java - getSharedPreferences 上的 Android SharedPreferences NullPointerException

Question

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

加载“设置” Activity 后，出现空指针异常。

来自用户设置的 fragment :

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

处理持久化的类:

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

您的Persistence 类不应是Activity。你应该使它成为一个普通类，并将其 onCreate 的代码放在这个普通类构造函数中。

把它改成这样:

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 Activity 中，您实例化此 Persistence 类，如下所示:

storage = new Persistence(this);