diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/DocumentReference.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/DocumentReference.java
index cde2b8a3e3c..44c29e88ef1 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/DocumentReference.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/DocumentReference.java
@@ -529,7 +529,7 @@ public boolean equals(Object o) {
     if (this == o) {
       return true;
     }
-    if (o == null || getClass() != o.getClass()) {
+    if (!(o instanceof DocumentReference)) {
       return false;
     }
 
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/Query.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/Query.java
index bb6143edcce..e80e28c9f87 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/Query.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/Query.java
@@ -896,7 +896,7 @@ public boolean equals(Object o) {
     if (this == o) {
       return true;
     }
-    if (o == null || getClass() != o.getClass()) {
+    if (!(o instanceof Query)) {
       return false;
     }
 
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/SetOptions.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/SetOptions.java
index 4c302738906..adde4525994 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/SetOptions.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/SetOptions.java
@@ -20,8 +20,9 @@
 import android.support.annotation.Nullable;
 import com.google.firebase.annotations.PublicApi;
 import com.google.firebase.firestore.model.mutation.FieldMask;
-import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 /**
  * An options object that configures the behavior of set() calls. By providing one of the SetOptions
@@ -78,13 +79,13 @@ public static SetOptions merge() {
   @NonNull
   @PublicApi
   public static SetOptions mergeFields(List<String> fields) {
-    List<com.google.firebase.firestore.model.FieldPath> fieldPaths = new ArrayList<>();
+    Set<com.google.firebase.firestore.model.FieldPath> fieldPaths = new HashSet<>();
 
     for (String field : fields) {
       fieldPaths.add(FieldPath.fromDotSeparatedPath(field).getInternalPath());
     }
 
-    return new SetOptions(true, FieldMask.fromCollection(fieldPaths));
+    return new SetOptions(true, FieldMask.fromSet(fieldPaths));
   }
 
   /**
@@ -100,13 +101,13 @@ public static SetOptions mergeFields(List<String> fields) {
   @NonNull
   @PublicApi
   public static SetOptions mergeFields(String... fields) {
-    List<com.google.firebase.firestore.model.FieldPath> fieldPaths = new ArrayList<>();
+    Set<com.google.firebase.firestore.model.FieldPath> fieldPaths = new HashSet<>();
 
     for (String field : fields) {
       fieldPaths.add(FieldPath.fromDotSeparatedPath(field).getInternalPath());
     }
 
-    return new SetOptions(true, FieldMask.fromCollection(fieldPaths));
+    return new SetOptions(true, FieldMask.fromSet(fieldPaths));
   }
 
   /**
@@ -121,13 +122,13 @@ public static SetOptions mergeFields(String... fields) {
   @NonNull
   @PublicApi
   public static SetOptions mergeFieldPaths(List<FieldPath> fields) {
-    List<com.google.firebase.firestore.model.FieldPath> fieldPaths = new ArrayList<>();
+    Set<com.google.firebase.firestore.model.FieldPath> fieldPaths = new HashSet<>();
 
     for (FieldPath field : fields) {
       fieldPaths.add(field.getInternalPath());
     }
 
-    return new SetOptions(true, FieldMask.fromCollection(fieldPaths));
+    return new SetOptions(true, FieldMask.fromSet(fieldPaths));
   }
 
   @Override
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/core/EventManager.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/core/EventManager.java
index e74f837e496..38665628a45 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/core/EventManager.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/core/EventManager.java
@@ -137,6 +137,7 @@ public void onError(Query query, Status error) {
     queries.remove(query);
   }
 
+  @Override
   public void handleOnlineStateChange(OnlineState onlineState) {
     this.onlineState = onlineState;
     for (QueryListenersInfo info : queries.values()) {
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/core/UserData.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/core/UserData.java
index 00cc3969799..0cb0f614903 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/core/UserData.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/core/UserData.java
@@ -29,9 +29,9 @@
 import com.google.firebase.firestore.model.value.ObjectValue;
 import com.google.firebase.firestore.util.Assert;
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.List;
-import java.util.SortedSet;
-import java.util.TreeSet;
+import java.util.Set;
 import java.util.regex.Pattern;
 import javax.annotation.Nullable;
 
@@ -72,8 +72,8 @@ public static class ParseAccumulator {
      */
     private final Source dataSource;
 
