firebase 를 연동해서 구글 계정을 인증하는 부분에서 오류가 발생합니다..

Question

구글 계정인증에 성공하면 파이어베이스와 연동되는 부분에 대한 코드를 작성 후 인증이 정상적으로 되는지 체크하려는데 앱이 계속해서 중지됩니다. 라는 내용과 함께 오류가 발생했습니다. 책자를 참고한 것이라 버전 문제인지 무엇인지 갈피가 잡히지 않아 질문드립니다.

 

일단 오류내용은

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.firebasechatexam, PID: 30923
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.firebasechatexam/com.example.firebasechatexam.MainActivity}: android.content.ActivityNotFoundException: Unable to find explicit activity class {com.example.firebasechatexam/com.example.firebasechatexam.SignInActivity}; have you declared this activity in your AndroidManifest.xml?
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: android.content.ActivityNotFoundException: Unable to find explicit activity class {com.example.firebasechatexam/com.example.firebasechatexam.SignInActivity}; have you declared this activity in your AndroidManifest.xml?
        at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:2005)
        at android.app.Instrumentation.execStartActivity(Instrumentation.java:1673)
        at android.app.Activity.startActivityForResult(Activity.java:4586)
        at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:767)
        at android.app.Activity.startActivityForResult(Activity.java:4544)
        at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:754)
        at android.app.Activity.startActivity(Activity.java:4905)
        at android.app.Activity.startActivity(Activity.java:4873)
        at com.example.firebasechatexam.MainActivity.onCreate(MainActivity.java:48)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

이러한 내용이고

이 부분에서 제가 작성한 MainActicity.java:48 의 내용은

 

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // 파이어베이스 인증 초기화
        mFirebaseAuth = FirebaseAuth.getInstance();
        mFirebaseUser = mFirebaseAuth.getCurrentUser();

        if(mFirebaseUser == null){
        // 인증이 안 되었다면 인증 화면으로 이동
        startActivity(new Intent(this, SignInActivity.class)); <--48번줄
        finish();
        return;
    }
        else{
        mUsername = mFirebaseUser.getDisplayName();
        if(mFirebaseUser.getPhotoUrl() != null){
            mPhotoUrl = mFirebaseUser.getPhotoUrl().toString();
        }
    }
    mMessageRecyclerView = findViewById(R.id.message_recycler_view);
}
}