在路径上找不到类

我正在使用facebook sdk for android，我正在关注一个教程 . 当我运行我的代码时，它说java.lang.RuntimeException：无法实例化活动ComponentInfo {com.example.ftester / com.example.ftester.MainActivity}：java.lang.ClassNotFoundException：没有找到类“com.example . ftester.MainActivity“on path：DexPathList [[zip file”/data/app/com.example.ftester-2.apk"],nativeLibraryDirectories=[/data/app-lib/com.example.ftester-2,/ vendor / lib，/ system / lib]]

public class MainActivity extends FragmentActivity {


private static final int SPLASH = 0;
private static final int SELECTION = 1;
private static final int FRAGMENT_COUNT = SELECTION +1;
private boolean isResumed = false;
private UiLifecycleHelper uiHelper;

private Fragment[] fragments = new Fragment[FRAGMENT_COUNT];

private Session.StatusCallback callback = 
        new Session.StatusCallback() {
        @Override
        public void call(Session session, 
                SessionState state, Exception exception) {
            onSessionStateChange(session, state, exception);
        }
    };
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    uiHelper = new UiLifecycleHelper(this, callback);
    uiHelper.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

     FragmentManager fm = getSupportFragmentManager();
        fragments[SPLASH] = fm.findFragmentById(R.id.splashFragment);
        fragments[SELECTION] = fm.findFragmentById(R.id.selectionFragment);

        FragmentTransaction transaction = fm.beginTransaction();
        for(int i = 0; i < fragments.length; i++) {
            transaction.hide(fragments[i]);
        }
        transaction.commit();


}





private void showFragment(int fragmentIndex, boolean addToBackStack) {
    FragmentManager fm = getSupportFragmentManager();
    FragmentTransaction transaction = fm.beginTransaction();
    for (int i = 0; i < fragments.length; i++) {
        if (i == fragmentIndex) {
            transaction.show(fragments[i]);
        } else {
            transaction.hide(fragments[i]);
        }
    }
    if (addToBackStack) {
        transaction.addToBackStack(null);
    }
    transaction.commit();
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
protected void onResumeFragments() {
    super.onResumeFragments();
    Session session = Session.getActiveSession();

    if (session != null && session.isOpened()) {
        // if the session is already open,
        // try to show the selection fragment
        showFragment(SELECTION, false);
    } else {
        // otherwise present the splash screen
        // and ask the person to login.
        showFragment(SPLASH, false);
    }
 }

private void onSessionStateChange(Session session, SessionState state, Exception exception) {
    // Only make changes if the activity is visible
    if (isResumed) {
         FragmentManager manager = getSupportFragmentManager();
        // Get the number of entries in the back stack
        int backStackSize = manager.getBackStackEntryCount();
        // Clear the back stack
        for (int i = 0; i < backStackSize; i++) {
            manager.popBackStack();
        }
        if (state.isOpened()) {
            // If the session state is open:
            // Show the authenticated fragment
            showFragment(SELECTION, false);
        } else if (state.isClosed()) {
            // If the session state is closed:
            // Show the login fragment
            showFragment(SPLASH, false);
        }
    }
}
@Override
public void onResume() {
    super.onResume();
    uiHelper.onResume();
    isResumed = true;
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    uiHelper.onActivityResult(requestCode, resultCode, data);
}

@Override
public void onDestroy() {
    super.onDestroy();
    uiHelper.onDestroy();
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    uiHelper.onSaveInstanceState(outState);
}






@Override
public void onPause() {
    super.onPause();
    uiHelper.onPause();
    isResumed = false;
}
     }




<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.ftester"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="17" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.example.ftester.MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
      </activity>
    </application>

   </manifest>