java–Android–迭代List时的null对象引用

我正在尝试将在Inner类中创建的List’usrs’复制到不同的列表’team_memebers’.
复制后,我尝试在FOR循环中迭代’team_memebers’,但是我得到’null object reference’错误.
‘users’列表包含返回的对象,通过调试打印进行测试.

public class ListNodeActivity extends AppCompatActivity
{
private ParseObject parse_task=null;
private List 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

() {
public void done(List

usrs, ParseException e) {
if (e == null) {
team_memebers = new ArrayList(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) 


解决方法:

您的问题是您在复制列表后没有迭代.
query.findInBackground()是异步回调,因此不会立即执行.因为你的迭代循环在回调下面是PLACED,并不意味着它将在回调执行后执行.只需将你的循环置于回调中,如下所示：

query.findInBackground(new FindCallback

() {
public void done(List

usrs, ParseException e) {
if (e == null) {
team_memebers = new ArrayList(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
}
}
});