-    /** Accumulates a list of the field paths found while parsing the data. */
-    private final SortedSet<FieldPath> fieldMask;
+    /** Accumulates a set of the field paths found while parsing the data. */
+    private final Set<FieldPath> fieldMask;
 
     /** Accumulates a list of field transforms found while parsing the data. */
     private final ArrayList<FieldTransform> fieldTransforms;
@@ -81,7 +81,7 @@ public static class ParseAccumulator {
     /** @param dataSource Indicates what kind of API method this data came from. */
     public ParseAccumulator(Source dataSource) {
       this.dataSource = dataSource;
-      this.fieldMask = new TreeSet<>();
+      this.fieldMask = new HashSet<>();
       this.fieldTransforms = new ArrayList<>();
     }
 
@@ -135,7 +135,7 @@ void addToFieldTransforms(FieldPath fieldPath, TransformOperation transformOpera
      */
     public ParsedSetData toMergeData(ObjectValue data) {
       return new ParsedSetData(
-          data, FieldMask.fromCollection(fieldMask), unmodifiableList(fieldTransforms));
+          data, FieldMask.fromSet(fieldMask), unmodifiableList(fieldTransforms));
     }
 
     /**
@@ -181,7 +181,7 @@ public ParsedSetData toSetData(ObjectValue data) {
      */
     public ParsedUpdateData toUpdateData(ObjectValue data) {
       return new ParsedUpdateData(
-          data, FieldMask.fromCollection(fieldMask), unmodifiableList(fieldTransforms));
+          data, FieldMask.fromSet(fieldMask), unmodifiableList(fieldTransforms));
     }
   }
 
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/core/ViewSnapshot.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/core/ViewSnapshot.java
index a05cffea965..1e05a3d8e98 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/core/ViewSnapshot.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/core/ViewSnapshot.java
@@ -118,11 +118,11 @@ public boolean excludesMetadataChanges() {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public final boolean equals(Object o) {
     if (this == o) {
       return true;
     }
-    if (o == null || getClass() != o.getClass()) {
+    if (!(o instanceof ViewSnapshot)) {
       return false;
     }
 
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/local/IndexedQueryEngine.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/local/IndexedQueryEngine.java
index 1dab574afff..ab734d95e31 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/local/IndexedQueryEngine.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/local/IndexedQueryEngine.java
@@ -22,7 +22,6 @@
 import com.google.firebase.firestore.core.Filter;
 import com.google.firebase.firestore.core.Filter.Operator;
 import com.google.firebase.firestore.core.IndexRange;
-import com.google.firebase.firestore.core.IndexRange.Builder;
 import com.google.firebase.firestore.core.NaNFilter;
 import com.google.firebase.firestore.core.NullFilter;
 import com.google.firebase.firestore.core.Query;
@@ -216,7 +215,7 @@ static IndexRange extractBestIndexRange(Query query) {
    * filter. The determined {@code IndexRange} is likely overselective and requires post-filtering.
    */
   private static IndexRange convertFilterToIndexRange(Filter filter) {
-    Builder indexRange = IndexRange.builder().setFieldPath(filter.getField());
+    IndexRange.Builder indexRange = IndexRange.builder().setFieldPath(filter.getField());
     if (filter instanceof RelationFilter) {
       RelationFilter relationFilter = (RelationFilter) filter;
       FieldValue filterValue = relationFilter.getValue();
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/BasePath.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/BasePath.java
index 2edc7875e3b..4da80b39d99 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/BasePath.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/BasePath.java
@@ -161,13 +161,11 @@ public int length() {
   }
 
   @Override
-  @SuppressWarnings("unchecked")
-  public boolean equals(Object o) {
-    if (o == null) {
-      return false;
+  public final boolean equals(Object o) {
+    if (this == o) {
+      return true;
     }
-    // The cast is not unchecked because of the class equality check.
-    return getClass() == o.getClass() && compareTo((B) o) == 0;
+    return (o instanceof BasePath) && compareTo((B) o) == 0;
   }
 
   @Override
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/Document.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/Document.java
index 83eeaa28412..44dfb3af8c4 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/Document.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/Document.java
@@ -23,7 +23,7 @@
  * Represents a document in Firestore with a key, version, data and whether the data has local
  * mutations applied to it.
  */
-public class Document extends MaybeDocument {
+public final class Document extends MaybeDocument {
 
