空指针异常

Question

我试图做

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我试图创建一个PreferenceActivity它承载一个开关按钮，如果我可以启用或禁用全球定位系统。布局和一切工作完美，我添加了代码后，交换机做了一些我得到一个NullPointerException，这不应该存在我的眼睛。

问题

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你能提供给我一个awnser为什么这个空指针是有=）有些codesnippets将是有益的。

代码

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PrefsActivity：

private Switch enable_gps; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     addPreferencesFromResource(R.xml.prefs); 

     enable_gps = (Switch) findViewById(R.id.switch1); 

     ActionBar actionBar = this.getActionBar(); 
     // So kann android.R.id.home benützt werden 
     actionBar.setDisplayHomeAsUpEnabled(true); 

     if (getPref() == false) { <--- Here I get a Nullpointer 
      // enable_gps.setEnabled(false); 
      enable_gps.setChecked(false); 
     } else { 
      enable_gps.setChecked(true); 
     } 

     enable_gps.setOnClickListener(new View.OnClickListener() { 

      public void onClick(View v) { 
       if (getPref() == false) { 

        GPSService.actionStart(getApplicationContext()); 
        setPref(true); 

       } else if (getPref() == true) { 

        GPSService.actionStop(getApplicationContext()); 
        setPref(false); 

       } 
      } 
     }); 

    } 

getPref和setPref

的logcat：

07-26 09:47:02.991: E/AndroidRuntime(21729): FATAL EXCEPTION: main 
07-26 09:47:02.991: E/AndroidRuntime(21729): java.lang.RuntimeException: Unable to start activity ComponentInfo{de.ivocore/de.ivocore.preferences.PrefsActivity}: java.lang.NullPointerException 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.app.ActivityThread.access$600(ActivityThread.java:130) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.os.Handler.dispatchMessage(Handler.java:99) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.os.Looper.loop(Looper.java:137) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.app.ActivityThread.main(ActivityThread.java:4745) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at java.lang.reflect.Method.invokeNative(Native Method) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at java.lang.reflect.Method.invoke(Method.java:511) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at dalvik.system.NativeStart.main(Native Method) 
07-26 09:47:02.991: E/AndroidRuntime(21729): Caused by: java.lang.NullPointerException 
07-26 09:47:02.991: E/AndroidRuntime(21729): at de.ivocore.preferences.PrefsActivity.onCreate(PrefsActivity.java:35) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.app.Activity.performCreate(Activity.java:5008) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079) 
07-26 09:47:02.991: E/AndroidRuntime(21729): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023) 
07-26 09:47:02.991: E/AndroidRuntime(21729): ... 11 more 

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感谢您的帮助提前

问候

野生动物园

Answer 1

添加此语句

setContentView(R.layout.main); <----- 

//主要是你已经宣布一个XML layour R.id.switch1

enable_gps = (Switch) findViewById(R.id.switch1); 
            ^^^^^^^^^^^^ 

private boolean getPref() { 
     boolean isStarted=false;<------ 
     // SharedPreferences instanzieren 
     SharedPreferences mPrefs = this.getSharedPreferences(LOG_TAG, 0); 
     return isStarted = mPrefs.getBoolean(PREF_STARTED, false); 
    } 

Answer 2

您正在访问这一观点enable_gps = (Switch) findViewById(R.id.switch1);无需调用setContentView();所以这里enable_gps是null，而你试图调用enable_gps.setChecked(false);因此它是造成NPE ..