NullPointerException：嘗試調用虛方法...對空對象引用

Question

當我啓動我的SettingsActivity時，出現以下錯誤，但我不知道爲什麼，因爲我一直都這樣做。

錯誤：

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) 

驗證碼：

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); 
} 

}

如果需要其他任何東西，我將它張貼！

Answer 1

您需要在設置活動佈局後完成您的UI工作。

@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) { 

     } 
    }); 
}