   /** Describes the `hasPendingWrites` state of a document. */
   public enum DocumentState {
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentKey.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentKey.java
index f4e3b7b5e09..173edfaf96c 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentKey.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentKey.java
@@ -23,7 +23,7 @@
 import java.util.List;
 
 /** DocumentKey represents the location of a document in the Firestore database. */
-public class DocumentKey implements Comparable<DocumentKey> {
+public final class DocumentKey implements Comparable<DocumentKey> {
 
   public static final String KEY_FIELD_NAME = "__name__";
 
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentSet.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentSet.java
index 66dd581cf1d..47a10987bc1 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentSet.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentSet.java
@@ -30,7 +30,7 @@
  * An immutable set of documents (unique by key) ordered by the given comparator or ordered by key
  * by default if no document is present.
  */
-public class DocumentSet implements Iterable<Document> {
+public final class DocumentSet implements Iterable<Document> {
 
   /** Returns an empty DocumentSet sorted by the given comparator, then by keys. */
   public static DocumentSet emptySet(final Comparator<Document> comparator) {
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/NoDocument.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/NoDocument.java
index 74b6d6cad54..df83be54a54 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/NoDocument.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/NoDocument.java
@@ -15,7 +15,7 @@
 package com.google.firebase.firestore.model;
 
 /** Represents that no documents exists for the key at the given version. */
-public class NoDocument extends MaybeDocument {
+public final class NoDocument extends MaybeDocument {
 
   private boolean hasCommittedMutations;
 
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/UnknownDocument.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/UnknownDocument.java
index dc8cf0d9142..892ffb2d712 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/UnknownDocument.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/UnknownDocument.java
@@ -18,7 +18,7 @@
  * A class representing an existing document whose data is unknown (e.g. a document that was updated
  * without a known base document).
  */
-public class UnknownDocument extends MaybeDocument {
+public final class UnknownDocument extends MaybeDocument {
   public UnknownDocument(DocumentKey key, SnapshotVersion version) {
     super(key, version);
   }
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/ArrayTransformOperation.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/ArrayTransformOperation.java
index 89e10ec3364..f8f1c7abd8c 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/ArrayTransformOperation.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/ArrayTransformOperation.java
@@ -51,6 +51,7 @@ public FieldValue applyToRemoteDocument(FieldValue previousValue, FieldValue tra
   }
 
   @Override
+  @SuppressWarnings("EqualsGetClass") // subtype-sensitive equality is intended.
   public boolean equals(Object o) {
     if (this == o) {
       return true;
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/FieldMask.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/FieldMask.java
index 0cbef886ccd..7b0f8ee9fb2 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/FieldMask.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/FieldMask.java
@@ -15,7 +15,7 @@
 package com.google.firebase.firestore.model.mutation;
 
 import com.google.firebase.firestore.model.FieldPath;
-import java.util.Collection;
+import java.util.Set;
 
 /**
  * Provides a set of fields that can be used to partially patch a document. The FieldMask is used in
@@ -25,14 +25,14 @@
  * companion ObjectValue, the field is deleted. foo.bar - Overwrites only the field bar of the
  * object foo. If foo is not an object, foo is replaced with an object containing foo.
  */
-public class FieldMask {
-  public static FieldMask fromCollection(Collection<FieldPath> mask) {
+public final class FieldMask {
+  public static FieldMask fromSet(Set<FieldPath> mask) {
     return new FieldMask(mask);
   }
 
-  private final Collection<FieldPath> mask;
+  private final Set<FieldPath> mask;
 
-  private FieldMask(Collection<FieldPath> mask) {
+  private FieldMask(Set<FieldPath> mask) {
     this.mask = mask;
   }
 
@@ -49,6 +49,11 @@ public boolean equals(Object o) {
     return mask.equals(fieldMask.mask);
   }
 
+  @Override
+  public String toString() {
+    return "FieldMask{mask=" + mask.toString() + "}";
+  }
+
   /**
    * Verifies that 'fieldPath' is included by at least one field in this field mask.
    *
@@ -69,7 +74,7 @@ public int hashCode() {
     return mask.hashCode();
   }
 
