

@@ -23,23 +23,56 @@
 
 /*
  * @test
- * @bug 4593209
+ * @bug 4593209 8001667
  * @summary Reverse comparator was subtly broken
  * @author Josh bloch
  */
 
 import java.util.*;
+import java.io.*;
 
 public class ReverseOrder {
+    static byte[] serialBytes(Object o) {
+        try {
+            ByteArrayOutputStream bos = new ByteArrayOutputStream();
+            ObjectOutputStream oos = new ObjectOutputStream(bos);
+            oos.writeObject(o);
+            oos.flush();
+            oos.close();
+            return bos.toByteArray();
+        } catch (Throwable t) {
+            throw new Error(t);
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    static <T> T serialClone(T o) {
+        try {
+            ObjectInputStream ois = new ObjectInputStream
+                (new ByteArrayInputStream(serialBytes(o)));
+            T clone = (T) ois.readObject();
+            return clone;
+        } catch (Throwable t) {
+            throw new Error(t);
+        }
+    }
+
     public static void main(String[] args) throws Exception {
         Foo[] a = { new Foo(2), new Foo(3), new Foo(1) };
         List list = Arrays.asList(a);
-        Collections.sort(list, Collections.reverseOrder());
+        Comparator cmp = Collections.reverseOrder();
+        Collections.sort(list, cmp);
 
         Foo[] golden = { new Foo(3), new Foo(2), new Foo(1) };
         List goldenList = Arrays.asList(golden);
         if (!list.equals(goldenList))
             throw new Exception(list.toString());
+
+        Comparator clone = serialClone(cmp);
+        List list2 = Arrays.asList(a);
+        Collections.sort(list2, clone);
+        if (!list2.equals(goldenList))
+            throw new Exception(list.toString());
     }
 }
 

