Add emptiesFirst/Last to Comparators for easily comparing java.util.O…

…ptional

values.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146801689

guava-tests/test/com/google/common/collect/ComparatorsTest.java

@@ -17,11 +17,13 @@
 package com.google.common.collect;
 
 import static java.util.Arrays.asList;
+import static java.util.Comparator.comparing;
 
 import com.google.common.collect.testing.Helpers;
 import com.google.common.testing.EqualsTester;
 import java.util.Collections;
 import java.util.Comparator;
+import java.util.Optional;
 import junit.framework.TestCase;
 
 /**
@@ -69,4 +71,13 @@ public void testIsInStrictOrder() {
     assertTrue(Comparators.isInStrictOrder(Collections.singleton(1), Ordering.natural()));
     assertTrue(Comparators.isInStrictOrder(Collections.<Integer>emptyList(), Ordering.natural()));
   }
+
+  public void testEmptiesFirst() {
+    Optional<String> empty = Optional.empty();
+    Optional<String> abc = Optional.of("abc");
+    Optional<String> z = Optional.of("z");
+
+    Comparator<Optional<String>> comparator = Comparators.emptiesFirst(comparing(String::length));
+    Helpers.testComparator(comparator, empty, z, abc);
+  }
 }

guava/src/com/google/common/collect/Comparators.java

@@ -22,6 +22,7 @@
 import com.google.common.annotations.GwtCompatible;
 import java.util.Comparator;
 import java.util.Iterator;
+import java.util.Optional;
 
 /**
  * Provides static methods for working with {@link Comparator} instances. For many other helpful
@@ -104,4 +105,30 @@ public static <T> boolean isInStrictOrder(
     }
     return true;
   }
+
+  /**
+   * Returns a comparator of {@link Optional} values which treats {@link Optional#empty} as less
+   * than all other values, and orders the rest using {@code valueComparator} on the contained
+   * value.
+   *
+   * @since 22.0
+   */
+  @Beta
+  public static <T> Comparator<Optional<T>> emptiesFirst(Comparator<T> valueComparator) {
+    checkNotNull(valueComparator);
+    return Comparator.comparing(o -> o.orElse(null), Comparator.nullsFirst(valueComparator));
+  }
+
+  /**
+   * Returns a comparator of {@link Optional} values which treats {@link Optional#empty} as greater
+   * than all other values, and orders the rest using {@code valueComparator} on the contained
+   * value.
+   *
+   * @since 22.0
+   */
+  @Beta
+  public static <T> Comparator<Optional<T>> emptiesLast(Comparator<T> valueComparator) {
+    checkNotNull(valueComparator);
+    return Comparator.comparing(o -> o.orElse(null), Comparator.nullsLast(valueComparator));
+  }
 }