-  public Collection<FieldPath> getMask() {
+  public Set<FieldPath> getMask() {
     return mask;
   }
 }
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/RemoteSerializer.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/RemoteSerializer.java
index fc4eb36bd30..fd2e85967af 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/RemoteSerializer.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/RemoteSerializer.java
@@ -98,8 +98,10 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 
 /** Serializer that converts to and from Firestore API protos. */
 public final class RemoteSerializer {
@@ -532,11 +534,11 @@ private DocumentMask encodeDocumentMask(FieldMask mask) {
 
   private FieldMask decodeDocumentMask(DocumentMask mask) {
     int count = mask.getFieldPathsCount();
-    List<FieldPath> paths = new ArrayList<>(count);
+    Set<FieldPath> paths = new HashSet<>(count);
     for (int i = 0; i < count; i++) {
       paths.add(FieldPath.fromServerFormat(mask.getFieldPaths(i)));
     }
-    return FieldMask.fromCollection(paths);
+    return FieldMask.fromSet(paths);
   }
 
   private DocumentTransform.FieldTransform encodeFieldTransform(FieldTransform fieldTransform) {
diff --git a/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/WatchChange.java b/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/WatchChange.java
index e2a882cc797..565c85afa2e 100644
--- a/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/WatchChange.java
+++ b/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/WatchChange.java
@@ -38,7 +38,7 @@ private WatchChange() {
    * A document change represents a change document and a list of target ids to which this change
    * applies. If the document has been deleted, the deleted document will be provided.
    */
-  public static class DocumentChange extends WatchChange {
+  public static final class DocumentChange extends WatchChange {
     // TODO: figure out if we can actually use arrays here for efficiency
     /** The new document applies to all of these targets. */
     private final List<Integer> updatedTargetIds;
@@ -137,7 +137,7 @@ public int hashCode() {
    * An ExistenceFilterWatchChange applies to the targets and is required to verify the current
    * client state against expected state sent from the server.
    */
-  public static class ExistenceFilterWatchChange extends WatchChange {
+  public static final class ExistenceFilterWatchChange extends WatchChange {
     private final int targetId;
 
     private final ExistenceFilter existenceFilter;
@@ -177,7 +177,7 @@ public enum WatchTargetChangeType {
   }
 
   /** The state of a target has changed. This can mean removal, addition, current or reset. */
-  public static class WatchTargetChange extends WatchChange {
+  public static final class WatchTargetChange extends WatchChange {
     private final WatchTargetChangeType changeType;
     private final List<Integer> targetIds;
     private final ByteString resumeToken;
diff --git a/firebase-firestore/src/test/java/com/google/firebase/firestore/local/LocalSerializerTest.java b/firebase-firestore/src/test/java/com/google/firebase/firestore/local/LocalSerializerTest.java
index db743396b29..a4429a0b27d 100644
--- a/firebase-firestore/src/test/java/com/google/firebase/firestore/local/LocalSerializerTest.java
+++ b/firebase-firestore/src/test/java/com/google/firebase/firestore/local/LocalSerializerTest.java
@@ -17,7 +17,7 @@
 import static com.google.firebase.firestore.testutil.TestUtil.deleteMutation;
 import static com.google.firebase.firestore.testutil.TestUtil.deletedDoc;
 import static com.google.firebase.firestore.testutil.TestUtil.doc;
-import static com.google.firebase.firestore.testutil.TestUtil.field;
+import static com.google.firebase.firestore.testutil.TestUtil.fieldMask;
 import static com.google.firebase.firestore.testutil.TestUtil.key;
 import static com.google.firebase.firestore.testutil.TestUtil.map;
 import static com.google.firebase.firestore.testutil.TestUtil.setMutation;
@@ -33,7 +33,6 @@
 import com.google.firebase.firestore.model.NoDocument;
 import com.google.firebase.firestore.model.SnapshotVersion;
 import com.google.firebase.firestore.model.UnknownDocument;
-import com.google.firebase.firestore.model.mutation.FieldMask;
 import com.google.firebase.firestore.model.mutation.Mutation;
 import com.google.firebase.firestore.model.mutation.MutationBatch;
 import com.google.firebase.firestore.model.mutation.PatchMutation;
@@ -71,7 +70,7 @@ public void testEncodesMutationBatch() {
         new PatchMutation(
             key("bar/baz"),
             TestUtil.wrapObject(map("a", "b", "num", 1)),
-            FieldMask.fromCollection(asList(field("a"))),
+            fieldMask("a"),
             com.google.firebase.firestore.model.mutation.Precondition.exists(true));
     Mutation del = deleteMutation("baz/quux");
     Timestamp writeTime = Timestamp.now();
diff --git a/firebase-firestore/src/test/java/com/google/firebase/firestore/model/MutationTest.java b/firebase-firestore/src/test/java/com/google/firebase/firestore/model/MutationTest.java
index d686c4829ef..67c1fba0ac4 100644
--- a/firebase-firestore/src/test/java/com/google/firebase/firestore/model/MutationTest.java
+++ b/firebase-firestore/src/test/java/com/google/firebase/firestore/model/MutationTest.java
@@ -18,6 +18,7 @@
 import static com.google.firebase.firestore.testutil.TestUtil.deletedDoc;
 import static com.google.firebase.firestore.testutil.TestUtil.doc;
 import static com.google.firebase.firestore.testutil.TestUtil.field;
+import static com.google.firebase.firestore.testutil.TestUtil.fieldMask;
 import static com.google.firebase.firestore.testutil.TestUtil.key;
 import static com.google.firebase.firestore.testutil.TestUtil.map;
 import static com.google.firebase.firestore.testutil.TestUtil.mutationResult;
@@ -110,7 +111,7 @@ public void testDeletesValuesFromTheFieldMask() {
     Document baseDoc = doc("collection/key", 0, data);
 
