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feat: add Gmail list messages sample
Add Gmail sample for listing messages and corresponding unit test. Based on the `list_messages` sample from Python.
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‎gmail/snippets/src/main/java/ListMessages.java‎

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// Copyright 2026 Google LLC
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// [START gmail_list_messages]
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import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
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import com.google.api.client.googleapis.json.GoogleJsonResponseException;
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import com.google.api.client.http.HttpRequestInitializer;
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import com.google.api.client.json.gson.GsonFactory;
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import com.google.api.services.gmail.Gmail;
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import com.google.api.services.gmail.GmailScopes;
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import com.google.api.services.gmail.model.ListMessagesResponse;
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import com.google.api.services.gmail.model.Message;
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import com.google.auth.http.HttpCredentialsAdapter;
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import com.google.auth.oauth2.GoogleCredentials;
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import java.io.IOException;
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import java.security.GeneralSecurityException;
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import java.util.Collections;
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import java.util.List;
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/*
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* Class to demonstrate the use of Gmail List Messages API
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*/
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public class ListMessages {
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/**
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* Lists the user's Gmail messages.
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*
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* @param service Authorized Gmail API instance.
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* @return List of Messages in the user's mailbox.
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* @throws IOException if the request to retrieve messages fails.
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* @throws GoogleJsonResponseException if the execution of the request fails.
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*/
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public static List<Message> listMessages(Gmail service) throws IOException {
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try {
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final long maxMessages = 10L;
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final String userId = "me";
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ListMessagesResponse results =
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service
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.users()
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.messages()
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.list(userId)
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.setLabelIds(Collections.singletonList("INBOX"))
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.setMaxResults(maxMessages)
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.execute();
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List<Message> messages = results.getMessages();
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if (messages == null || messages.isEmpty()) {
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System.out.println("No messages found.");
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return messages;
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}
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System.out.println("Messages:");
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for (Message message : messages) {
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/*TODO(developer) - Consider batch requests if you need to retrieve many messages at once.
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See https://developers.google.com/workspace/gmail/api/guides/batch for implementation guides. */
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Message msg = service.users().messages().get(userId, message.getId()).execute();
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System.out.printf("- Message ID: %s%n Snippet: %s%n", message.getId(), msg.getSnippet());
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}
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return messages;
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} catch (GoogleJsonResponseException e) {
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// TODO(developer) - handle error appropriately
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System.err.println("An error occurred: " + e);
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throw e;
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}
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}
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private static Gmail getGmailService() throws IOException, GeneralSecurityException {
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/* Load pre-authorized user credentials from the environment.
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TODO(developer) - See https://developers.google.com/identity for
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guides on implementing OAuth2 for your application. */
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GoogleCredentials credentials =
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GoogleCredentials.getApplicationDefault().createScoped(GmailScopes.GMAIL_READONLY);
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HttpRequestInitializer requestInitializer = new HttpCredentialsAdapter(credentials);
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// Create the gmail API client
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return new Gmail.Builder(
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GoogleNetHttpTransport.newTrustedTransport(),
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GsonFactory.getDefaultInstance(),
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requestInitializer)
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.setApplicationName("Gmail samples")
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.build();
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}
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}
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// [END gmail_list_messages]

‎gmail/snippets/src/test/java/TestListMessages.java‎

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// Copyright 2026 Google LLC
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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import com.google.api.client.googleapis.json.GoogleJsonResponseException;
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import com.google.api.client.http.HttpTransport;
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import com.google.api.client.http.LowLevelHttpRequest;
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import com.google.api.client.http.LowLevelHttpResponse;
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import com.google.api.client.json.Json;
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import com.google.api.client.json.gson.GsonFactory;
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import com.google.api.client.testing.http.MockHttpTransport;
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import com.google.api.client.testing.http.MockLowLevelHttpRequest;
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import com.google.api.client.testing.http.MockLowLevelHttpResponse;
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import com.google.api.services.gmail.Gmail;
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import com.google.api.services.gmail.model.Message;
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import org.junit.Test;
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import java.io.IOException;
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import java.util.List;
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import static org.junit.Assert.assertEquals;
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import static org.junit.Assert.assertNotNull;
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import static org.junit.Assert.assertThrows;
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import static org.junit.Assert.assertTrue;
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// Unit test case for gmail list messages snippet
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public class TestListMessages {
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@Test
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public void testListMessagesReturnsMessages() throws IOException {
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HttpTransport transport =
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new MockHttpTransport() {
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@Override
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public LowLevelHttpRequest buildRequest(String method, String url) {
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return new MockLowLevelHttpRequest() {
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@Override
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public LowLevelHttpResponse execute() {
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MockLowLevelHttpResponse response = new MockLowLevelHttpResponse();
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response.setStatusCode(200);
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response.setContentType(Json.MEDIA_TYPE);
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// Mock response for listing messages in the inbox
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if (url.contains("/messages") && url.contains("labelIds=INBOX")) {
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response.setContent("{\"messages\": [{\"id\": \"12345\"}]}");
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} else if (url.contains("/messages/12345")) {
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response.setContent(
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"{\"id\": \"12345\", \"snippet\": \"This is a test snippet.\"}");
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} else {
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response.setStatusCode(404);
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}
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return response;
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}
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};
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}
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};
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Gmail service = getMockGmailService(transport);
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List<Message> result = ListMessages.listMessages(service);
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assertNotNull("The returned message list should not be null", result);
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assertEquals("The returned list should contain exactly one message", 1, result.size());
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assertEquals("The message ID should match the mock data", "12345", result.get(0).getId());
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}
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@Test
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public void testListMessagesHandlesApiError() {
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// Mock transport simulates a 404 Not Found error
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HttpTransport errorTransport =
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new MockHttpTransport() {
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@Override
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public LowLevelHttpRequest buildRequest(String method, String url) {
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return new MockLowLevelHttpRequest() {
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@Override
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public LowLevelHttpResponse execute() {
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MockLowLevelHttpResponse response = new MockLowLevelHttpResponse();
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response.setStatusCode(404);
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response.setContentType(Json.MEDIA_TYPE);
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response.setContent("{\"error\": {\"code\": 404, \"message\": \"Not Found\"}}");
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return response;
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}
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};
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}
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};
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Gmail service = getMockGmailService(errorTransport);
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assertThrows(GoogleJsonResponseException.class, () -> ListMessages.listMessages(service));
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}
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@Test
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public void testListMessagesHandlesEmptyResults() throws IOException {
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// Mock transport returns empty messages array
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HttpTransport emptyTransport =
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new MockHttpTransport() {
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@Override
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public LowLevelHttpRequest buildRequest(String method, String url) {
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return new MockLowLevelHttpRequest() {
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@Override
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public LowLevelHttpResponse execute() {
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MockLowLevelHttpResponse response = new MockLowLevelHttpResponse();
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response.setStatusCode(200);
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response.setContentType(Json.MEDIA_TYPE);
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response.setContent("{\"messages\": []}");
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return response;
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}
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};
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}
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};
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Gmail service = getMockGmailService(emptyTransport);
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List<Message> result = ListMessages.listMessages(service);
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assertNotNull("Should return an empty list when no messages found", result);
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assertTrue("Returned list should be empty", result.isEmpty());
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}
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private Gmail getMockGmailService(HttpTransport transport) {
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return new Gmail.Builder(transport, GsonFactory.getDefaultInstance(), request -> {})
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.setApplicationName("Gmail API Snippets")
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.build();
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}
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}

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