Document that ComparisonChain is mostly obsolete.

If you are able to use the Java 8 API, you should almost certainly be using the `Comparator` methods introduced there. They are less error-prone and more succinct. In particular they avoid slip-ups like `compare(a.foo(), a.foo())` or `compare(a.foo(), b.bar())`.

RELNOTES=Documented that `ComparisonChain` is mostly obsolete.
PiperOrigin-RevId: 475862634

Author: eamonnmcmanus

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

@@ -16,11 +16,20 @@
 
 package com.google.common.collect;
 
+import static com.google.common.base.MoreObjects.toStringHelper;
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.Truth.assertWithMessage;
+import static java.lang.Integer.signum;
+import static java.util.Comparator.comparing;
+import static java.util.Comparator.naturalOrder;
+import static java.util.Comparator.nullsLast;
 
 import com.google.common.annotations.GwtCompatible;
+import com.google.common.primitives.Booleans;
+import java.util.Comparator;
 import junit.framework.AssertionFailedError;
 import junit.framework.TestCase;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
  * Unit test for {@link ComparisonChain}.
@@ -118,4 +127,95 @@ public void testCompareTrueFirst() {
     assertThat(ComparisonChain.start().compareTrueFirst(false, true).result()).isGreaterThan(0);
     assertThat(ComparisonChain.start().compareTrueFirst(false, false).result()).isEqualTo(0);
   }
+
+  enum TriState {
+    FALSE,
+    MAYBE,
+    TRUE,
+  }
+
+  static class Foo {
+    private final String aString;
+    private final int anInt;
+    private final @Nullable TriState anEnum;
+
+    Foo(String aString, int anInt, @Nullable TriState anEnum) {
+      this.aString = aString;
+      this.anInt = anInt;
+      this.anEnum = anEnum;
+    }
+
+    @Override
+    public String toString() {
+      return toStringHelper(this)
+          .add("aString", aString)
+          .add("anInt", anInt)
+          .add("anEnum", anEnum)
+          .toString();
+    }
+  }
+
+  /** Validates that the Comparator equivalent we document is correct. */
+  public void testComparatorEquivalent() {
+    Comparator<Foo> comparatorUsingComparisonChain =
+        (a, b) ->
+            ComparisonChain.start()
+                .compare(a.aString, b.aString)
+                .compare(a.anInt, b.anInt)
+                .compare(a.anEnum, b.anEnum, Ordering.natural().nullsLast())
+                .result();
+    Comparator<Foo> comparatorUsingComparatorMethods =
+        comparing((Foo foo) -> foo.aString)
+            .thenComparing(foo -> foo.anInt)
+            .thenComparing(foo -> foo.anEnum, nullsLast(naturalOrder()));
+    ImmutableList<Foo> instances =
+        ImmutableList.of(
+            new Foo("a", 1, TriState.TRUE),
+            new Foo("a", 2, TriState.TRUE),
+            new Foo("b", 1, TriState.FALSE),
+            new Foo("b", 1, TriState.TRUE),
+            new Foo("b", 1, null));
+    for (Foo a : instances) {
+      for (Foo b : instances) {
+        int comparedUsingComparisonChain = signum(comparatorUsingComparisonChain.compare(a, b));
+        int comparedUsingComparatorMethods = signum(comparatorUsingComparatorMethods.compare(a, b));
+        assertWithMessage("%s vs %s", a, b)
+            .that(comparedUsingComparatorMethods)
+            .isEqualTo(comparedUsingComparisonChain);
+      }
+    }
+  }
+
+  static class Bar {
+    private final boolean isBaz;
+
+    Bar(boolean isBaz) {
+      this.isBaz = isBaz;
+    }
+
+    boolean isBaz() {
+      return isBaz;
+    }
+  }
+
+  /**
+   * Validates that {@link Booleans#trueFirst()} and {@link Booleans#falseFirst()} can be used with
+   * {@link Comparator} when replacing {@link ComparisonChain#compareTrueFirst} and {@link
+   * ComparisonChain#compareFalseFirst}, as we document.
+   */
+  public void testTrueFirstFalseFirst() {
+    Bar trueBar = new Bar(true);
+    Bar falseBar = new Bar(false);
+
+    assertThat(ComparisonChain.start().compareTrueFirst(trueBar.isBaz(), falseBar.isBaz()).result())
+        .isLessThan(0);
+    Comparator<Bar> trueFirstComparator = comparing(Bar::isBaz, Booleans.trueFirst());
+    assertThat(trueFirstComparator.compare(trueBar, falseBar)).isLessThan(0);
+
+    assertThat(
+            ComparisonChain.start().compareFalseFirst(falseBar.isBaz(), trueBar.isBaz()).result())
+        .isLessThan(0);
+    Comparator<Bar> falseFirstComparator = comparing(Bar::isBaz, Booleans.falseFirst());
+    assertThat(falseFirstComparator.compare(falseBar, trueBar)).isLessThan(0);
+  }
 }

