刚刚开始ndk的hello world,出现如下问题,求大神解答

08-23 10:41:44.384: E/AndroidRuntime(24885): FATAL EXCEPTION: main
08-23 10:41:44.384: E/AndroidRuntime(24885): java.lang.UnsatisfiedLinkError: Native method not found: com.ndk.GetString.getStr:()Ljava/lang/String;
08-23 10:41:44.384: E/AndroidRuntime(24885): at com.ndk.GetString.getStr(Native Method)
08-23 10:41:44.384: E/AndroidRuntime(24885): at com.ndk.MainActivity.onCreate(MainActivity.java:19)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.app.Activity.performCreate(Activity.java:5122)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
08-23 10:41:44.384: E/AndroidRuntime(24885): at com.qihoo360.mobilesafe.loader.c.callActivityOnCreate(SourceFile:98)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2283)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2371)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.app.ActivityThread.access$600(ActivityThread.java:164)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1348)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.os.Handler.dispatchMessage(Handler.java:99)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.os.Looper.loop(Looper.java:153)
08-23 10:41:44.384: E/AndroidRuntime(24885): at android.app.ActivityThread.main(ActivityThread.java:5314)
08-23 10:41:44.384: E/AndroidRuntime(24885): at java.lang.reflect.Method.invokeNative(Native Method)
08-23 10:41:44.384: E/AndroidRuntime(24885): at java.lang.reflect.Method.invoke(Method.java:511)
08-23 10:41:44.384: E/AndroidRuntime(24885): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
08-23 10:41:44.384: E/AndroidRuntime(24885): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606)
08-23 10:41:44.384: E/AndroidRuntime(24885): at dalvik.system.NativeStart.main(Native Method)