Use @Serial on serialVersionUID fields.

[`@Serial`](https://docs.oracle.com/en/java/javase/23/docs/api/java.base/java/io/Serial.html) was added in Java 14, but it has only `SOURCE` retention, so it requires Java 14+ only [at _build_ time](#6549). Guava continues to support Java 8 at runtime, and it already has required a higher version (though perhaps only Java 9+?) to build.

Still, if this change causes people trouble, it is easy enough for us to include `@Serial` only in our Google-internal source or even to back out the change entirely. (Part of the purpose of this change is to identify any such trouble before we add any features that require newer versions of Java to build "for real.")

Alternatively, if this change goes well, we can consider using `@Serial` on _all_ eligible APIs.

RELNOTES=n/a
PiperOrigin-RevId: 676818238

android/guava-testlib/src/com/google/common/collect/testing/AbstractIteratorTester.java

@@ -27,6 +27,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
+import java.io.Serial;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -109,15 +110,15 @@ void assertPermitted(Exception exception) {
       }
     }
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
   }
 
   private static final class UnknownElementException extends RuntimeException {
     private UnknownElementException(Collection<?> expected, Object actual) {
       super("Returned value '" + actual + "' not found. Remaining elements: " + expected);
     }
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
   }
 
   /**

android/guava-testlib/src/com/google/common/collect/testing/BaseComparable.java

@@ -19,6 +19,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
+import java.io.Serial;
 import java.io.Serializable;
 import org.jspecify.annotations.Nullable;
 
@@ -56,5 +57,5 @@ public int compareTo(BaseComparable o) {
     return s.compareTo(o.s);
   }
 
-  @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+  @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
 }

android/guava-testlib/src/com/google/common/collect/testing/DerivedComparable.java

@@ -19,6 +19,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
+import java.io.Serial;
 
 /**
  * Simple derived class to verify that we handle generics correctly.
@@ -31,5 +32,5 @@ public DerivedComparable(String s) {
     super(s);
   }
 
-  @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+  @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
 }

android/guava-testlib/src/com/google/common/collect/testing/SafeTreeMap.java

@@ -18,6 +18,7 @@
 
 import com.google.common.annotations.GwtIncompatible;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.AbstractSet;
 import java.util.Collection;
@@ -325,5 +326,5 @@ public String toString() {
     return delegate.toString();
   }
 
-  private static final long serialVersionUID = 0L;
+  @Serial private static final long serialVersionUID = 0L;
 }

android/guava-testlib/src/com/google/common/collect/testing/SafeTreeSet.java

@@ -18,6 +18,7 @@
 
 import com.google.common.annotations.GwtIncompatible;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.Collection;
 import java.util.Comparator;
@@ -253,5 +254,5 @@ public String toString() {
     return delegate.toString();
   }
 
-  private static final long serialVersionUID = 0L;
+  @Serial private static final long serialVersionUID = 0L;
 }

android/guava-testlib/src/com/google/common/collect/testing/features/ConflictingRequirementsException.java

@@ -19,6 +19,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
+import java.io.Serial;
 import java.util.Set;
 
 /**
@@ -51,5 +52,5 @@ public String getMessage() {
     return super.getMessage() + " (source: " + source + ")";
   }
 
-  @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+  @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
 }

android/guava-testlib/src/com/google/common/collect/testing/features/TesterRequirements.java

@@ -21,6 +21,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
+import java.io.Serial;
 import java.util.Collections;
 import java.util.Set;
 import org.jspecify.annotations.Nullable;
@@ -80,5 +81,5 @@ public String toString() {
     return "{TesterRequirements: present=" + presentFeatures + ", absent=" + absentFeatures + "}";
   }
 
-  @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+  @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
 }

android/guava-testlib/src/com/google/common/collect/testing/google/BiMapInverseTester.java