test/java/util/ComparatorsTest.java

+/*
+ * Copyright (c) 2012, 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 8001667
+ * @run testng ComparatorsTest
+ */
+
+import java.util.Comparator;
+import java.util.Comparators;
+import java.util.AbstractMap;
+import java.util.Map;
+import org.testng.annotations.Test;
+
+import java.util.function.Function;
+import java.util.function.ToIntFunction;
+import java.util.function.ToLongFunction;
+import java.util.function.ToDoubleFunction;
+
+import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertTrue;
+import static org.testng.Assert.assertSame;
+
+/**
+ * Unit tests for helper methods in Comparators
+ */
+@Test(groups = "unit")
+public class ComparatorsTest {
+    private static class Thing {
+        public final int intField;
+        public final long longField;
+        public final double doubleField;
+        public final String stringField;
+
+        private Thing(int intField, long longField, double doubleField, String stringField) {
+            this.intField = intField;
+            this.longField = longField;
+            this.doubleField = doubleField;
+            this.stringField = stringField;
+        }
+
+        public int getIntField() {
+            return intField;
+        }
+
+        public long getLongField() {
+            return longField;
+        }
+
+        public double getDoubleField() {
+            return doubleField;
+        }
+
+        public String getStringField() {
+            return stringField;
+        }
+    }
+
+    private final int[] intValues = { -2, -2, -1, -1, 0, 0, 1, 1, 2, 2 };
+    private final long[] longValues = { -2, -2, -1, -1, 0, 0, 1, 1, 2, 2 };
+    private final double[] doubleValues = { -2, -2, -1, -1, 0, 0, 1, 1, 2, 2 };
+    private final String[] stringValues = { "a", "a", "b", "b", "c", "c", "d", "d", "e", "e" };
+    private final int[] comparisons = { 0, -1, 0, -1, 0, -1, 0, -1, 0 };
+
+    private<T> void assertComparisons(T[] things, Comparator<T> comp, int[] comparisons) {
+        for (int i=0; i<comparisons.length; i++) {
+            assertEquals(comparisons.length + 1, things.length);
+            assertEquals(comparisons[i], comp.compare(things[i], things[i+1]));
+            assertEquals(-comparisons[i], comp.compare(things[i+1], things[i]));
+        }
+    }
+
+    public void testIntComparator() {
+        Thing[] things = new Thing[intValues.length];
+        for (int i=0; i<intValues.length; i++)
+            things[i] = new Thing(intValues[i], 0L, 0.0, null);
+        Comparator<Thing> comp = Comparators.comparing(new ToIntFunction<ComparatorsTest.Thing>() {
+            @Override
+            public int applyAsInt(Thing thing) {
+                return thing.getIntField();
+            }
+        });
+
+        assertComparisons(things, comp, comparisons);
+    }
+
+    public void testLongComparator() {
+        Thing[] things = new Thing[longValues.length];
+        for (int i=0; i<longValues.length; i++)
+            things[i] = new Thing(0, longValues[i], 0.0, null);
+        Comparator<Thing> comp = Comparators.comparing(new ToLongFunction<ComparatorsTest.Thing>() {
+            @Override
+            public long applyAsLong(Thing thing) {
+                return thing.getLongField();
+            }
+        });
+
+        assertComparisons(things, comp, comparisons);
+    }
+
+    public void testDoubleComparator() {
+        Thing[] things = new Thing[doubleValues.length];
+        for (int i=0; i<doubleValues.length; i++)
+            things[i] = new Thing(0, 0L, doubleValues[i], null);
+        Comparator<Thing> comp = Comparators.comparing(new ToDoubleFunction<ComparatorsTest.Thing>() {
+            @Override
+            public double applyAsDouble(Thing thing) {
+                return thing.getDoubleField();
+            }
+        });
+
+        assertComparisons(things, comp, comparisons);
+    }
+
+    public void testComparing() {
+        Thing[] things = new Thing[doubleValues.length];
+        for (int i=0; i<doubleValues.length; i++)
+            things[i] = new Thing(0, 0L, 0.0, stringValues[i]);
+        Comparator<Thing> comp = Comparators.comparing(new Function<Thing, String>() {
+            @Override
+            public String apply(Thing thing) {
+                return thing.getStringField();
+            }
+        });
+
+        assertComparisons(things, comp, comparisons);
+    }
+
+    public void testNaturalOrderComparator() {
+        Comparator<String> comp = Comparators.naturalOrder();
+
+        assertComparisons(stringValues, comp, comparisons);
