[Android] NDK: No implementation found for native

Problem:
I met an error when I build a simple ndk app.
error message as below:

04-12 14:35:30.009: WARN/dalvikvm(538): No implementation found for native Lcom/example/MyActivity;.stringFromJNI ()Ljava/lang/String;
04-12 14:35:30.018: DEBUG/AndroidRuntime(538): Shutting down VM
04-12 14:35:30.018: WARN/dalvikvm(538): threadid=1: thread exiting with uncaught exception (group=0x40015560)
04-12 14:35:30.048: ERROR/AndroidRuntime(538): FATAL EXCEPTION: main
        java.lang.UnsatisfiedLinkError: stringFromJNI
        at com.example.MyActivity.stringFromJNI(Native Method)
        at com.example.MyActivity.onCreate(MyActivity.java:16)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
        at android.app.ActivityThread.access$1500(ActivityThread.java:117)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:123)
        at android.app.ActivityThread.main(ActivityThread.java:3683)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:507)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
        at dalvik.system.NativeStart.main(Native Method)

Solve:
It cause by wrong function name of native code.

my native code, shown that package is com.play.MyActivity

#include <string.h>
#include <jni.h>
jstring Java_com_play_MyActivity_stringFromJNI( JNIEnv* env, jobject thiz )
{
    return (*env)->NewStringUTF(env, "Hello from JNI by myself!");
}

my source code, shown that package is com.example.MyActivity

package com.example;
import ...

public class MyActivity extends Activity
{
    /** code **/
}

I modified the native code to correct package then problem is sloved.

#include <string.h>
#include <jni.h>
jstring Java_com_example_MyActivity_stringFromJNI( JNIEnv* env, jobject thiz )
{
    return (*env)->NewStringUTF(env, "Hello from JNI by myself!");
}