Fix collection tests for latest guava-testlib update (#2786)

With the fix in the latest guava-testlib version null support is now properly
checked. But it expects that if a collection does not permit null, then
creating it with null should fail as well, see guava-testlib's:
- MapCreationTester.testCreateWithNullValueUnsupported()
- CollectionCreationTester.testCreateWithNull_unsupported()

However, the previous implementation for the JsonArray and JsonObject test
was using methods which implicitly converted null to JsonNull.

gson/src/test/java/com/google/gson/JsonArrayAsListSuiteTest.java

@@ -42,10 +42,14 @@ public Iterable<JsonElement> order(List<JsonElement> insertionOrder) {
     @Override
     public List<JsonElement> create(Object... elements) {
       JsonArray array = new JsonArray();
+      // This is not completely accurate: Because there is no way to directly construct JsonArray or
+      // its List view with existing elements, this has to add the elements individually with
+      // `List#add`
+      var list = array.asList();
       for (Object element : elements) {
-        array.add((JsonElement) element);
+        list.add((JsonElement) element);
       }
-      return array.asList();
+      return list;
     }
   }
 
@@ -54,8 +58,6 @@ public static Test suite() {
     return ListTestSuiteBuilder.using(new ListGenerator())
         .withFeatures(
             CollectionSize.ANY,
-            // Note: There is current a Guava bug which causes 'null additions' to not be tested if
-            // 'null queries' is enabled, see https://github.com/google/guava/issues/7401
             CollectionFeature.ALLOWS_NULL_QUERIES,
             CollectionFeature.RESTRICTS_ELEMENTS, // List only allows JsonElement
             CollectionFeature.SUPPORTS_ADD,

gson/src/test/java/com/google/gson/JsonObjectAsMapSuiteTest.java

@@ -34,11 +34,15 @@ public SampleElements<Entry<String, JsonElement>> samples() {
     @Override
     public Map<String, JsonElement> create(Object... elements) {
       JsonObject object = new JsonObject();
+      // This is not completely accurate: Because there is no way to directly construct JsonObject
+      // or its Map view with existing entries, this has to add the entries individually with
+      // `Map#put`
+      var map = object.asMap();
       for (Object element : elements) {
         var entry = (Entry<?, ?>) element;
-        object.add((String) entry.getKey(), (JsonElement) entry.getValue());
+        map.put((String) entry.getKey(), (JsonElement) entry.getValue());
       }
-      return object.asMap();
+      return map;
     }
 
     @SuppressWarnings("unchecked")
@@ -70,8 +74,6 @@ public static Test suite() {
     return MapTestSuiteBuilder.using(new MapGenerator())
         .withFeatures(
             CollectionSize.ANY,
-            // Note: There is current a Guava bug which causes 'null additions' to not be tested if
-            // 'null queries' is enabled, see https://github.com/google/guava/issues/7401
             MapFeature.ALLOWS_ANY_NULL_QUERIES,
             MapFeature.RESTRICTS_KEYS, // Map only allows String keys
             MapFeature.RESTRICTS_VALUES, // Map only allows JsonElement values

gson/src/test/java/com/google/gson/internal/LinkedTreeMapSuiteTest.java

@@ -31,6 +31,8 @@ public MapGenerator(boolean allowNullValues) {
 
     @Override
     protected Map<String, String> create(Entry<String, String>[] entries) {
+      // This is not completely accurate: Because LinkedTreeMap has no constructor which accepts
+      // existing entries, this has to add the entries individually with `Map#put`
       var map = new LinkedTreeMap<String, String>(allowNullValues);
       for (var entry : entries) {
         map.put(entry.getKey(), entry.getValue());
@@ -47,8 +49,6 @@ private static Feature<?>[] createFeatures(Feature<?>... additionalFeatures) {
         new ArrayList<Feature<?>>(
             List.of(
                 CollectionSize.ANY,
-                // Note: There is current a Guava bug which causes 'null additions' to not be tested
-                // if 'null queries' is enabled, see https://github.com/google/guava/issues/7401
                 MapFeature.ALLOWS_ANY_NULL_QUERIES,
                 MapFeature.RESTRICTS_KEYS, // Map only allows comparable keys
                 MapFeature.SUPPORTS_PUT,
@@ -74,7 +74,8 @@ public static Test suite() {
             .named("nullValues=false")
             .createTestSuite();
 
-    TestSuite testSuite = new TestSuite("LinkedTreeMap");
+    // Use qualified class name to make it easier to find this test class in the IDE
+    TestSuite testSuite = new TestSuite(LinkedTreeMapSuiteTest.class.getName());
     testSuite.addTest(nullValuesSuite);
     testSuite.addTest(nonNullValuesSuite);