fixup! addressed comments.

pom.xml

@@ -101,6 +101,7 @@
     <beamSurefireArgline />
 
     <!-- If updating dependencies, please update any relevant javadoc offlineLinks -->
+    <apache.commons.lang.version>3.5</apache.commons.lang.version>
     <apex.kryo.version>2.24.0</apex.kryo.version>
     <avro.version>1.8.1</avro.version>
     <bigquery.version>v2-rev295-1.22.0</bigquery.version>
@@ -452,6 +453,12 @@
         <version>${project.version}</version>
       </dependency>
 
+      <dependency>
+        <groupId>org.apache.commons</groupId>
+        <artifactId>commons-lang3</artifactId>
+        <version>${apache.commons.lang.version}</version>
+      </dependency>
+
       <dependency>
         <groupId>io.grpc</groupId>
         <artifactId>grpc-all</artifactId>

sdks/java/core/pom.xml

@@ -374,7 +374,6 @@
     <dependency>
       <groupId>org.apache.commons</groupId>
       <artifactId>commons-lang3</artifactId>
-      <version>3.5</version>
     </dependency>
 
     <dependency>

sdks/java/core/src/main/java/org/apache/beam/sdk/io/fs/ResourceId.java

@@ -46,6 +46,19 @@ public interface ResourceId {
    * <p>This {@code ResourceId} should represents a directory.
    *
    * <p>It is up to each file system to resolve in their own way.
+   *
+   * <p>Resolving special characters:
+   * <ul>
+   *   <li>{@code resourceId.resolve("..", StandardResolveOptions.RESOLVE_DIRECTORY)} returns
+   *   the parent directory of this {@code ResourceId}.
+   *   <li>{@code resourceId.resolve("{@literal *}", StandardResolveOptions.RESOLVE_FILE)} returns
+   *   a {@code ResourceId} which matches all files in this {@code ResourceId}.
+   *   <li>{@code resourceId.resolve("{@literal *}", StandardResolveOptions.RESOLVE_DIRECTORY)}
+   *   returns a {@code ResourceId} which matches all directories in this {@code ResourceId}.
+   * </ul>
+   *
+   * @throws IllegalArgumentException if other contains illegal characters or is an illegal name.
+   * It is recommended that callers use common characters, such as [_a-zA-Z0-9-], in {@code other}.
    */
   ResourceId resolve(String other, ResolveOptions resolveOptions);
 

sdks/java/core/src/main/java/org/apache/beam/sdk/util/gcsfs/GcsPath.java

@@ -440,7 +440,7 @@ public Path resolveSibling(Path other) {
   }
 
   @Override
-  public GcsPath resolveSibling(String other) {
+  public Path resolveSibling(String other) {
     if (getNameCount() < 2) {
       throw new UnsupportedOperationException("Can't resolve the sibling of a root path: " + this);
     }

sdks/java/io/google-cloud-platform/src/test/java/org/apache/beam/sdk/io/gcp/storage/GcsResourceIdTest.java

@@ -76,8 +76,7 @@ public void testResolveHandleBadInputs() throws Exception {
   @Test
   public void testResolveInvalidInputs() throws Exception {
     thrown.expect(IllegalArgumentException.class);
-    thrown.expectMessage(
-        "The resolved file: [tmp/] should not end with '/'.");
+    thrown.expectMessage("The resolved file: [tmp/] should not end with '/'.");
     toResourceIdentifier("gs://my_bucket/").resolve("tmp/", StandardResolveOptions.RESOLVE_FILE);
   }
 
@@ -93,15 +92,27 @@ public void testResolveInvalidNotDirectory() throws Exception {
 
   @Test
   public void testGetCurrentDirectory() throws Exception {
-    // Tests for local files without the scheme.
+    // Tests gcs paths.
     assertEquals(
         toResourceIdentifier("gs://my_bucket/tmp dir/"),
         toResourceIdentifier("gs://my_bucket/tmp dir/").getCurrentDirectory());
 
-    // Tests path with unicode
+    // Tests path with unicode.
     assertEquals(
         toResourceIdentifier("gs://my_bucket/输出 目录/"),
         toResourceIdentifier("gs://my_bucket/输出 目录/文件01.txt").getCurrentDirectory());
+
+    // Tests bucket with no ending '/'.
+    assertEquals(
+        toResourceIdentifier("gs://my_bucket/"),
+        toResourceIdentifier("gs://my_bucket").getCurrentDirectory());
+  }
+
+  @Test
+  public void testInvalidGcsPath() throws Exception {
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("Invalid GCS URI: gs://");
+    toResourceIdentifier("gs://");
   }
 
   @Test