Java 更改片段和使用回调时的NullPointerException

我在更改片段方面有一个问题，我使用操作栏中的微调器来完成此操作，有时还使用AlertDialog中的按钮。当我按下按钮时，我有空点异常。 这是我的接口回调代码：

public interface Callback {
    public void load(String name);
}

这是我的第一个片段的代码，我在OnCreateOptionMenu中使用微调器：

private Callback callback;

@Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        Log.d("TAG", "onAttach");
        try {
        callback = (Callback) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement interface");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        callback = null;
    }

public void onCreateOptionsMenu(Menu menu, MenuInflater inflate) {
        menu.clear();
        inflate.inflate(R.menu.kfu, menu);
        spinner = (Spinner) menu.findItem(R.id.action_spinner).getActionView()
                .findViewById(R.id.actionbar_spinner_change_map);
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity()
                .getBaseContext(),
                android.R.layout.simple_spinner_dropdown_item, actions);
        spinner.setAdapter(adapter);
        spinner.setOnItemSelectedListener(new OnItemSelectedListener() {

            @Override
            public void onItemSelected(AdapterView<?> parent, View view,
                    int position, long id) {
                Log.d("POSITION", "Position is = "+position);
                switch (position) {
                case 1: 
                    if (parent.getItemAtPosition(position).equals("VKNews")) {
                //      Log.d("Tag", callback.toString()+" "+ (callback == null) + " " + parent.getItemAtPosition(position));
                        callback.load("VKNews"); //305 line
                    }   
                }   
            }

            @Override
            public void onNothingSelected(AdapterView<?> parent) {
                // TODO Auto-generated method stub

            }
        });
    }

我的活动：

public class GeneralActivity extends Activity implements Callback {
...
    @Override
    public void load(String name) {
        FragmentTransaction transaction = getFragmentManager()
                .beginTransaction();
        Log.d("TAG", "transaction = " + transaction + " "
                + (transaction == null));
        if (name.equals("KFUNews")) {
            transaction.replace(R.id.general_fragment, kfu);
            transaction.commit();
            invalidateOptionsMenu();
        } else {
            transaction.replace(R.id.general_fragment, vkfrag);
            transaction.commit();
            invalidateOptionsMenu();
        }
    }
}

更新：它在Android 4.2.2和4.3、4.4上不起作用有三种可能：

在事件激发之前不会调用onAttach（），并且从不设置

回调

调用了onAttach（），但为

activity

在事件发生之前的某个时间点调用detach（）

---

该活动可能实现了错误的接口。回调是众多接口的名称。确保为接口使用唯一的名称或更改

GeneralActivity extends活动实现回调

到

GeneralActivity extends活动实现了ru.fosslabs.mobilestudent.fragment.KFUFragment.Callback
您的活动实现了回调接口吗？是的。公共类GeneralActivity扩展活动实现回调{
10-01 15:34:58.532: E/AndroidRuntime(2519): FATAL EXCEPTION: main
10-01 15:34:58.532: E/AndroidRuntime(2519): java.lang.NullPointerException
10-01 15:34:58.532: E/AndroidRuntime(2519):     at ru.fosslabs.mobilestudent.fragment.KFUFragment$3.onItemSelected(KFUFragment.java:305)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at android.widget.AdapterView.fireOnSelected(AdapterView.java:892)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at android.widget.AdapterView.access$200(AdapterView.java:49)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at android.widget.AdapterView$SelectionNotifier.run(AdapterView.java:860)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at android.os.Handler.handleCallback(Handler.java:730)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at android.os.Handler.dispatchMessage(Handler.java:92)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at android.os.Looper.loop(Looper.java:137)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at android.app.ActivityThread.main(ActivityThread.java:5103)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at java.lang.reflect.Method.invokeNative(Native Method)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at java.lang.reflect.Method.invoke(Method.java:525)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-01 15:34:58.532: E/AndroidRuntime(2519):     at dalvik.system.NativeStart.main(Native Method)

public class GeneralActivity extends Activity implements Callback {
...
    @Override
    public void load(String name) {
        FragmentTransaction transaction = getFragmentManager()
                .beginTransaction();
        Log.d("TAG", "transaction = " + transaction + " "
                + (transaction == null));
        if (name.equals("KFUNews")) {
            transaction.replace(R.id.general_fragment, kfu);
            transaction.commit();
            invalidateOptionsMenu();
        } else {
            transaction.replace(R.id.general_fragment, vkfrag);
            transaction.commit();
            invalidateOptionsMenu();
        }
    }
}