具有共享首选项的空对象引用

Question

我已经把头撞在键盘上几个小时了。我想在我的应用程序中为单独的统计信息活动存储一个递增的整数。我正在尝试使用共享首选项来实现此目的。然而，在我最近的一次尝试中，这是我得到的最远的一次尝试，该程序抛出了一个异常。

我的代码：

public class LootChest extends AppCompatActivity {
    public static final String prefName = "prefsFile";
    SharedPreferences settings = getApplicationContext().getSharedPreferences(prefName, 0); //line 25
    int rollCountS = settings.getInt("Roll Count", 0);
    int rollCount = 0; //to be incremented
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_loot_chest);
    }
    public void openChest(View v) {
        SharedPreferences.Editor editor = settings.edit();
        editor.putInt("Roll Count", rollCount);
        rollCount = rollCountS + 1;

例外情况：

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 at android.content.ContextWrapper
.getSharedPreferences(ContextWrapper.java:171)
at net.zingrook.mobiloot.LootChest.<init>(LootChest.java:25)

我已经阅读了数十个关于实现这一点的线程，并查看了Android文档，我没有想法。感谢您的任何帮助。

Answer 1

您正在初始化共享首选项对象，然后才能开始活动生命周期。

将初始化移至onCreate

public static final String prefName = "prefsFile";
    SharedPreferences settings;  //line 25
    int rollCountS;
    int rollCount = 0;
@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_loot_chest);
        settings = getApplicationContext().getSharedPreferences(prefName, 0); 
    }

Answer 2

只需以这种方式更改您的代码即可。

public class LootChest extends AppCompatActivity {
    public static final String prefName = "prefsFile";
    SharedPreferences settings;  //line 25
    int rollCountS;
    int rollCount = 0; //to be incremented
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_loot_chest);
        settings = getApplicationContext().getSharedPreferences(prefName, 0);
        rollCountS = settings.getInt("Roll Count", 0);
    }

Answer 3

我的观点

public class LootChest extends AppCompatActivity {
public static final String prefName = "prefsFile";
SharedPreferences settings;
SharedPreferences.Editor editor;
int rollCountS;
int rollCount = 0;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_loot_chest);
    settings=getApplicationContext().getSharedPreferences(prefName, 0);
    editor = settings.edit();
    rollCountS= settings.getInt("Roll Count", 0);
}
public void openChest(View v) {
    editor.putInt("Roll Count", rollCount);
    rollCount = rollCountS + 1;