NullPointerException：ヌルオブジェクトリファレンスで仮想メソッドを呼び出そうとしました...

Question

SettingsActivityを開始すると、次のエラーが表示されますが、私はすべてのことをいつものように行っています。

ERROR：

FATAL EXCEPTION: main Process: com.android.niklasvlach.vertretung, PID: 9175 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.niklasvlach.vertretung/com.android.niklasvlach.vertretung.SettingsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setOnItemSelectedListener(android.widget.AdapterView$OnItemSelectedListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2358) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2420) at android.app.ActivityThread.access$900(ActivityThread.java:154) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5294) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setOnItemSelectedListener(android.widget.AdapterView$OnItemSelectedListener)' on a null object reference at com.android.niklasvlach.vertretung.SettingsActivity.onCreate(SettingsActivity.java:28) at android.app.Activity.performCreate(Activity.java:5990) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2311) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2420)  at android.app.ActivityThread.access$900(ActivityThread.java:154)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:135)  at android.app.ActivityThread.main(ActivityThread.java:5294)  at java.lang.reflect.Method.invoke(Native Method)  at java.lang.reflect.Method.invoke(Method.java:372)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699) 

コード：

public class SettingsActivity extends AppCompatActivity { 

Spinner select; 
EditText username; 
EditText password; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    select = (Spinner) findViewById(R.id.selectclass); 
    username = (EditText) findViewById(R.id.username_view); 
    password = (EditText) findViewById(R.id.password_view); 

    select.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() { 

     public void onItemSelected(AdapterView<?> arg0, View arg1, int pos, long text) { 
      Toast.makeText(SettingsActivity.this,"" + text,Toast.LENGTH_LONG).show(); 

     } 

     public void onNothingSelected(AdapterView<?> arg0) { 

     } 
    }); 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_settings); 
} 

}

他の何かが必要な場合、私はそれを投稿します！

Answer 1

アクティビティのレイアウトを設定した後でUI作業を行う必要があります。

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_settings); 

    select = (Spinner) findViewById(R.id.selectclass); 
    username = (EditText) findViewById(R.id.username_view); 
    password = (EditText) findViewById(R.id.password_view); 

    select.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() { 

     public void onItemSelected(AdapterView<?> arg0, View arg1, int pos, long text) { 
      Toast.makeText(SettingsActivity.this,"" + text,Toast.LENGTH_LONG).show(); 

     } 

     public void onNothingSelected(AdapterView<?> arg0) { 

     } 
    }); 
}