Java 空对象引用 Android 上的共享首选项

Shared Preferences on a Null Object Reference Android

提问by Simbilli Dev

I am new to android developing maybe its a silly question but please help me. I am getting this error while trying to save an int value.

我是 android 开发的新手，也许这是一个愚蠢的问题，但请帮助我。尝试保存 int 值时出现此错误。

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

引起：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)”

And here is my code

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);

And for write

而对于写

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

I just want to set a SharedPreferences and want to read it at the first and write it inside the Activity. How can I solve it?

我只想设置一个 SharedPreferences 并想首先读取它并将其写入活动中。我该如何解决？

EDIT

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();
}

I managed to save preferences with a different way but I couldn't manage to read it in MainActivity extends Activity class.

我设法以不同的方式保存首选项，但我无法在 MainActivity extends Activity 类中读取它。

log :

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

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

采纳答案by Anand Singh

This is an example of SharedPreferences

这是一个例子 SharedPreferences

To save namein SharedPreferences:

要保存name在 SharedPreferences 中：

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

To get namefrom SharedPreferences:

name从 SharedPreferences获取：

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

For more details visit: http://developer.android.com/training/basics/data-storage/shared-preferences.html

更多详情请访问：http: //developer.android.com/training/basics/data-storage/shared-preferences.html

Updated Code:

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();
}

回答by Ashwin S Ashok

You can implement it like this

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();
    }
}

getCount() will return -1 if no value is set.

如果没有设置值，getCount() 将返回 -1。

回答by Saman Arbabi

This question was for the "Simplify Networking with RetroFit" android course. I was getting this error because i didnt have my BaseApplication class in the Android Manifest...Everything is good now

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