Android

Question

我正在尝试将在内部 class 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 'null object reference' 错误。 'users' 列表包含返回的对象，通过调试打印测试。

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

你的问题是你在复制列表后没有迭代。 query.findInBackground() 是一个异步回调，因此不会立即执行。因为您的迭代循环在该回调下方 PLACED ，并不意味着它会在回调执行后执行。只需像这样将循环放入回调中：

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
        }
    }
});

Android - 迭代 List 时的空对象引用

Android - null object reference when iterating List

java

nullreferenceexception