onAttach 中无法初始化接口对象？

Question

我创建了一个接口 DataPasserInterface 来将数据从片段传递到它的宿主 activity。我在 onAttach 中初始化了界面，如下面的代码所示，问题是，在运行时，应用程序崩溃了，我收到下面发布的 logCat 错误。

不知道为什么this.dataPasserObj = (DataPasserInterface) activity;会报错？ logcat 说，接口对象不能转换？！

界面:

private DataPasserInterface dataPasserObj;
...
...
...
 public interface DataPasserInterface {

void onTopicsListChosen(ArrayList<String> chosenTopicsList);

}

onAttach:

@Override
public void onAttach(Activity activity) {
    // TODO Auto-generated method stub
    super.onAttach(activity);
    Log.w(TAG, "@onAttach().");

    this.dataPasserObj = (DataPasserInterface) activity;
    this.designList = new ArrayList<ItemDesign>();
}

logcat:

 E/AndroidRuntime(7828): FATAL EXCEPTION: main
 E/AndroidRuntime(7828): Process: com.example.mqtt_designlayout_02, PID: 7828
 E/AndroidRuntime(7828): java.lang.ClassCastException: com.example.mqtt_designlayout_02.SettingsActivity cannot be cast to com.example.mqtt_designlayout_02.DataPasserInterface
 E/AndroidRuntime(7828):    at com.example.mqtt_designlayout_02.Eco_Settings_Frag.onAttach(Eco_Settings_Frag.java:95)

更新

onCreate() 的 activity 托管片段，此 activity 实现接口

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.w(TAG, "@onCreate().");

    this.mqttSettingsDB = new MQTT_Settings_DB(this);
    this.sqliteDB = this.mqttSettingsDB.getWritableDatabase();

    if (this.fragList == null) {
        this.fragList = new ArrayList<Fragment>();

        this.fragList.add(new MQTT_Settings_Frag());
        this.fragList.add(new Eco_Settings_Frag());
        this.fragList.add(new Logger_Settings_Frag());
    }

    initviews(R.layout.settings_activity);
    setUpActionBar();
    this.viewPager.setOnPageChangeListener(viewPagerListener);
}

Answer 1

ClassCastException: com.example.mqtt_designlayout_02.SettingsActivity

可能没有在 SettingsActivity

中实现 DataPasserInterface 接口

建议在 Eco_Settings_Frag class 中创建构造函数以获取 DataPasserInterface 接口并初始化其他值：

public Eco_Settings_Frag(Activity activity){
    this.dataPasserObj = (DataPasserInterface) activity;
    this.designList = new ArrayList<ItemDesign>();
}

并在创建 Eco_Settings_Frag class 的对象时传递 SettingsActivity 上下文：

 .....
 this.fragList.add(new Eco_Settings_Frag(SettingsActivity.this));
 this.fragList.add(new Logger_Settings_Frag());
 .....

也在SettingsActivityActivity

中实现DataPasserInterface接口

onAttach 中无法初始化接口对象？

an interface object cannt be initialized in onAttach?

java

android

interface