Question

我是Android的新手，可能是一个愚蠢的问题，但请帮助我。我在尝试保存int值时遇到此错误。

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference

这是我的代码

SharedPreferences prefs = this.getSharedPreferences("com.example.app", Context.MODE_PRIVATE);
String value = "com.example.app.value";
int i = prefs.getInt(value, 0);

写作

prefs.edit().putInt(number, i).apply();

我只想设置一个SharedPreferences，并希望在第一次读取它并将其写入Activity中。你能告诉我如何解决或提出如何制作它的其他建议吗？感谢

修改

public class MainActivity extends Activity {
SharedPreferences sharedpreferences;
public static final String MyPREFERENCES = "myprefs";
public static final  String value = "key";

int i = sharedpreferences.getInt(value, 0);

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    sharedpreferences = getSharedPreferences(MyPREFERENCES, Context.MODE_PRIVATE);
}
public void sendMessage(View view) {
    i += 1;    
    SharedPreferences.Editor editor = sharedpreferences.edit();
    editor.putInt(value, i);
    editor.apply();
}

我设法以不同的方式保存pereferences但我无法在MainActivity扩展Activity类中读取它。

日志：Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int android.content.SharedPreferences.getInt(java.lang.String, int)' on a null object reference

Answer 1

这是SharedPreferences

的示例

在SharedPreferences中保存name：

SharedPreferences sharedPreferences = PreferenceManager
                .getDefaultSharedPreferences(this);
        Editor editor = sharedPreferences.edit();
        editor.putString(key, name);
        editor.apply();

从SharedPreferences获取name：

 SharedPreferences sharedPreferences = PreferenceManager
                .getDefaultSharedPreferences(this);
        String name = sharedPreferences.getString(key, "default value");

有关详细信息，请访问：http://developer.android.com/training/basics/data-storage/shared-preferences.html

更新代码：

public class MainActivity extends Activity {
SharedPreferences sharedpreferences;
public static final String MyPREFERENCES = "myprefs";
public static final  String value = "key";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    sharedpreferences = getSharedPreferences(MyPREFERENCES, Context.MODE_PRIVATE);
    int i = sharedpreferences.getInt(value, 0);
    //use the value of i where needed.
}
public void saveMessage(View view) {
    i += 1;    
    SharedPreferences.Editor editor = sharedpreferences.edit();
    editor.putInt(value, i);
    editor.apply();
}

Answer 2

你可以像这样实现它

public class PreferenceClass {
    public static final String PREFERENCE_NAME = "PREFERENCE_DATA";
    private final SharedPreferences sharedpreferences;

    public PreferenceClass(Context context) {
         sharedpreferences = context.getSharedPreferences(PREFERENCE_NAME, Context.MODE_PRIVATE);
    }

    public int getCount() {
         int count = sharedpreferences.getInt("count", -1);
         return count;
    }

    public void setCount(int count) {
         SharedPreferences.Editor editor = sharedpreferences.edit();
         editor.putInt("count", count);
         editor.commit();
    }

    public void clearCount() {
         SharedPreferences.Editor editor = sharedpreferences.edit();
         editor.remove("count");
         editor.commit();
    }
}

如果没有设置值，

getCount（）将返回-1。

Answer 3

这个问题是针对“使用RetroFit简化网络”的android课程。我收到此错误是因为我在Android Manifest中没有我的BaseApplication类...现在一切都很好

Null对象参考Android上的共享首选项

3 个答案: