在片段中使用SharedPreferences时获取NullPointerException

Question

我正在使用TabLayout，并且我有3个片段。当我尝试从SharedPreferences获取值时，第一个Fragment中出现NullPointerException。 Context对象有什么问题吗？请帮忙。感谢 logcat的：

java.lang.NullPointerException: Attempt to invoke virtual method 
'android.content.res.Resources android.content.Context.getResources()' on a 
null object reference 
/me.karolis.notsecretproject W/System.err: at android.widget.Toast.<init> 
(Toast.java:102) 
/me.karolis.notsecretproject W/System.err: at 
android.widget.Toast.makeText(Toast.java:260) 
/me.karolis.notsecretproject W/System.err:  at 
me.karolis.notsecretproject.fragments.MainFragment$1.onItemClick 
(MainFragment.java:63)me.karolis.notsecretproject.fragments. 
MainFragment$1.onItemClick(MainFragment.java:63) 

我的偏好帮助器类：

public class PreferencesHelper { 

private SharedPreferences sharedPreferences; 
private SharedPreferences.Editor editor; 

public PreferencesHelper(Context context) { 
    this.sharedPreferences = context.getApplicationContext().getSharedPreferences("myPrefs", Context.MODE_PRIVATE); 
    this.editor = sharedPreferences.edit(); 
} 

public boolean getIsDoingChallenge() { 
    return sharedPreferences.getBoolean("isDoingChallenge", false); 
} 

public void setIsDoingChallenge(boolean tof) { 
    editor.putBoolean("isDoingChallenge", tof).apply(); 
} 

}

第一个片段（其中的问题是）：

public class MainFragment extends android.support.v4.app.Fragment { 

@Nullable 
@Override 
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.fragment_main, container, false); 

    listView = (ListView) view.findViewById(R.id.fragment_main_list_view); 

    listView.setAdapter(adapter); 
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
     @Override 
     public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
      try { 
       DatabaseHandler db = new DatabaseHandler(getContext().getApplicationContext()); 
       PreferencesHelper preferenceHelper = new PreferencesHelper(getContext().getApplicationContext()); 
       boolean tof = preferenceHelper.getIsDoingChallenge(); 
       Toast.makeText(mainActivity, "" + tof, Toast.LENGTH_SHORT).show(); 
      } catch (Exception e) { 
       e.printStackTrace(); 
      } 
     } 
    }); 

    return view; 
} 

数据库（这是我创建了我的SharedPreferences）：

public class DatabaseHandler extends SQLiteOpenHelper { 

@Override 
public void onCreate(SQLiteDatabase database) { 
    SharedPreferences sharedPreferences = context.getApplicationContext().getSharedPreferences("myPrefs", Context.MODE_PRIVATE); 
    SharedPreferences.Editor editor = sharedPreferences.edit(); 
    editor.putBoolean("isDoingChallenge", false).apply(); 
} 

Answer 1

尝试改变：editor.putBoolean("isDoingChallenge", false).apply();

有：editor.putBoolean("isDoingChallenge", false).commit();

Answer 2

问题是与你的mainActivity变量 - 它为空。

logcat显示您正尝试创建Toast，因为上下文为空，因此无法使用getResources()。

如果您致电getActivity()而不是使用mainActivity - 它应该修复崩溃。

Answer 3

您正在通过构造函数getContext().getApplicationContext()到PreferencesHelper构造函数并再次调用context.getApplicationContext()。

尝试更改PreferencesHelper构造函数，如下所示。

public PreferencesHelper(Context context) { 
    this.sharedPreferences = context.getSharedPreferences("myPrefs", Context.MODE_PRIVATE); 
    this.editor = sharedPreferences.edit(); 
}