First, the error is reported because the third-party classes are not found. Solution: Right-click the project Properties -------> Build Path -------> Order And Export to check the third-party jar package. Then Clear your project and re-compile and run it to solve the above problem. For details, see the above steps and then Clear the project and compile and run it. By the way, I posted the source code of Sina OAuth authentication to the buddies who needed it and downloaded it: Sina's jar package is required for authentication. Signpost-commonshttp4-1.2.1.1.jar signpost-core-1.2.1.1.jar: jar package download ============================ file name: AuthActivity. java ========================== package OAuth4Sina.com; import java. util. sortedSet; import oauth. signpost. OAuthProvider; import oauth. signpost. basic. defaultOAuthProvider; import oauth. signpost. commonshttp. commonsHttpOAuthConsumer; import oauth. signpost. exception. OAuthCommunicationException; import oauth. signpost. exception. OAuthExpectationFailedException; import oauth. signpost. exception. OAuthMessageSignerException; import oauth. signpost. exception. OAuthNotAuthorizedException; import android. app. activity; import android. content. intent; import android.net. uri; import android. OS. bundle; import android. view. view; import android. widget. button; import android. widget. textView;/***** <p> Title: AuthActivity. java </p> * <p> Description: Implementation of OAuth authentication </p> * <p> Copyright: Copyright (c) 2011 </p> * <p> Company: CTX Teachnology </p> * <p> CreateTime: 2012-12-22 04:21:04 </p> ** @ author YGC * @ version V1.0 * @ since JDK1.6 */public class AuthActivity extends Activity {CommonsHttpOAuthConsumer httpOauthConsumer; OAuthProvider httpOauthprovider; @ Override public void onCreate (Bundle savedInstanceState) {super. onCreate (savedInstanceState); setContentView (R. layout. main); Button btn2 = (Button) findViewById (R. id. btn); btn2.setOnClickListener (new View. onClickListener () {@ Override public void onClick (View v) {try {String consumerKey = "434313181"; // replace it with your own key String consumerSecret = "secret "; // also your own Secret String callBackUrl = "myapp: // AuthActivity"; httpOauthConsumer = new consumer (consumerKey, consumerSecret); httpOauthprovider = new DefaultOAuthProvider ("http://api.t.sina.com.cn/oauth/request_token", "http://api.t.sina.com.cn/oauth/access_token ", "http://api.t.sina.com.cn/oauth/authorize"); String authUrl = httpOauthprovider. retrieveRequestToken (httpOauthConsumer, callBackUrl); startActivity (new Intent (Intent. ACTION_VIEW, Uri. parse (authUrl); int B = 4;} catch (Exception e) {String s = e. getMessage () ;}}) ;}@ Override protected void onNewIntent (Intent intent) {super. onNewIntent (intent); Uri uri = intent. getData (); String verifier = uri. getQueryParameter (oauth. signpost. OAuth. OAUTH_VERIFIER); try {httpOauthprovider. setOAuth10a (true); httpOauthprovider. retrieveAccessToken (httpOauthConsumer, verifier);} catch (OAuthMessageSignerException ex) {ex. printStackTrace ();} catch (OAuthNotAuthorizedException ex) {ex. printStackTrace ();} catch (OAuthExpectationFailedException ex) {ex. printStackTrace ();} catch (OAuthCommunicationException ex) {ex. printStackTrace ();} SortedSet <String> user_id = httpOauthprovider. getResponseParameters (). get ("user_id"); String userId = user_id.first (); String userKey = httpOauthConsumer. getToken (); String userSecret = httpOauthConsumer. getTokenSecret (); TextView text = (TextView) findViewById (R. id. text); text. setText ("suerId:" + userId + "/userKey:" + userKey + "/userSecret:" + userSecret );}}