@@ -25,6 +25,7 @@
 import com.google.common.collect.testing.Helpers;
 import com.google.common.collect.testing.features.CollectionFeature;
 import com.google.common.testing.SerializableTester;
+import java.io.Serial;
 import java.io.Serializable;
 import java.lang.reflect.Method;
 import java.util.Collections;
@@ -68,7 +69,7 @@ private static class BiMapPair<K, V> implements Serializable {
       this.backward = original.inverse();
     }
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
   }
 
   /**

android/guava-testlib/test/com/google/common/testing/SerializableTesterTest.java

@@ -16,6 +16,7 @@
 
 package com.google.common.testing;
 
+import java.io.Serial;
 import java.io.Serializable;
 import junit.framework.AssertionFailedError;
 import junit.framework.TestCase;
@@ -77,11 +78,11 @@ public void testObjectWhichIsEqualButChangesClass() {
   }
 
   private static class ClassWhichDoesNotImplementEquals implements Serializable {
-    private static final long serialVersionUID = 1L;
+    @Serial private static final long serialVersionUID = 1L;
   }
 
   private static class ClassWhichIsAlwaysEqualButHasDifferentHashcodes implements Serializable {
-    private static final long serialVersionUID = 2L;
+    @Serial private static final long serialVersionUID = 2L;
 
     @SuppressWarnings("EqualsHashCode")
     @Override
@@ -91,7 +92,7 @@ public boolean equals(@Nullable Object other) {
   }
 
   private static class ObjectWhichIsEqualButChangesClass implements Serializable {
-    private static final long serialVersionUID = 1L;
+    @Serial private static final long serialVersionUID = 1L;
 
     @Override
     public boolean equals(@Nullable Object other) {

android/guava-tests/test/com/google/common/base/FunctionsTest.java

@@ -27,6 +27,7 @@
 import com.google.common.testing.EqualsTester;
 import com.google.common.testing.NullPointerTester;
 import com.google.common.testing.SerializableTester;
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.Map;
 import junit.framework.TestCase;
@@ -368,7 +369,7 @@ public void testConstantSerializable() {
 
   private static class CountingSupplier implements Supplier<Integer>, Serializable {
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
 
     private int value;
 

android/guava-tests/test/com/google/common/base/PredicatesTest.java

@@ -28,6 +28,7 @@
 import com.google.common.testing.EqualsTester;
 import com.google.common.testing.NullPointerTester;
 import com.google.common.testing.SerializableTester;
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -61,7 +62,8 @@ public boolean apply(@Nullable Integer i) {
 
   /** Instantiable predicate with reasonable hashCode() and equals() methods. */
   static class IsOdd implements Predicate<@Nullable Integer>, Serializable {
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0x150ddL;
+    @GwtIncompatible @J2ktIncompatible @Serial
+    private static final long serialVersionUID = 0x150ddL;
 
     @Override
     public boolean apply(@Nullable Integer i) {
@@ -718,7 +720,7 @@ public void testIn_serialization() {
 
   public void testIn_handlesNullPointerException() {
     class CollectionThatThrowsNullPointerException<T> extends ArrayList<T> {
-      @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 1L;
+      @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 1L;
 
       @Override
       public boolean contains(@Nullable Object element) {
@@ -733,7 +735,7 @@ public boolean contains(@Nullable Object element) {
 
   public void testIn_handlesClassCastException() {
     class CollectionThatThrowsClassCastException<T> extends ArrayList<T> {
-      @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 1L;
+      @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 1L;
 
       @Override
       public boolean contains(@Nullable Object element) {

android/guava-tests/test/com/google/common/base/SuppliersTest.java

@@ -30,6 +30,7 @@
 import com.google.common.testing.ClassSanityTester;
 import com.google.common.testing.EqualsTester;
 import java.io.NotSerializableException;
+import java.io.Serial;
 import java.io.Serializable;
 import java.time.Duration;
 import java.util.ArrayList;
@@ -74,11 +75,11 @@ public Integer get() {
   }
 
   static class SerializableCountingSupplier extends CountingSupplier implements Serializable {
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0L;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0L;
   }
 
   static class SerializableThrowingSupplier extends ThrowingSupplier implements Serializable {
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0L;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0L;
   }
 
   static void checkMemoize(CountingSupplier countingSupplier, Supplier<Integer> memoizedSupplier) {

android/guava-tests/test/com/google/common/collect/Base.java

@@ -19,6 +19,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
+import java.io.Serial;
 import java.io.Serializable;
 import org.jspecify.annotations.NullUnmarked;
 import org.jspecify.annotations.Nullable;
@@ -54,5 +55,5 @@ public int compareTo(Base o) {
     return s.compareTo(o.s);
   }
 
