Add test coverage

Author: RanVaknin

services-custom/dynamodb-enhanced/src/test/java/software/amazon/awssdk/enhanced/dynamodb/mapper/StaticTableMetadataTest.java

@@ -21,6 +21,7 @@
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.sameInstance;
 import static software.amazon.awssdk.enhanced.dynamodb.TableMetadata.primaryIndexName;
 
 import java.util.Collection;
@@ -359,6 +360,43 @@ public void builderReuse_independentValidation() {
         assertThat(metadata2.indexPartitionKeys("gsi1"), contains("key1", "key2"));
     }
 
+    @Test
+    public void indexPartitionKeys_shouldReturnCachedPartitionKeysList() {
+        StaticTableMetadata metadata = StaticTableMetadata.builder()
+                                                                 .addIndexPartitionKey(primaryIndexName(),
+                                                                                       ATTRIBUTE_NAME,
+                                                                                       AttributeValueType.S)
+                                                                 .build();
+        List<String> first = metadata.indexPartitionKeys(primaryIndexName());
+        List<String> second = metadata.indexPartitionKeys(primaryIndexName());
+
+        assertThat(first, sameInstance(second));
+    }
+
+    @Test
+    public void indexSortKeys_shouldReturnCachedSortKeysList() {
+        StaticTableMetadata metadata = StaticTableMetadata.builder()
+                                                          .addIndexSortKey(primaryIndexName(),
+                                                                                ATTRIBUTE_NAME,
+                                                                                AttributeValueType.S)
+                                                          .build();
+        List<String> first = metadata.indexSortKeys(primaryIndexName());
+        List<String> second = metadata.indexSortKeys(primaryIndexName());
+
+        assertThat(first, sameInstance(second));
+    }
+
+    @Test
+    public void indexSortKeys_shouldReturnUnmodifiableList() {
+        StaticTableMetadata metadata = StaticTableMetadata.builder()
+                                                          .addIndexSortKey(primaryIndexName(),
+                                                                           ATTRIBUTE_NAME,
+                                                                           AttributeValueType.S)
+                                                          .build();
+        List<String> result = metadata.indexSortKeys(primaryIndexName());
+        assertThatThrownBy(() -> result.add("foo")).isInstanceOf(UnsupportedOperationException.class);
+    }
+
     @Test
     public void getIndexKeys_partitionAndSort() {
         TableMetadata tableMetadata = StaticTableMetadata.builder()