SharedPreferences Android 中的 NullPointerException [英] NullPointerException in SharedPreferences Android

问题描述

我第一次使用 sharedPreferences，我似乎无法克服这个错误.我有一个子菜单，应该允许用户设置他们的区域.这应该打开正确的区域活动，并在再次打开应用程序时存储和调用.我已经绕了很多圈，所以有些代码会有点奇怪.我一直专注于从美国(默认)更改为英国.

My first time using sharedPreferences and i can't seem to get past this error. I have a submenu that is supposed to allow the user to set their region. This should open the correct region activity and be stored and recalled when the app is opened again. I've been going round in circles many times so some of the code will be a bit wierd. I've been focusing on changing from US (default)to UK.

在 DDMS 中，我得到了这个:

In the DDMS I'm getting this:

05-13 11:22:39.344: ERROR/AndroidRuntime(960): java.lang.NullPointerException
05-13 11:22:39.344: ERROR/AndroidRuntime(960):     at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:146)
05-13 11:22:39.344: ERROR/AndroidRuntime(960):     at com.silifeform.android.Prefs.editRegion(Prefs.java:29)
05-13 11:22:39.344: ERROR/AndroidRuntime(960):     at com.silifeform.android.dog.onOptionsItemSelected(dog.java:344)

我的代码是这样的:

public class Prefs extends Activity {
public static final String PREFS_NAME="LocalePrefs";
private String region;
public boolean isUk;
public boolean isUs;
public boolean isEu;

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

    //restore prefs
    SharedPreferences settings= getSharedPreferences(PREFS_NAME,0);
    String myRegion = settings.getString(region,"us");

    this.region=myRegion;
    changeLocale(getRegion());
}

public void editRegion(String sregion) {
// The error occurs here:
    SharedPreferences settings = getSharedPreferences(PREFS_NAME,0);
    SharedPreferences.Editor ed = settings.edit();
    ed.clear();
    ed.putString(region,sregion);
    ed.commit();
}

public String getRegion(){
        SharedPreferences settings= getSharedPreferences(PREFS_NAME,0);
        String myRegion = settings.getString(region,"us");
        String gRegion=myRegion;
        return gRegion;
    }

public void changeLocale(String locale){
    try{
    String l= locale;
    if(l==("us")){
        this.isUs=true;
        Toast.makeText(this, "Us region P selected", Toast.LENGTH_SHORT).show();
        Intent intent = new Intent(Prefs.this, dog.class);
        startActivity(intent);
    }
    if(l.equals("uk")){
        this.isUk=true;
        //Toast.makeText(this, "UK region selected", Toast.LENGTH_SHORT).show();
        Intent intent = new Intent(Prefs.this, cat.class);
        startActivity(intent);
    }
            }catch (NullPointerException e){
        //what to do here?
        finish();
    }
}

@Override
protected void onStop() {
    super.onStop();
    SharedPreferences settings = getPreferences(0);
    SharedPreferences.Editor ed = settings.edit();
    ed.putString(region,region);
    ed.commit();

}

我在狗类中的子菜单代码如下所示:

My submenu code in the dog class looks like this:

public boolean onOptionsItemSelected(MenuItem item){
    Prefs pob = new Prefs();
    switch (item.getItemId()){
    //-------Options menu----------
    case R.id.about:
        //Toast.makeText(this, "About menu", Toast.LENGTH_SHORT).show();
        //showAbout();  
        return true;
    case R.id.locale:
        //Toast.makeText(this, "Locale menu", Toast.LENGTH_SHORT).show();
        return true;

    //-----Sub menu----------

                case R.id.uk_item:
        Toast.makeText(this, "UK selected", Toast.LENGTH_SHORT).show();
        pob.editRegion("uk");
        pob.changeLocale("uk");
        finish();
        return true;
    case R.id.us_item:
        Toast.makeText(this, "US already selected", Toast.LENGTH_SHORT).show();
        pob.changeLocale("us");
        //finish();
        return true;
    default :
        return super.onOptionsItemSelected(item);

        }

谢谢

推荐答案

getSharedPreferences() 只能在 onCreate() 已在 上被调用后调用活动.您的 Prefs 类奇怪地用于 onOptionsItemSelected().通常，您不会自己实例化活动.如果您的 Prefs 类不是实际的 Activity 而是用于访问首选项的辅助类，请将 Context 对象传递给您的方法，如下所示:

getSharedPreferences() can only be called after onCreate() has been called on an Activity. Your Prefs class is strangely used in onOptionsItemSelected(). In general, you don't instantiate activities yourself. If your Prefs class is not an actual Activity but a helper class for accessing preferences, pass a Context object to your methods, like this:

public void editRegion(Context ctx, String sregion) {
    SharedPreferences settings = ctx.getSharedPreferences(PREFS_NAME,0);
    SharedPreferences.Editor ed = settings.edit();
    ed.clear();
    ed.putString(region,sregion);
    ed.commit();
}

并通过以下方式调用它:

and call it by:

pob.editRegion(this, "uk");

来自您的 onOptionsItemSelected() 方法.

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