Addressing review comments:

 - separate positive and negative unit tests
 - fix formatting
 - reuse s3 client for both positive and negative tests

airbyte-integrations/connectors/destination-s3/src/main/java/io/airbyte/integrations/destination/s3/S3Destination.java

@@ -36,13 +36,14 @@ public class S3Destination extends BaseConnector implements Destination {
   private static final Logger LOGGER = LoggerFactory.getLogger(S3Destination.class);
   private final S3DestinationConfigFactory configFactory;
 
-  public S3Destination () {
+  public S3Destination() {
     this.configFactory = new S3DestinationConfigFactory();
   }
 
-  public S3Destination (final S3DestinationConfigFactory configFactory) {
+  public S3Destination(final S3DestinationConfigFactory configFactory) {
     this.configFactory = configFactory;
   }
+
   public static void main(final String[] args) throws Exception {
     new IntegrationRunner(new S3Destination()).run(args);
   }

airbyte-integrations/connectors/destination-s3/src/main/java/io/airbyte/integrations/destination/s3/S3DestinationConfig.java

@@ -52,14 +52,14 @@ public S3DestinationConfig(final String endpoint,
   }
 
   public S3DestinationConfig(final String endpoint,
-      final String bucketName,
-      final String bucketPath,
-      final String bucketRegion,
-      final String accessKeyId,
-      final String secretAccessKey,
-      final Integer partSize,
-      final S3FormatConfig formatConfig,
-      final AmazonS3 s3Client) {
+                             final String bucketName,
+                             final String bucketPath,
+                             final String bucketRegion,
+                             final String accessKeyId,
+                             final String secretAccessKey,
+                             final Integer partSize,
+                             final S3FormatConfig formatConfig,
+                             final AmazonS3 s3Client) {
     this.endpoint = endpoint;
     this.bucketName = bucketName;
     this.bucketPath = bucketPath;
@@ -72,13 +72,13 @@ public S3DestinationConfig(final String endpoint,
   }
 
   public S3DestinationConfig(final String endpoint,
-      final String bucketName,
-      final String bucketPath,
-      final String bucketRegion,
-      final String accessKeyId,
-      final String secretAccessKey,
-      final Integer partSize,
-      final S3FormatConfig formatConfig) {
+                             final String bucketName,
+                             final String bucketPath,
+                             final String bucketRegion,
+                             final String accessKeyId,
+                             final String secretAccessKey,
+                             final Integer partSize,
+                             final S3FormatConfig formatConfig) {
     this(endpoint, bucketName, bucketPath, bucketRegion, accessKeyId, secretAccessKey, partSize, formatConfig, null);
   }
 
@@ -141,7 +141,7 @@ public S3FormatConfig getFormatConfig() {
   }
 
   public AmazonS3 getS3Client() {
-    if(this.s3Client !=null) {
+    if (this.s3Client != null) {
       return this.s3Client;
     }
 

airbyte-integrations/connectors/destination-s3/src/main/java/io/airbyte/integrations/destination/s3/S3DestinationConfigFactory.java

@@ -1,9 +1,15 @@
+/*
+ * Copyright (c) 2021 Airbyte, Inc., all rights reserved.
+ */
+
 package io.airbyte.integrations.destination.s3;
 
 import com.fasterxml.jackson.databind.JsonNode;
 
 public class S3DestinationConfigFactory {
+
   public S3DestinationConfig getS3DestinationConfig(final JsonNode config) {
     return S3DestinationConfig.getS3DestinationConfig(config);
   }
+
 }

airbyte-integrations/connectors/destination-s3/src/test/java/io/airbyte/integrations/destination/s3/S3DestinationTest.java

@@ -6,17 +6,16 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertTrue;
-
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.when;
 
-import com.amazonaws.services.s3.model.AmazonS3Exception;
 import com.amazonaws.services.s3.AmazonS3;
+import com.amazonaws.services.s3.model.AmazonS3Exception;
 import com.amazonaws.services.s3.model.InitiateMultipartUploadRequest;
 import com.amazonaws.services.s3.model.InitiateMultipartUploadResult;
 import com.amazonaws.services.s3.model.ListObjectsRequest;
@@ -34,21 +33,16 @@
 public class S3DestinationTest {
 
   private AmazonS3 s3;
-  private AmazonS3 s3NoAccess;
   private S3DestinationConfig config;
 
   @BeforeEach
   public void setup() {
-    // S3 client mock for successful operations
     s3 = mock(AmazonS3.class);
     final InitiateMultipartUploadResult uploadResult = mock(InitiateMultipartUploadResult.class);
     final UploadPartResult uploadPartResult = mock(UploadPartResult.class);
     when(s3.uploadPart(any(UploadPartRequest.class))).thenReturn(uploadPartResult);
     when(s3.initiateMultipartUpload(any(InitiateMultipartUploadRequest.class))).thenReturn(uploadResult);
 
-    // S3 client mock that throws Access Denied exception for listObjects operation
-    s3NoAccess = mock(AmazonS3.class);
-    doThrow(new AmazonS3Exception("Access Denied")).when(s3NoAccess).listObjects(any(ListObjectsRequest.class));
     config = new S3DestinationConfig(
         "fake-endpoint",
         "fake-bucket",
@@ -60,8 +54,12 @@ public void setup() {
   }
 
   @Test
-  public void checksS3NoListObjectPermission() {
-    final S3Destination destinationFail = new S3Destination(new S3DestinationConfigFactory () {
+  /**
+   * Test that check will fail if IAM user does not have listObjects permission
+   */
+  public void checksS3WithoutListObjectPermission() {
+    final S3Destination destinationFail = new S3Destination(new S3DestinationConfigFactory() {
+
       public S3DestinationConfig getS3DestinationConfig(final JsonNode config) {
         return new S3DestinationConfig(
             "fake-endpoint",
@@ -71,15 +69,23 @@ public S3DestinationConfig getS3DestinationConfig(final JsonNode config) {
             "fake-accessKeyId",
             "fake-secretAccessKey",
             S3DestinationConfig.DEFAULT_PART_SIZE_MB,
-            null, s3NoAccess);
+            null, s3);
       }
+
     });
-    AirbyteConnectionStatus status = destinationFail.check(null);
+    doThrow(new AmazonS3Exception("Access Denied")).when(s3).listObjects(any(ListObjectsRequest.class));
+    final AirbyteConnectionStatus status = destinationFail.check(null);
     assertEquals(Status.FAILED, status.getStatus(), "Connection check should have failed");
     assertTrue(status.getMessage().indexOf("Access Denied") > 0, "Connection check returned wrong failure message");
+  }
+
+  @Test
+  /**
+   * Test that check will succeed when IAM user has all required permissions
+   */
+  public void checksS3WithListObjectPermission() {
+    final S3Destination destinationSuccess = new S3Destination(new S3DestinationConfigFactory() {
 
-    // Test that check succeeds when IAM user has listObjects permission
-    final S3Destination destinationSuccess = new S3Destination(new S3DestinationConfigFactory () {
       public S3DestinationConfig getS3DestinationConfig(final JsonNode config) {
         return new S3DestinationConfig(
             "fake-endpoint",
@@ -91,8 +97,9 @@ public S3DestinationConfig getS3DestinationConfig(final JsonNode config) {
             S3DestinationConfig.DEFAULT_PART_SIZE_MB,
             null, s3);
       }
+
     });
-    status = destinationSuccess.check(null);
+    final AirbyteConnectionStatus status = destinationSuccess.check(null);
     assertEquals(Status.SUCCEEDED, status.getStatus(), "Connection check should have succeeded");
   }