空对象上的共享首选项引用 Android

2022-09-03 06:35:47

我是机器人开发的新手,也许这是一个愚蠢的问题,但请帮助我。我在尝试保存int值时收到此错误。

Caused by: java.lang.NullPointerException: 尝试在 null 对象引用上调用虚拟方法 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)'

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);

对于写入

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

我只想设置一个共享首选项,并希望首先阅读它并将其写入活动。我该如何解决?

编辑

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首选项,但我无法在MainActivity扩展活动类中阅读它。

日志:

Caused by: java.lang.NullPointerException: 尝试在空对象引用上调用接口方法 'int android.content.SharedPreferences.getInt(java.lang.String, int)'


答案 1

这是一个示例SharedPreferences

要在“共享首选项”中存储:name

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

要从共享首选项获取:name

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

欲了解更多详情,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();
}

答案 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();
    }
}

如果未设置值,getCount() 将返回 -1。


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