+    }
+
+    public void testReverseComparator() {
+        Comparator<String> cmpr = Comparators.reverseOrder();
+        Comparator<String> cmp = cmpr.reverseOrder();
+
+        assertEquals(cmp.reverseOrder(), cmpr);
+        assertEquals(0, cmp.compare("a", "a"));
+        assertEquals(0, cmpr.compare("a", "a"));
+        assertTrue(cmp.compare("a", "b") < 0);
+        assertTrue(cmpr.compare("a", "b") > 0);
+        assertTrue(cmp.compare("b", "a") > 0);
+        assertTrue(cmpr.compare("b", "a") < 0);
+    }
+
+    public void testReverseComparator2() {
+        Comparator<String> cmp = (s1, s2) -> s1.length() - s2.length();
+        Comparator<String> cmpr = cmp.reverseOrder();
+
+        assertEquals(cmpr.reverseOrder(), cmp);
+        assertEquals(0, cmp.compare("abc", "def"));
+        assertEquals(0, cmpr.compare("abc", "def"));
+        assertTrue(cmp.compare("abcd", "def") > 0);
+        assertTrue(cmpr.compare("abcd", "def") < 0);
+        assertTrue(cmp.compare("abc", "defg") < 0);
+        assertTrue(cmpr.compare("abc", "defg") > 0);
+    }
+
+    @Test(expectedExceptions=NullPointerException.class)
+    public void testReverseComparatorNPE() {
+        Comparator<String> cmp = Comparators.reverseOrder(null);
+    }
+
+    public void testComposeComparator() {
+        // Longer string in front
+        Comparator<String> first = (s1, s2) -> s2.length() - s1.length();
+        Comparator<String> second = Comparators.naturalOrder();
+        Comparator<String> composed = Comparators.compose(first, second);
+
+        assertTrue(composed.compare("abcdefg", "abcdef") < 0);
+        assertTrue(composed.compare("abcdef", "abcdefg") > 0);
+        assertTrue(composed.compare("abcdef", "abcdef") == 0);
+        assertTrue(composed.compare("abcdef", "ghijkl") < 0);
+        assertTrue(composed.compare("ghijkl", "abcdefg") > 0);
+    }
+
+    private <K, V> void assertPairComparison(K k1, V v1, K k2, V v2,
+                                        Comparator<Map.Entry<K, V>> ck,
+                                        Comparator<Map.Entry<K, V>> cv) {
+        final Map.Entry<K, V> p11 = new AbstractMap.SimpleImmutableEntry<>(k1, v1);
+        final Map.Entry<K, V> p12 = new AbstractMap.SimpleImmutableEntry<>(k1, v2);
+        final Map.Entry<K, V> p21 = new AbstractMap.SimpleImmutableEntry<>(k2, v1);
+        final Map.Entry<K, V> p22 = new AbstractMap.SimpleImmutableEntry<>(k2, v2);
+
+        assertTrue(ck.compare(p11, p11) == 0);
+        assertTrue(ck.compare(p12, p11) == 0);
+        assertTrue(ck.compare(p11, p12) == 0);
+        assertTrue(ck.compare(p12, p22) < 0);
+        assertTrue(ck.compare(p12, p21) < 0);
+        assertTrue(ck.compare(p21, p11) > 0);
+        assertTrue(ck.compare(p21, p12) > 0);
+
+        assertTrue(cv.compare(p11, p11) == 0);
+        assertTrue(cv.compare(p12, p11) > 0);
+        assertTrue(cv.compare(p11, p12) < 0);
+        assertTrue(cv.compare(p12, p22) == 0);
+        assertTrue(cv.compare(p12, p21) > 0);
+        assertTrue(cv.compare(p21, p11) == 0);
+        assertTrue(cv.compare(p21, p12) < 0);
+
+        Comparator<Map.Entry<K, V>> cmp = Comparators.compose(ck, cv);
+        assertTrue(cmp.compare(p11, p11) == 0);
+        assertTrue(cmp.compare(p12, p11) > 0);
+        assertTrue(cmp.compare(p11, p12) < 0);
+        assertTrue(cmp.compare(p12, p22) < 0);
+        assertTrue(cmp.compare(p12, p21) < 0);
+        assertTrue(cmp.compare(p21, p11) > 0);
+        assertTrue(cmp.compare(p21, p12) > 0);
+
+        cmp = Comparators.compose(cv, ck);
+        assertTrue(cmp.compare(p11, p11) == 0);
+        assertTrue(cmp.compare(p12, p11) > 0);
+        assertTrue(cmp.compare(p11, p12) < 0);
+        assertTrue(cmp.compare(p12, p22) < 0);
+        assertTrue(cmp.compare(p12, p21) > 0);
+        assertTrue(cmp.compare(p21, p11) > 0);
+        assertTrue(cmp.compare(p21, p12) < 0);
+    }
+
+    public void testKVComparatorable() {
+        assertPairComparison(1, "ABC", 2, "XYZ",
+                         Comparators.<Integer, String>naturalOrderKeys(),
+                         Comparators.<Integer, String>naturalOrderValues());
+    }
+
+    private static class People {
+        final String firstName;
+        final String lastName;
+        final int age;
+
+        People(String first, String last, int age) {
+            firstName = first;
+            lastName = last;
+            this.age = age;
+        }
+
+        String getFirstName() { return firstName; }
+        String getLastName() { return lastName; }
+        int getAge() { return age; }
+        long getAgeAsLong() { return (long) age; };
+        double getAgeAsDouble() { return (double) age; };
+    }
+
+    private final People people[] = {
+        new People("John", "Doe", 34),
+        new People("Mary", "Doe", 30),
+        new People("Maria", "Doe", 14),
+        new People("Jonah", "Doe", 10),
+        new People("John", "Cook", 54),
+        new People("Mary", "Cook", 50),
+    };
+
+    public void testKVComparators() {
+        // Comparator<People> cmp = Comparators.naturalOrder(); // Should fail to compiler as People is not comparable
+        // We can use simple comparator, but those have been tested above.
+        // Thus choose to do compose for some level of interation.
+        Comparator<People> cmp1 = Comparators.comparing((Function<People, String>) People::getFirstName);
+        Comparator<People> cmp2 = Comparators.comparing((Function<People, String>) People::getLastName);
+        Comparator<People> cmp = Comparators.compose(cmp1, cmp2);
+
+        assertPairComparison(people[0], people[0], people[1], people[1],
+                         Comparators.<People, People>byKey(cmp),
+                         Comparators.<People, People>byValue(cmp));
+
+    }
+
+    private <T> void assertComparison(Comparator<T> cmp, T less, T greater) {
+        assertTrue(cmp.compare(less, greater) < 0, "less");
+        assertTrue(cmp.compare(less, less) == 0, "equal");
+        assertTrue(cmp.compare(greater, less) > 0, "greater");
+    }
+
+    public void testComparatorDefaultMethods() {
+        Comparator<People> cmp = Comparators.comparing((Function<People, String>) People::getFirstName);
+        Comparator<People> cmp2 = Comparators.comparing((Function<People, String>) People::getLastName);
+        // reverseOrder
+        assertComparison(cmp.reverseOrder(), people[1], people[0]);
+        // thenComparing(Comparator)
+        assertComparison(cmp.thenComparing(cmp2), people[0], people[1]);
+        assertComparison(cmp.thenComparing(cmp2), people[4], people[0]);
+        // thenComparing(Function)
+        assertComparison(cmp.thenComparing(People::getLastName), people[0], people[1]);
+        assertComparison(cmp.thenComparing(People::getLastName), people[4], people[0]);
+        // thenComparing(ToIntFunction)
+        assertComparison(cmp.thenComparing(People::getAge), people[0], people[1]);
+        assertComparison(cmp.thenComparing(People::getAge), people[1], people[5]);
+        // thenComparing(ToLongFunction)
+        assertComparison(cmp.thenComparing(People::getAgeAsLong), people[0], people[1]);
+        assertComparison(cmp.thenComparing(People::getAgeAsLong), people[1], people[5]);
+        // thenComparing(ToDoubleFunction)
+        assertComparison(cmp.thenComparing(People::getAgeAsDouble), people[0], people[1]);
+        assertComparison(cmp.thenComparing(People::getAgeAsDouble), people[1], people[5]);
+    }
+
+    public void testGreaterOf() {
+        // lesser
+        assertSame(Comparators.greaterOf(Comparators.comparing(
+                                    (Function<People, String>) People::getFirstName))
+                              .apply(people[0], people[1]),
+                   people[1]);
+        // euqal
+        assertSame(Comparators.greaterOf(Comparators.comparing(
+                                    (Function<People, String>) People::getLastName))
+                              .apply(people[0], people[1]),
+                   people[0]);
+        // greater
+        assertSame(Comparators.greaterOf(Comparators.comparing(
+                                    (ToIntFunction<People>) People::getAge))
+                              .apply(people[0], people[1]),
+                   people[0]);
+    }
+
+    public void testLesserOf() {
+        // lesser
+        assertSame(Comparators.lesserOf(Comparators.comparing(
+                                    (Function<People, String>) People::getFirstName))
+                              .apply(people[0], people[1]),
+                   people[0]);
+        // euqal
+        assertSame(Comparators.lesserOf(Comparators.comparing(
+                                    (Function<People, String>) People::getLastName))
+                              .apply(people[0], people[1]),
+                   people[0]);
+        // greater
+        assertSame(Comparators.lesserOf(Comparators.comparing(
+                                    (ToIntFunction<People>) People::getAge))
+                              .apply(people[0], people[1]),
+                   people[1]);
+    }
+}
\ No newline at end of file