If the reason is not in the predefined ERROR_CODES it would be more helpful to use the reason String that came in as an parameter than UNKNOWN_ERROR. UNKNOWN_ERROR could be used for null or empty Strings.

Author: JosRoseboom

src/main/java/com/google/firebase/messaging/TopicManagementResponse.java

@@ -102,8 +102,11 @@ public static class Error {
 
     private Error(int index, String reason) {
       this.index = index;
-      this.reason = ERROR_CODES.containsKey(reason)
-        ? ERROR_CODES.get(reason) : UNKNOWN_ERROR;
+      if (reason == null || reason.trim().isEmpty()) {
+        this.reason = UNKNOWN_ERROR;
+      } else {
+        this.reason = ERROR_CODES.getOrDefault(reason, reason);
+      }
     }
 
     /**

src/test/java/com/google/firebase/messaging/InstanceIdClientImplTest.java

@@ -404,7 +404,30 @@ public void testTopicManagementResponseWithEmptyList() {
 
   @Test
   public void testTopicManagementResponseErrorToString() {
-    GenericJson json = new GenericJson().set("error", "test error");
+    GenericJson json = new GenericJson().set("error", "INVALID_ARGUMENT");
+    ImmutableList<GenericJson> jsonList = ImmutableList.of(json);
+
+    TopicManagementResponse topicManagementResponse = new TopicManagementResponse(jsonList);
+
+    String expected = "[Error{index=0, reason=invalid-argument}]";
+    assertEquals(expected, topicManagementResponse.getErrors().toString());
+  }
+
+  @Test
+  public void testTopicManagementResponseErrorNotInErrorCodes() {
+    String myError = "myError";
+    GenericJson json = new GenericJson().set("error", myError);
+    ImmutableList<GenericJson> jsonList = ImmutableList.of(json);
+
+    TopicManagementResponse topicManagementResponse = new TopicManagementResponse(jsonList);
+
+    String expected = "[Error{index=0, reason=" + myError + "}]";
+    assertEquals(expected, topicManagementResponse.getErrors().toString());
+  }
+
+  @Test
+  public void testTopicManagementResponseErrorUnknown() {
+    GenericJson json = new GenericJson().set("error", "");
     ImmutableList<GenericJson> jsonList = ImmutableList.of(json);
 
     TopicManagementResponse topicManagementResponse = new TopicManagementResponse(jsonList);
@@ -443,7 +466,7 @@ private void checkTopicManagementRequest(
     assertEquals(1, result.getFailureCount());
     assertEquals(1, result.getErrors().size());
     assertEquals(1, result.getErrors().get(0).getIndex());
-    assertEquals("unknown-error", result.getErrors().get(0).getReason());
+    assertEquals("error_reason", result.getErrors().get(0).getReason());
 
     ByteArrayOutputStream out = new ByteArrayOutputStream();
     request.getContent().writeTo(out);