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

@@ -24,7 +24,10 @@
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
- * A utility for performing a chained comparison statement. For example:
+ * A utility for performing a chained comparison statement. <b>Note:</b> Java 8+ users should
+ * generally prefer the methods in {@link Comparator}; see <a href="#java8">below</a>.
+ *
+ * <p>Example usage of {@code ComparisonChain}:
  *
  * <pre>{@code
  * public int compareTo(Foo that) {
@@ -52,6 +55,37 @@
  * "https://github.com/google/guava/wiki/CommonObjectUtilitiesExplained#comparecompareto">{@code
  * ComparisonChain}</a>.
  *
+ * <h4 id="java8">Java 8+ equivalents</h4>
+ *
+ * If you are using Java version 8 or greater, you should generally use the static methods in {@link
+ * Comparator} instead of {@code ComparisonChain}. The example above can be implemented like this:
+ *
+ * <pre>{@code
+ * import static java.util.Comparator.comparing;
+ * import static java.util.Comparator.nullsLast;
+ * import static java.util.Comparator.naturalOrder;
+ *
+ * ...
+ *   private static final Comparator<Foo> COMPARATOR =
+ *       comparing((Foo foo) -> foo.aString)
+ *           .thenComparing(foo -> foo.anInt)
+ *           .thenComparing(foo -> foo.anEnum, nullsLast(naturalOrder()));}
+ *
+ *   {@code @Override}{@code
+ *   public int compareTo(Foo that) {
+ *     return COMPARATOR.compare(this, that);
+ *   }
+ * }</pre>
+ *
+ * <p>With method references it is more succinct: {@code comparing(Foo::aString)} for example.
+ *
+ * <p>Using {@link Comparator} avoids certain types of bugs, for example when you meant to write
+ * {@code .compare(a.foo, b.foo)} but you actually wrote {@code .compare(a.foo, a.foo)} or {@code
+ * .compare(a.foo, b.bar)}. {@code ComparisonChain} also has a potential performance problem that
+ * {@code Comparator} doesn't: it evaluates all the parameters of all the {@code .compare} calls,
+ * even when the result of the comparison is already known from previous {@code .compare} calls.
+ * That can be expensive.
+ *
  * @author Mark Davis
  * @author Kevin Bourrillion
  * @since 2.0
@@ -243,6 +277,12 @@ public final ComparisonChain compare(Boolean left, Boolean right) {
    * Compares two {@code boolean} values, considering {@code true} to be less than {@code false},
    * <i>if</i> the result of this comparison chain has not already been determined.
    *
+   * <p>Java 8+ users: you can get the equivalent from {@link Booleans#trueFirst()}. For example:
+   *
+   * <pre>
+   * Comparator.comparing(Foo::isBar, {@link Booleans#trueFirst()})
+   * </pre>
+   *
    * @since 12.0
    */
   public abstract ComparisonChain compareTrueFirst(boolean left, boolean right);
@@ -251,6 +291,12 @@ public final ComparisonChain compare(Boolean left, Boolean right) {
    * Compares two {@code boolean} values, considering {@code false} to be less than {@code true},
    * <i>if</i> the result of this comparison chain has not already been determined.
    *
+   * <p>Java 8+ users: you can get the equivalent from {@link Booleans#falseFirst()}. For example:
+   *
+   * <pre>
+   * Comparator.comparing(Foo::isBar, {@link Booleans#falseFirst()})
+   * </pre>
+   *
    * @since 12.0 (present as {@code compare} since 2.0)
    */
   public abstract ComparisonChain compareFalseFirst(boolean left, boolean right);