android - 为什么我会在这一行收到 NullPointerException？

Question

我的代码以前运行良好，现在抛出 NPE。它也不在一条新线上，所以我很困惑。有人能指出我正确的方向吗？

问题线在大约一半的位置被识别出来。

public class FavouritesActivity extends Activity {

private String mFavourites;
ArrayAdapter <String> adapter;
List<String> list;

private ListView lv;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_favourites);

    lv = (ListView) findViewById(R.id.My_Favourites);

    SharedPreferences preferences = this.getSharedPreferences("MyPreferences", Context.MODE_PRIVATE);
    list = new ArrayList<String>();         
    Map<String, ?> prefsMap = preferences.getAll();
    for (Map.Entry<String, ?> entry: prefsMap.entrySet()) {
        list.add(entry.getValue().toString());   //THIS IS THE PROBLEM <<<<<<<<<<<<<<<<<<
    } 

    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, list); 

    lv.setAdapter(adapter);

     lv.setOnItemLongClickListener(new OnItemLongClickListener() {
            // setting onItemLongClickListener and passing the position to the function
                      @Override
            public boolean onItemLongClick(AdapterView<?> arg0, View arg1,
                    int position, long arg3) {
                String mFavourites =(lv.getItemAtPosition(position).toString());
                removeItemFromList(position);   

                return true;
            }
        });

}

     protected void removeItemFromList(int position) {


            final int deletePosition = position;

            AlertDialog.Builder alert = new AlertDialog.Builder(
                    FavouritesActivity.this);

            alert.setTitle("Delete");
            alert.setMessage("Do you want delete this item?");
            alert.setPositiveButton("YES", new DialogInterface.OnClickListener() {

                @Override
                public void onClick(DialogInterface dialog, int which) {
                    // TOD O Auto-generated method stub

                        // main code on after clicking yes
                        list.remove(deletePosition);

                        Operator();


                        adapter.notifyDataSetChanged();
                        adapter.notifyDataSetInvalidated();


                }
            });
            alert.setNegativeButton("CANCEL", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    // TODO Auto-generated method stub
                    dialog.dismiss();
                }
            });


            alert.show();


        }

 protected void Operator() {
     SharedPreferences preferences = this.getSharedPreferences("MyPreferences", Context.MODE_PRIVATE);

        SharedPreferences.Editor editor = preferences.edit();

        editor.remove(mFavourites);
        editor.commit();

    }

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.my_supplements, menu);
    return true;
}   
}

这是 LogCat:

01-10 14:08:59.850: E/AndroidRuntime(2194): FATAL EXCEPTION: main
01-10 14:08:59.850: E/AndroidRuntime(2194): Process: com.LifeSchematics.msg, PID: 2194
01-10 14:08:59.850: E/AndroidRuntime(2194): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.LifeSchematics.msg/com.LifeSchematics.msg.FavouritesActivity}: java.lang.NullPointerException
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.os.Handler.dispatchMessage(Handler.java:102)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.os.Looper.loop(Looper.java:136)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.app.ActivityThread.main(ActivityThread.java:5017)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at java.lang.reflect.Method.invokeNative(Native Method)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at java.lang.reflect.Method.invoke(Method.java:515)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at dalvik.system.NativeStart.main(Native Method)
01-10 14:08:59.850: E/AndroidRuntime(2194): Caused by: java.lang.NullPointerException
01-10 14:08:59.850: E/AndroidRuntime(2194):     at com.LifeSchematics.msg.FavouritesActivity.onCreate(MySupplementsActivity.java:40)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.app.Activity.performCreate(Activity.java:5231)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
01-10 14:08:59.850: E/AndroidRuntime(2194):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)

Answer 1

您会得到 NullPointerException，因为某些首选项值存储为 null 而 entry.getValue() 为 null。所以在调用 toString() 之前添加 null 检查

for (Map.Entry<String, ?> entry: prefsMap.entrySet()) {
    final Object value = entry.getValue();
    list.add(value == null ? "null" : value.toString());
} 

或者

for (Map.Entry<String, ?> entry: prefsMap.entrySet()) {
    final Object value = entry.getValue();
    if (value != null) {
        list.add(value.toString());
    }
}