Android studio 在片段中调用SharedReferences

我有一个名为SharePrefManager的类。这是密码 包com.example.lms02

public class SharedPrefManager {

private static final String SHARED_PREF_NAME = "volleyregisterlogin";
private static final String KEY_USERNAME = "keyusername";
private static final String KEY_EMAIL = "keyemail";
private static final String KEY_GENDER = "keygender";
private static final String KEY_ID = "keyid";
private static final String KEY_STDID = "keystdid";
private static final String KEY_USER_STATUS = "keyuserstatus";
private static SharedPrefManager mInstance;
private static Context ctx;

private SharedPrefManager(Context context) {
    ctx = context;
}
public static synchronized SharedPrefManager getInstance(Context context) {
    if (mInstance == null) {
        mInstance = new SharedPrefManager(context);
    }
    return mInstance;
}

//this method will store the user data in shared preferences
public void userLogin(User user) {
    SharedPreferences sharedPreferences = ctx.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);
    SharedPreferences.Editor editor = sharedPreferences.edit();
    editor.putInt(KEY_ID, user.getId());
    editor.putString(KEY_STDID,user.getStdid());
    editor.putString(KEY_USERNAME, user.getName());
    editor.putString(KEY_EMAIL, user.getEmail());
    editor.putString(KEY_GENDER, user.getGender());
    editor.putString(KEY_USER_STATUS, user.getStatus());
    editor.apply();
}

//this method will checker whether user is already logged in or not
public boolean isLoggedIn() {
    SharedPreferences sharedPreferences = ctx.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);
    return sharedPreferences.getString(KEY_USERNAME, null) != null;
}

//this method will give the logged in user
public User getUser() {
    SharedPreferences sharedPreferences = ctx.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);
    return new User(
            sharedPreferences.getInt(KEY_ID, -1),
            sharedPreferences.getString(KEY_STDID, null),
            sharedPreferences.getString(KEY_USERNAME, null),
            sharedPreferences.getString(KEY_EMAIL, null),
            sharedPreferences.getString(KEY_GENDER, null),
            sharedPreferences.getString(KEY_USER_STATUS, null)
    );
}

//this method will logout the user
public void logout() {
    SharedPreferences sharedPreferences = ctx.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);
    SharedPreferences.Editor editor = sharedPreferences.edit();
    editor.clear();
    editor.apply();
    ctx.startActivity(new Intent(ctx, LoginActivity.class));
}
}

我用的是碎片。这是我片段的代码

public class HomeFragment extends Fragment {

TextView txtvwuserid;
User user = SharedPrefManager.getInstance(getContext()).getUser();
public HomeFragment() {
    // Required empty public constructor
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_home, container, false);
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    txtvwuserid = getActivity().findViewById(R.id.txtvwstdid);
    txtvwuserid.setText("Hy");
}
}

问题发生在HomeFragment中 User User=SharedPrefManager.getInstance（getContext（））.getUser（）

显示的错误是： java.lang.NullPointerException:尝试调用虚拟方法“android.content.SharedReferences” 空对象引用上的android.content.Context.getSharedReferences（java.lang.String，int）“”

我的问题是如何访问SharedPrefManager的值以及错误的含义

问题已解决：

根据碎片的生命周期。我们必须在创建片段之后放置所有代码。我做错的是在创建片段之前声明用户对象。像

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);

    user = SharedPrefManager.getInstance(getActivity()).getUser(); //Placing here

}

这对我有用。干杯