NullPointerException异常与片段接口监听器 [英] NullPointerException with Fragment Interface Listener
问题描述
我很抱歉，我敢肯定，这将是简单的，但我不能看到我要去哪里错在这里。
I'm sorry, I'm sure that this will be simple but I just cannot see where I'm going wrong here.
我有一个片段（刀具列表），可以在两个窗格中设置了启动一些其他的片段（工具）的。有上述＆放大器坐在活动;我使用它来管理删除工具片段时，他们的关闭按钮是pssed $ P $。然后，从弹出的backstack一个工具，如果存在的话。
I've got a Fragment (a tool list) that can start a number of other fragments (tools) in a two pane set up. There is an activity that sits above & I am using it to manage the removal of tool fragments when their 'close button' is pressed. It then pops a tool from the backstack if one exists.
但奇怪的是，我有这个工作一个片段之内，但是剪切和放大器;粘贴code到另一个片段不起作用＆安培;抛出NPE。任何想法，为什么很多AP preciated。
The strange thing is, that I have this working within one fragment but the 'cut & pasted' code into another fragment doesn't work & throws a NPE. Any ideas why much appreciated.
该活动片段：
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;
public class MyActivity extends FragmentActivity implements OnFragmentButtonClickedListener {
private final static int CLOSE_BUTTON = 0;
...
public void onFragmentButtonClicked(int button, Fragment fragId) {
Log.i(TAG, "onFragmentButton");
// process closing of fragment
if (button == CLOSE_BUTTON) {
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.remove(fragId);
ft.commit();
}
// then restore most recent fragment from backstack
if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
getSupportFragmentManager().popBackStack();
}
}
}
这个片段如下：
import android.support.v4.app.Fragment;
public class FirstFragment extends Fragment implements OnClickListener{
OnFragmentButtonClickedListener fragmentButtonListener;
private final static int CLOSE_BUTTON = 0;
// empty constructor
public FirstFragment() {};
// Activity must implement this interface
public interface OnFragmentButtonClickedListener {
public void onFragmentButtonClicked (int button, Fragment fragId);
}
@Override
public void onAttach(Activity activity) {
Log.i(TAG, "onAttach");
super.onAttach(activity);
try {
fragmentButtonListener = (OnFragmentButtonClickedListener) activity;
} catch (ClassCastException e) {
Log.w(TAG, (activity.toString() + " must implement OnFragmentButtonClickedListener"));
}
}
...
public void onClick(View v) {
Log.i(TAG, "onClick");
switch (v.getId()) {
case R.id.btn_st_close:
//close activity
eraseData(prefs);
Log.i(TAG, "Frag Id = " + String.valueOf(getFragmentManager().findFragmentById(getId())));
fragmentButtonListener.onFragmentButtonClicked(CLOSE_BUTTON, getFragmentManager().findFragmentById(getId()));
break;
}
}
不过，这一次在onclick方法呼吁fragmentButtonListener的onFragmentButtonClicked（）方法时抛出NPE。
But, this one throws a NPE in the onClick method when calling the onFragmentButtonClicked() method on the fragmentButtonListener.
import android.support.v4.app.Fragment;
public class SecondFragment extends Fragment implements OnClickListener {
OnFragmentButtonClickedListener fragmentButtonListener;
private final static int CLOSE_BUTTON = 0;
// empty constructor
public SecondFragment() {};
// Activity must implement this interface
public interface OnFragmentButtonClickedListener {
public void onFragmentButtonClicked (int button, Fragment fragId);
}
@Override
public void onAttach(Activity activity) {
Log.i(TAG, "onAttach");
super.onAttach(activity);
try {
fragmentButtonListener = (OnFragmentButtonClickedListener) activity;
} catch (ClassCastException e) {
Log.w(TAG, (activity.toString() + " must implement OnFragmentButtonClickedListener"));
}
}
...
public void onClick(View v) {
Log.i(TAG, "OnClick");
switch (v.getId()) {
case R.id.btn_wc_close:
//close activity
eraseData(prefs);
Log.i(TAG, "Frag Id = " + String.valueOf(getFragmentManager().findFragmentById(getId())));
try {
fragmentButtonListener.onFragmentButtonClicked(CLOSE_BUTTON, getFragmentManager().findFragmentById(getId()));
}
catch (Exception e) {
Log.i(TAG, "Exception: " + e);
}
break;
}
}
从logcat中，我可以看到片段编号不为空，所以我很困惑。
From logcat, I can see that the fragment Id is not null so I'm very confused.
08-20 09:13:49.096: I/Second Fragment(1691): onCreate
08-20 09:13:49.096: I/Second Fragment(1691): Frag Id = SecondFragment{41355d68 #1 id=0x7f090093}
08-20 09:13:49.106: I/Second Fragment(1691): onCreateView
08-20 09:13:49.406: I/Second Fragment(1691): onActivityCreated
08-20 09:13:49.560: I/Second Fragment(1691): onClick
0 09:13:51.766: I/Second Fragment(1691): Frag Id = SecondFragment{41355d68 #1 id=0x7f090093}
08-20 09:13:51.766: W/dalvikvm(1691): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
08-20 09:13:51.826: E/AndroidRuntime(1691): FATAL EXCEPTION: main
08-20 09:13:51.826: E/AndroidRuntime(1691): java.lang.NullPointerException
08-20 09:13:51.826: E/AndroidRuntime(1691): at SecondFragment.onClick(IceTurbFragment.java:136)
08-20 09:13:51.826: E/AndroidRuntime(1691): at android.view.View.performClick(View.java:4084)
08-20 09:13:51.826: E/AndroidRuntime(1691): at android.view.View$PerformClick.run(View.java:16966)
08-20 09:13:51.826: E/AndroidRuntime(1691): at android.os.Handler.handleCallback(Handler.java:615)
08-20 09:13:51.826: E/AndroidRuntime(1691): at android.os.Handler.dispatchMessage(Handler.java:92)
08-20 09:13:51.826: E/AndroidRuntime(1691): at android.os.Looper.loop(Looper.java:137)
08-20 09:13:51.826: E/AndroidRuntime(1691): at android.app.ActivityThread.main(ActivityThread.java:4745)
08-20 09:13:51.826: E/AndroidRuntime(1691): at java.lang.reflect.Method.invokeNative(Native Method)
08-20 09:13:51.826: E/AndroidRuntime(1691): at java.lang.reflect.Method.invoke(Method.java:511)
08-20 09:13:51.826: E/AndroidRuntime(1691): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
08-20 09:13:51.826: E/AndroidRuntime(1691): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-20 09:13:51.826: E/AndroidRuntime(1691): at dalvik.system.NativeStart.main(Native Method)
感谢您的帮助。
推荐答案
我可以看到你定义2接口与在每个片段中相同的名称。
I can see you define 2 interface with the same name in each fragment.
public interface OnFragmentButtonClickedListener {
public void onFragmentButtonClicked (int button, Fragment fragId);
}
但活动可以实现这个名字只有一个接口（在这种情况下，我相信你的活动实施第一个片段的接口），也许第二个片段的这项事业的错误空指针异常。
But the activity can implement only one interface with this name (in this case, I believe your activity implement interface of first fragment) and maybe this cause error null pointer exception of second fragment.
//where is OnFragmentButtonClickedListener come from?
public class MyActivity extends FragmentActivity implements FirstFragment OnFragmentButtonClickedListener
试着写OnFragmentButtonClickedListener 2片段（也许在另一个文件）
Try to write OnFragmentButtonClickedListener outside of 2 fragments ( maybe in another file)
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