java - 空对象引用 Android 上的共享首选项

Question

我是 android 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);

和写

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

我只是想设置一个SharedPreferences，想先读取它，然后写到Activity里面。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();
}

我设法用不同的方式保存首选项，但我无法在 MainActivity extends 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的一个例子

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");

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();
}