在Android上实现java接口

Question

在我的Android应用程序上实现java界面有问题..。当事件被调用时，应用程序就会崩溃。具体来说，在在线"listener.callBack(String.valueOf(stringListValue.get(position)));“上，这是Logcat：

03-29 15:23:38.659  12568-12568/com.robertot.timereport E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.robertot.timereport, PID: 12568
    java.lang.NullPointerException
            at com.robertot.timereport.com.robertot.timereport.pages.MainActivity$2.onItemSelected(MainActivity.java:231)
            at android.widget.AdapterView.fireOnSelected(AdapterView.java:893)
            at android.widget.AdapterView.access$200(AdapterView.java:48)
            at android.widget.AdapterView$SelectionNotifier.run(AdapterView.java:861)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5081)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
            at dalvik.system.NativeStart.main(Native Method)

这是MainActivity：

    public class MainActivity extends FragmentActivity
        {
            //[...]

            private OnSelectItemSpinner listener;

            public interface OnSelectItemSpinner
            {
                public void callBack(String idjob);
            }

            @Override
            protected void onCreate(Bundle savedInstanceState) { //[...] 
}

        //[...]
        ArrayAdapter<String> spnAdapt = new ArrayAdapter<String> (this, android.R.layout.simple_spinner_item, stringListName);
                    spnAdapt.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
                    spnJobList.setAdapter(spnAdapt);
                    spnJobList.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener()
                    {
                        @Override
                        public void onItemSelected(AdapterView<?> arg0, View arg1,int position, long id)
                        {
                            switch (FragmentTag)
                            {
                                //Summary - Refresh ListView
                                case 0:
//CRASH HERE...
                                    listener.callBack(String.valueOf(stringListValue.get(position)));
                                    break;
                            }
                        }

                        @Override
                        public void onNothingSelected(AdapterView<?> arg0){}
                    });
    //[...]

这是我调用接口时的片段：

public class SummaryFragment extends Fragment implements MainActivity.OnSelectItemSpinner 
{

@Override
    public void callBack(String idjob)
    {
        //do work
    }
}

什么时候我错了？

谢谢！

Answer 1

如果要在活动和片段之间进行通信，必须注意以下提示:要允许片段与其 activity 通信，可以在片段类中定义接口，并在Activity中实现它。该片段在其onAttach()生命周期方法中捕获接口实现，然后可以调用接口方法以便与活动通信。

请参阅活性与片段间的通信

如果您想反向通信，可以执行以下代码：

public class MainFragment extends Fragment {

    public void callBack() {
        // Do SomeThing
    }
}


public class MainActivity extends Activity { 

    public void callFragmentCallBack() {
        MainFragment fragment = (MainFragment) getFragmentManager().findFragmentById(R.id.fragment_main);
        fragment.callBack(); // This line is important
    }
}

Answer 2

您得到NullPointerException是因为您没有在任何地方初始化listener。

OnSelectItemSpinner listener = new OnSelectItemSpinner()
{
    @Override
    public void callBack(String idjob)
    {
        // Do smth
    }
};