Android-迭代列表时的空对象引用

Question

我试图将在内部类中创建的列表'usrs'复制到另一个列表'team_memebers'。复制后，我尝试在FOR循环中迭代“ team_memebers”，但出现“空对象引用”错误。“用户”列表包含通过调试打印进行测试的返回对象。

public class ListNodeActivity extends AppCompatActivity
{
    private ParseObject parse_task=null;
    private List<String> team_memebers=null;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list_node);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

    query.findInBackground(new FindCallback<ParseObject>() {
        public void done(List<ParseObject> usrs, ParseException e) {
            if (e == null) {

                team_memebers = new ArrayList<String>(usrs.size());
                for (ParseObject prso:usrs) {
                    team_memebers.add(new String(prso.getString("Username")));
                }
            } else {//handle the error
            }
        }
    });

    for (String str:team_memebers)
    {
        empolyeeSpinnerAdapter.add(str);
    }
}

堆栈跟踪

 FATAL EXCEPTION: main
 Process: il.ac.shenkar.david.todolistex2, PID: 14490
 java.lang.RuntimeException: Unable to start activity ComponentInfo{il.ac.shenkar.david.todolistex2/il.ac.shenkar.david.todolistex2.ListNodeActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
     at android.app.ActivityThread.-wrap11(ActivityThread.java)
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
     at android.os.Handler.dispatchMessage(Handler.java:102)
     at android.os.Looper.loop(Looper.java:148)
     at android.app.ActivityThread.main(ActivityThread.java:5417)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
  Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference
     at il.ac.shenkar.david.todolistex2.ListNodeActivity.onCreate(ListNodeActivity.java:116)
     at android.app.Activity.performCreate(Activity.java:6251)
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:148) 
     at android.app.ActivityThread.main(ActivityThread.java:5417) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

Answer 1

您的问题是您在复制列表后没有进行迭代。 query.findInBackground()是一个异步回调，因此不会立即执行。因为您的迭代循环位于该回调下方，但这并不意味着它将在回调执行后执行。只需将循环放入回调中，如下所示：

query.findInBackground(new FindCallback<ParseObject>() {
        public void done(List<ParseObject> usrs, ParseException e) {
            if (e == null) {

            team_memebers = new ArrayList<String>(usrs.size());
            for (ParseObject prso:usrs) {
                team_memebers.add(new String(prso.getString("Username")));
            }
            for (String str:team_memebers)
            {
                empolyeeSpinnerAdapter.add(str);
            }
        } else {//handle the error
        }
    }
});