Anwendung wurde gestoppt. Warum?

Question

Ich kann den Logcat-Bericht nicht dekodieren. Ich lerne Android und versuche, Absicht, Toast und benutzerdefinierten Toast in einzelne App zu implementieren.

Soweit ich weiß, habe ich es richtig gemacht. Ich lege Logcat-Trace an.

FATAL EXCEPTION: main 
Process: com.bt4u.customtoast, PID: 24490 
Theme: themes:{default=overlay:com.rr.neptune, iconPack:system, fontPkg:com.rr.neptune, com.android.systemui=overlay:com.rr.neptune, com.android.systemui.navbar=overlay:com.rr.neptune} 
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bt4u.customtoast/com.bt4u.customtoast.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2450) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2520) 
at android.app.ActivityThread.access$900(ActivityThread.java:153) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:5466) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
at com.bt4u.customtoast.MainActivity.onCreate(MainActivity.java:21) 
at android.app.Activity.performCreate(Activity.java:6251) 
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108) 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2403) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2520)  
at android.app.ActivityThread.access$900(ActivityThread.java:153)  
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363)  
at android.os.Handler.dispatchMessage(Handler.java:102)  
at android.os.Looper.loop(Looper.java:148)  
at android.app.ActivityThread.main(ActivityThread.java:5466)  
at java.lang.reflect.Method.invoke(Native Method)  
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)  
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)  

Haupttätigkeit

b1 = (Button)findViewById(R.id.button1); 
    b2 = (Button)findViewById(R.id.button2); 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    b1.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      Toast toast = Toast.makeText(MainActivity.this,"Just clicked",Toast.LENGTH_SHORT); 
      toast.setGravity(Gravity.BOTTOM|Gravity.RIGHT,0,0); 
     } 
    }); 

    b2.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      Intent i = new Intent(MainActivity.this,second.class); 
      startActivity(i); 
     } 
    }); 
    } 

mir auch sagen, wie das Problem zu finden, indem Sie die Log-Spur

Answer 1

mit Sie finden die Reihenfolge umkehren müssen Sie Ihre Tasten sind Einstellung:

super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_main); 
b1 = (Button)findViewById(R.id.button1); 
b2 = (Button)findViewById(R.id.button2); 

Dies ist sehr wichtig, da Sie zuerst setContentView anrufen müssen, weil das v iiew wird dann verwendet, um view.findViewById aufzurufen. Wenn Sie dies vor dem Festlegen der Ansicht versuchen, können Sie keine Elemente der Ansicht abrufen.

EDIT: Was Ihre zweite Frage, ersetzen:

Toast toast = Toast.makeText(MainActivity.this,"Just clicked",Toast.LENGTH_SHORT); 
     toast.setGravity(Gravity.BOTTOM|Gravity.RIGHT,0,0); 

mit:

Toast.makeText(MainActivity.this,"Just clicked",Toast.LENGTH_SHORT).setGravity(Gravity.BOTTOM|Gravity.RIGHT,0,0).show(); 

Answer 2

Sie rufen setContentView Methode falschen Ort

super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    b1 = (Button)findViewById(R.id.button1); 
     b2 = (Button)findViewById(R.id.button2); 



     b1.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       Toast toast = Toast.makeText(MainActivity.this,"Just clicked",Toast.LENGTH_SHORT); 
       toast.setGravity(Gravity.BOTTOM|Gravity.RIGHT,0,0); 
      } 
     }); 

     b2.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       Intent i = new Intent(MainActivity.this,second.class); 
       startActivity(i); 
      } 
     });