     DocumentKey key = key("collection/key");
-    FieldMask mask = FieldMask.fromCollection(Arrays.asList(field("foo.bar")));
+    FieldMask mask = fieldMask("foo.bar");
     Mutation patch = new PatchMutation(key, ObjectValue.emptyObject(), mask, Precondition.NONE);
 
     MaybeDocument patchDoc = patch.applyToLocalView(baseDoc, baseDoc, Timestamp.now());
diff --git a/firebase-firestore/src/testUtil/java/com/google/firebase/firestore/testutil/TestUtil.java b/firebase-firestore/src/testUtil/java/com/google/firebase/firestore/testutil/TestUtil.java
index d6d5384fd5f..27caae9bbdc 100644
--- a/firebase-firestore/src/testUtil/java/com/google/firebase/firestore/testutil/TestUtil.java
+++ b/firebase-firestore/src/testUtil/java/com/google/firebase/firestore/testutil/TestUtil.java
@@ -74,10 +74,13 @@
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
+import java.util.SortedSet;
+import java.util.TreeSet;
 import javax.annotation.Nullable;
 
 /** A set of utilities for tests */
@@ -109,6 +112,14 @@ public static ByteString byteString(int... bytes) {
     return ByteString.copyFrom(primitive);
   }
 
+  public static FieldMask fieldMask(String... fields) {
+    FieldPath[] mask = new FieldPath[fields.length];
+    for (int i = 0; i < fields.length; i++) {
+      mask[i] = field(fields[i]);
+    }
+    return FieldMask.fromSet(new HashSet<>(Arrays.asList(mask)));
+  }
+
   public static final Map<String, Object> EMPTY_MAP = new HashMap<>();
 
   public static FieldValue wrap(Object value) {
@@ -430,13 +441,15 @@ public static PatchMutation patchMutation(
 
     boolean merge = updateMask != null;
 
-    // We sort the fieldMaskPaths to make the order deterministic in tests.
-    Collections.sort(objectMask);
+    // We sort the fieldMaskPaths to make the order deterministic in tests. (Otherwise, when we
+    // flatten a Set to a proto repeated field, we'll end up comparing in iterator order and
+    // possibly consider {foo,bar} != {bar,foo}.)
+    SortedSet<FieldPath> fieldMaskPaths = new TreeSet<>(merge ? updateMask : objectMask);
 
     return new PatchMutation(
         key(path),
         objectValue,
-        FieldMask.fromCollection(merge ? updateMask : objectMask),
+        FieldMask.fromSet(fieldMaskPaths),
         merge ? Precondition.NONE : Precondition.exists(true));
   }