关于ViewModel使用的一个血案

关于ViewModel使用的一个血案: Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.demo043002.MainViewModel

在写一个关于ViewModel的demo,总是在运行期报错,百思不得其解:

Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.demo043002.MainViewModel
       at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:186)
       at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:164)
       at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:130)
       at com.example.demo043002.MainActivity.onCreate(MainActivity.kt:14)
       at android.app.Activity.performCreate(Activity.java:6975)
       at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
       at android.app.ActivityThread.-wrap11(Unknown Source:0) 
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
       at android.os.Handler.dispatchMessage(Handler.java:105) 
       at android.os.Looper.loop(Looper.java:164) 
       at android.app.ActivityThread.main(ActivityThread.java:6541) 
       at java.lang.reflect.Method.invoke(Native Method) 
       at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 

代码很简单:

class MainViewModel extends ViewModel {
}

class MainActivity : AppCompatActivity() {
    val TAG="MainActivity"
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val mainViewModel=ViewModelProvider(this,ViewModelProvider.NewInstanceFactory()).get(MainViewModel::class.java)
        Log.e(TAG,"mainViewModel==null:"+(mainViewModel==null))
    }
}

同志们,发现问题没???其实就是MainViewModel的权限没有用public!!!,教训惨重啊,又浪费了半天时间