当我运行应用程序时，即使代码看起来“合法”并且没有语法错误，它也会崩溃。我已经建立了两个类：
在第一堂课中，我将只预览调用两个导致应用程序崩溃的其他类。
HelpFragment.java
private void updateTab(String tabId, int placeholder) {
FragmentManager fm = getFragmentManager();
if (fm.findFragmentByTag(tabId) == null) {
Bundle bundl = new Bundle() ;
bundl.putString("tabId", tabId);
MyListFragment list_fragment = new MyListFragment();
list_fragment.setArguments(bundl);
fm.beginTransaction()
.replace(placeholder, list_fragment, tabId)
.commit();
}
}
我们正在调用的类（以及有问题的类）：
MyListFragment.java
import java.util.ArrayList;
import java.util.List;
import android.annotation.SuppressLint;
import android.content.Context;
import android.os.Bundle;
import android.app.ListFragment;
import android.app.LoaderManager.LoaderCallbacks;
import android.content.AsyncTaskLoader;
import android.content.Loader;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;
public class MyListFragment extends ListFragment implements
LoaderCallbacks<Void> {
private static final String TAG = "FragmentTabs";
private String mTag;
private MyAdapter mAdapter;
private ArrayList<String> mItems;
private LayoutInflater mInflater;
private int mPosition;
private int mTotal;
private static final String[] About = { "Lorem", "ipsum", "dolor", "sit",
"amet", "consectetur", "adipiscing", "elit", "Fusce", "pharetra",
"luctus", "sodales" };
private static final String[] FAQ = { "I", "II", "III", "IV", "V",
"VI", "VII", "VIII", "IX", "X", "XI", "XII", "XIII", "XIV", "XV" };
private static final String[] Tips = { "hello" , "bitch" , "ass" , "partners" , "screw", "you" ,"all", "peace" , "out"};
private static final int SLEEP = 1000;
private final int wordBarColor = R.color.word_bar;
private final int numberBarColor = R.color.number_bar;
public MyListFragment() {
mTag = getArguments().getString("tabId");
mTotal = HelpFragment.TAB_About.equals(mTag) ? About.length : FAQ.length;
}
// public MyListFragment(String tag) {
// mTag = tag;
// mTotal = HelpFragment.TAB_About.equals(mTag) ? About.length
// : About.length;
//
// Log.d(TAG, "Constructor: tag=" + tag);
// }
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
// this is really important in order to save the state across screen
// configuration changes for example
setRetainInstance(true);
mInflater = LayoutInflater.from(getActivity());
// you only need to instantiate these the first time your fragment is
// created; then, the method above will do the rest
if (mAdapter == null) {
mItems = new ArrayList<String>();
mAdapter = new MyAdapter(getActivity(), mItems);
}
getListView().setAdapter(mAdapter);
// initiate the loader to do the background work
getLoaderManager().initLoader(0, null, this);
}
@Override
public Loader<Void> onCreateLoader(int id, Bundle args) {
AsyncTaskLoader<Void> loader = new AsyncTaskLoader<Void>(getActivity()) {
@Override
public Void loadInBackground() {
try {
// simulate some time consuming operation going on in the
// background
Thread.sleep(SLEEP);
} catch (InterruptedException e) {
}
return null;
}
};
// somehow the AsyncTaskLoader doesn't want to start its job without
// calling this method
loader.forceLoad();
return loader;
}
@Override
public void onLoadFinished(Loader<Void> loader, Void result) {
// add the new item and let the adapter know in order to refresh the
// views
mItems.add(HelpFragment.TAB_About.equals(mTag) ? About[mPosition]
: FAQ[mPosition]);
mAdapter.notifyDataSetChanged();
// advance in your list with one step
mPosition++;
if (mPosition < mTotal - 1) {
getLoaderManager().restartLoader(0, null, this);
Log.d(TAG, "onLoadFinished(): loading next...");
} else {
Log.d(TAG, "onLoadFinished(): done loading!");
}
}
@Override
public void onLoaderReset(Loader<Void> loader) {
}
private class MyAdapter extends ArrayAdapter<String> {
public MyAdapter(Context context, List<String> objects) {
super(context, R.layout.list_item, R.id.text, objects);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View view = convertView;
Wrapper wrapper;
if (view == null) {
view = mInflater.inflate(R.layout.list_item, null);
wrapper = new Wrapper(view);
view.setTag(wrapper);
} else {
wrapper = (Wrapper) view.getTag();
}
wrapper.getTextView().setText(getItem(position));
wrapper.getBar().setBackgroundColor(
mTag == HelpFragment.TAB_About ? getResources().getColor(
wordBarColor) : getResources().getColor(
numberBarColor));
return view;
}
}
// use an wrapper (or view holder) object to limit calling the
// findViewById() method, which parses the entire structure of your
// XML in search for the ID of your view
private class Wrapper {
private final View mRoot;
private TextView mText;
private View mBar;
public Wrapper(View root) {
mRoot = root;
}
public TextView getTextView() {
if (mText == null) {
mText = (TextView) mRoot.findViewById(R.id.text);
}
return mText;
}
public View getBar() {
if (mBar == null) {
mBar = mRoot.findViewById(R.id.bar);
}
return mBar;
}
}
}
日志：
04-06 22:21:25.079: E/AndroidRuntime(5958): FATAL EXCEPTION: main
04-06 22:21:25.079: E/AndroidRuntime(5958): java.lang.NullPointerException
04-06 22:21:25.079: E/AndroidRuntime(5958): at com.example.poca2.MyListFragment. <init>(MyListFragment.java:46)
04-06 22:21:25.079: E/AndroidRuntime(5958): at com.example.poca2.HelpFragment.updateTab(HelpFragment.java:61)
04-06 22:21:25.079: E/AndroidRuntime(5958): at com.example.poca2.HelpFragment.onActivityCreated(HelpFragment.java:51)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.app.Fragment.performActivityCreated(Fragment.java:1707)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:921)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1075)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.app.BackStackRecord.run(BackStackRecord.java:682)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1455)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.app.FragmentManagerImpl$1.run(FragmentManager.java:441)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.os.Handler.handleCallback(Handler.java:730)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.os.Handler.dispatchMessage(Handler.java:92)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.os.Looper.loop(Looper.java:137)
04-06 22:21:25.079: E/AndroidRuntime(5958): at android.app.ActivityThread.main(ActivityThread.java:5419)
04-06 22:21:25.079: E/AndroidRuntime(5958): at java.lang.reflect.Method.invokeNative(Native Method)
04-06 22:21:25.079: E/AndroidRuntime(5958): at java.lang.reflect.Method.invoke(Method.java:525)
04-06 22:21:25.079: E/AndroidRuntime(5958): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
04-06 22:21:25.079: E/AndroidRuntime(5958): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
04-06 22:21:25.079: E/AndroidRuntime(5958): at dalvik.system.NativeStart.main(Native Method)