-  @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+  @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
 }

android/guava-tests/test/com/google/common/collect/Derived.java

@@ -19,6 +19,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
+import java.io.Serial;
 import org.jspecify.annotations.NullUnmarked;
 
 /** Simple derived class to verify that we handle generics correctly. */
@@ -29,5 +30,5 @@ public Derived(String s) {
     super(s);
   }
 
-  @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+  @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
 }

android/guava-tests/test/com/google/common/collect/HashMultisetTest.java

@@ -27,6 +27,7 @@
 import com.google.common.collect.testing.google.MultisetTestSuiteBuilder;
 import com.google.common.collect.testing.google.TestStringMultisetGenerator;
 import com.google.common.testing.SerializableTester;
+import java.io.Serial;
 import java.io.Serializable;
 import junit.framework.Test;
 import junit.framework.TestCase;
@@ -115,7 +116,7 @@ private static class MultisetHolder implements Serializable {
       this.member = multiset;
     }
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 1L;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 1L;
   }
 
   @J2ktIncompatible

android/guava-tests/test/com/google/common/collect/ImmutableMapTest.java

@@ -47,6 +47,7 @@
 import com.google.common.testing.NullPointerTester;
 import java.io.ByteArrayOutputStream;
 import java.io.ObjectOutputStream;
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.AbstractMap;
 import java.util.Collection;
@@ -873,7 +874,7 @@ public int hashCode() {
       return value;
     }
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 5;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 5;
   }
 
   public void testMutableValues() {

android/guava-tests/test/com/google/common/collect/ImmutableSortedMapTest.java

@@ -38,6 +38,7 @@
 import com.google.common.collect.testing.google.SortedMapGenerators.ImmutableSortedMapValueListGenerator;
 import com.google.common.testing.NullPointerTester;
 import com.google.common.testing.SerializableTester;
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.Collections;
 import java.util.Comparator;
@@ -676,7 +677,7 @@ public int hashCode() {
       return value;
     }
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 5;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 5;
   }
 
   public void testMutableValues() {

android/guava-tests/test/com/google/common/collect/LegacyComparable.java

@@ -21,6 +21,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
+import java.io.Serial;
 import java.io.Serializable;
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
@@ -70,5 +71,5 @@ public int hashCode() {
     return value.hashCode();
   }
 
-  @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+  @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
 }

android/guava-tests/test/com/google/common/collect/ListsTest.java

@@ -48,6 +48,7 @@
 import com.google.common.collect.testing.google.ListGenerators.CharactersOfStringGenerator;
 import com.google.common.testing.NullPointerTester;
 import com.google.common.testing.SerializableTester;
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -91,7 +92,7 @@ public Iterator<Integer> iterator() {
       return SOME_COLLECTION.iterator();
     }
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
   }
 
   private static final List<Integer> SOME_LIST = Lists.newArrayList(1, 2, 3, 4);
@@ -108,7 +109,7 @@ public String apply(Number n) {
       return String.valueOf(n);
     }
 
-    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
+    @GwtIncompatible @J2ktIncompatible @Serial private static final long serialVersionUID = 0;
   }
 
   @J2ktIncompatible

android/guava-tests/test/com/google/common/collect/LockHeldAssertingSet.java

@@ -19,6 +19,7 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 import static junit.framework.Assert.assertTrue;
 
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.Collection;
 import java.util.Set;
@@ -154,5 +155,5 @@ public <T> T[] toArray(T[] a) {
     return super.toArray(a);
   }
 
-  private static final long serialVersionUID = 0;
+  @Serial private static final long serialVersionUID = 0;
 }