SharedPreferences.Editor NPE

Question

我一直在与共享的首选项做斗争，我看过帖子和教程，但无法让它发挥作用。

我最终从android.dev复制了这段代码，它对我也不起作用。我唯一的改变就是

1. 扩展AppCompatActivity而不是活动。
2. 添加打印文件和
3. getSharedPreferences的部分。

它在onPause()中的SharedPreferences.Editor中使用NPE失败。我在onStart()中没有发现任何例外。

谁能告诉我我错过了什么吗？提前谢谢你！

public class MainActivity extends AppCompatActivity {
    static final int DAY_VIEW_MODE = 0;
    static final int WEEK_VIEW_MODE = 1;

    private SharedPreferences mPrefs;
    private int mCurViewMode;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        System.out.println("onCreate() ");
        SharedPreferences mPrefs = getSharedPreferences("MyPrefFile", 0);
        mCurViewMode = mPrefs.getInt("view_mode", DAY_VIEW_MODE);
    }

    protected void onPause() {
        super.onPause();

        System.out.println("onPause() ");
        SharedPreferences.Editor ed = mPrefs.edit();
        ed.putInt("view_mode", mCurViewMode);
        ed.commit();
    }
}

这是logcat，这是您请求的错误日志吗？对不起，我对这件事很陌生。(ug.有什么更好的方法来分享这个吗？)

java.lang.RuntimeException: Unable to pause activity {temperatureconverter.android.vogella.com.prefsfromandroiddev/temperatureconverter.android.vogella.com.prefsfromandroiddev.MainActivity}: java.lang.NullPointerException
  at android.app.ActivityThread.performPauseActivity(ActivityThread.java:2902)
  at android.app.ActivityThread.performPauseActivity(ActivityThread.java:2858)
  at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:2836)
  at android.app.ActivityThread.access$900(ActivityThread.java:140)
  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1254)
  at android.os.Handler.dispatchMessage(Handler.java:99)
  at android.os.Looper.loop(Looper.java:137)
  at android.app.ActivityThread.main(ActivityThread.java:4921)
  at java.lang.reflect.Method.invokeNative(Native Method)
  at java.lang.reflect.Method.invoke(Method.java:511)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1038)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:805)
  at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
  at temperatureconverter.android.vogella.com.prefsfromandroiddev.MainActivity.onPause(MainActivity.java:38)
  at android.app.Activity.performPause(Activity.java:5286)
  at android.app.Instrumentation.callActivityOnPause(Instrumentation.java:1240)
  at android.app.ActivityThread.performPauseActivity(ActivityThread.java:2889)
  at android.app.ActivityThread.performPauseActivity(ActivityThread.java:2858) 
  at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:2836) 
  at android.app.ActivityThread.access$900(ActivityThread.java:140) 
  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1254) 
  at android.os.Handler.dispatchMessage(Handler.java:99) 
  at android.os.Looper.loop(Looper.java:137) 
  at android.app.ActivityThread.main(ActivityThread.java:4921) 
  at java.lang.reflect.Method.invokeNative(Native Method) 
  at java.lang.reflect.Method.invoke(Method.java:511) 
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1038) 
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:805) 
  at dalvik.system.NativeStart.main(Native Method)

Answer 1

您的代码将在mPrefs函数中创建和初始化onCreate函数，因此它与oncreate外部的private SharedPreferences mPrefs;不一样，因此mPrefs;的作用域仅限于oncreate，不能在外部使用(将为null)。

因此，要使它对当前类全局化，请使用全局引用变量，不要在oncreate函数中再次声明它，以便它可以在onPause函数中使用，如下所示

 public class MainActivity extends AppCompatActivity {

    static final int DAY_VIEW_MODE = 0;
    static final int WEEK_VIEW_MODE = 1;

    private SharedPreferences mPrefs;
    private int mCurViewMode;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        System.out.println("onCreate() ");
       // you are creating an another mPrefs reference local to oncreate function
       //  which is different from the global one ,so remove this 
       // SharedPreferences mPrefs = getSharedPreferences("MyPrefFile", 0);
        mPrefs = getSharedPreferences("MyPrefFile", 0); // use this
        mCurViewMode = mPrefs.getInt("view_mode", DAY_VIEW_MODE);
    }

    protected void onPause() {
        super.onPause();

        System.out.println("onPause() ");
        // you can do the same for editor to make it accessible to all functions 
        SharedPreferences.Editor ed = mPrefs.edit();
        ed.putInt("view_mode", mCurViewMode);
        ed.commit();
    }
}

Answer 2

在onCreate(Bundle savedInstanceState)中使用这个

mPrefs = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());

而不是写作

SharedPreferences mPrefs = getSharedPreferences("MyPrefFile", 0);