GetSharedReferences上的java Android SharedReferences NullPointerException

Question

我正在尝试创建一个类，它将处理存储和检索两个用户设置“radius”和“cluster”

加载“设置”活动时，我得到一个空指针异常

来自用户设置的代码片段：

    storage = new Persistence();
    radius = (EditText) findViewById(R.id.etRadius);        
    radius.setText(String.valueOf(storage.getRadius()));  <-- Problem

处理持久性的类：

public class Persistence extends Activity { 

    private static final String PREFERENCES = "tourist_guide_preferences";
    private SharedPreferences settings;
    private SharedPreferences.Editor editor;

    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        settings = getSharedPreferences(PREFERENCES, 0);
        editor = settings.edit();
    }

    public int getRadius()
    {
        return settings.getInt("radius", 2000);
    }

    public int getClusterSize()
    {
        return settings.getInt("cluster", 50);
    }

    public void setRadius(int radius)
    {
        editor.putInt("radius", radius);
        editor.commit();
    }

    public void setClusterSize(int size)
    {
        editor.putInt("cluster", size);
        editor.commit();        
    }   
}

Answer 1

存储=新的持久性（）；这不会调用持久性活动的onCreate。 最好创建一个通用类。您可以创建一个上下文变量，并使用它来创建SharedReference实例。您应该从活动类调用此常规类

Answer 2

你的Persistence类不应该是Activity。您应该使它成为一个普通类，并将其onCreate的代码放入这个普通类构造函数中

将其更改为如下所示：

public class Persistence { 

    private static final String PREFERENCES = "tourist_guide_preferences";
    private SharedPreferences settings;
    private SharedPreferences.Editor editor;
    private Context context;


    public Persistence(Context context)
    {
        this.context = context;
        settings = context.getSharedPreferences(PREFERENCES, 0);
        editor = settings.edit();
    }

    public int getRadius()
    {
        return settings.getInt("radius", 2000);
    }

    public int getClusterSize()
    {
        return settings.getInt("cluster", 50);
    }

    public void setRadius(int radius)
    {
        editor.putInt("radius", radius);
        editor.commit();
    }

    public void setClusterSize(int size)
    {
        editor.putInt("cluster", size);
        editor.commit();        
    }   
}

在您的活动中，您可以这样实例化这个Persistence类：

storage = new Persistence(this);