SharedPreferences Android中的NullPointerException

Question

我第一次使用sharedPreferences,我似乎无法通过此错误.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进行存储和调用.我已经多次转圈,所以有些代码会有点奇怪.我一直专注于从美国(默认)改为英国.

在DDMS中,我得到了这个:

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dog类中的子菜单代码如下所示:

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

Answer 1

getSharedPreferences()只能在调用之后onCreate()调用Activity.你的Prefs班级用得很奇怪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帮助程序类,Context请将对象传递给您的方法,如下所示:

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:

pob.editRegion(this, "uk");

从你的onOptionsItemSelected()方法.

Answer 2

我有同样的事情,直到我进入斯蒂芬的答案,我才能得到我的问题的准确答案.但是我想让下一个更清楚:

我试图通过使用没有扩展ActivityClass 的外部类来访问活动之外的共享首选项.这让我一样 java.lang.NullPointerException.

我的主要活动是创建一个需要Context访问共享首选项的对象:

   ...
   ...
   MyClass myClass = new MyClass (this);

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }

正如你所看到的那样,MyClass创建是在调用onCreate之前完成的,这导致上下文被"半熟"并抛出java.lang.NullPointerException.

为什么"半烤"？在调试问题时,我有一个断点

mSharedPreferences = 
        context.getSharedPreferences(PREF_FILE, context.MODE_PRIVATE);

并且在检查上下文参数时,我能够看到它的值(其中一些反正)给出了错误的信念,即参数已完全初始化.

将myClass的初始化移动到onCreate事件中解决了以下问题:

   ...
   ...
   MyClass myClass;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        myClass = new MyClass (this);
    }