Android Studio：在我的智能手機上運行應用程序後，應用程序崩潰

Question

在每個應用程序啓動時，手機上都顯示一條錯誤消息：「應用程序已結束，很抱歉」。我現在能做什麼？我已經搜索了網絡，並試圖解決它，但我沒有找到答案。

這是我的logcat輸出：

FATAL EXCEPTION: main 
Process: mycompany.kopfrechnenti1907_1839, PID: 524 
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{mycompany.kopfrechnenti1907_1839/mycompany.kopfrechnenti1907_1839.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference 
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2363) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512) 
    at android.app.ActivityThread.access$900(ActivityThread.java:158) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5521) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:804) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:666) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference 
    at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:120) 
    at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:155) 
    at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:31) 
    at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:55) 
    at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:33) 
    at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:203) 
    at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185) 
    at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519) 
    at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190) 
    at mycompany.kopfrechnenti1907_1839.MainActivity.<init>(MainActivity.java:10) 
    at java.lang.Class.newInstance(Native Method) 
    at android.app.Instrumentation.newActivity(Instrumentation.java:1069) 
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2353) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512) 
    at android.app.ActivityThread.access$900(ActivityThread.java:158) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5521) 
    at java.lang.reflect.Method.invoke(Native Method) 

import android.os.Bundle; 
import android.support.v7.app.AppCompatActivity; 
import android.widget.RatingBar; 
import android.widget.Button; 

public class MainActivity extends AppCompatActivity { 

    Button btnplus = (Button)findViewById(R.id.btnplus); 
    RatingBar ratingbar = (RatingBar)findViewById(R.id.ratingBar); 


    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
    } 

    public void plusonclick() 
    { 
     if (ratingbar.getRating() == 1) 
     { 
      setContentView(R.layout.activity_plusrechnenlvl1); 
     } 
    } 

} 

我的英語水平是不是最好的，但我希望你能理解我:)

非常感謝您！

七月

Answer 1

你必須寫後

setContentView() 
Button btnplus = (Button)findViewById(R.id.btnplus); 
    RatingBar ratingbar = (RatingBar)findViewById(R.id.ratingBar); 

像下面按照onCreate。

public class MainActivity extends AppCompatActivity { 

    Button btnplus; 
    RatingBar ratingbar; 


    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

    btnplus = (Button)findViewById(R.id.btnplus); 
    ratingbar = (RatingBar)findViewById(R.id.ratingBar); 
    } 

    public void plusonclick() 
    { 
     if (ratingbar.getRating() == 1) 
     { 
      setContentView(R.layout.activity_plusrechnenlvl1); 
     } 
    } 

}