java - 无法恢复 Activity - NullPointerException

Question

我知道这是一个很常见的问题，但我四处搜索并尝试解决以下问题但没有成功。

特别是，以下是我从 logcat 收到的消息。

08-03 17:55:53.885: E/AndroidRuntime(1286): FATAL EXCEPTION: main
08-03 17:55:53.885: E/AndroidRuntime(1286): Process: com.dooba.beta, PID: 1286
08-03 17:55:53.885: E/AndroidRuntime(1286): java.lang.RuntimeException: Unable to resume activity {com.dooba.beta/com.dooba.beta.MatchingActivity}: java.lang.NullPointerException
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2788)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2817)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2250)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.os.Handler.dispatchMessage(Handler.java:102)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.os.Looper.loop(Looper.java:136)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.ActivityThread.main(ActivityThread.java:5017)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at java.lang.reflect.Method.invokeNative(Native Method)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at java.lang.reflect.Method.invoke(Method.java:515)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at dalvik.system.NativeStart.main(Native Method)
08-03 17:55:53.885: E/AndroidRuntime(1286): Caused by: java.lang.NullPointerException
08-03 17:55:53.885: E/AndroidRuntime(1286):     at com.dooba.beta.MatchingActivity.onResume(MatchingActivity.java:82)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1192)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.Activity.performResume(Activity.java:5310)
08-03 17:55:53.885: E/AndroidRuntime(1286):     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2778)
08-03 17:55:53.885: E/AndroidRuntime(1286):     ... 12 more

下面是 Activity 的代码

public class MatchingActivity extends Activity {

    protected ParseRelation<ParseUser> mFriendsRelation;
    protected ParseUser mCurrentUser;   
    protected List<ParseUser> mUsers;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        setContentView(R.layout.matching);
        // Show the Up button in the action bar.

    }

    @Override
    protected void onResume() {
        super.onResume();

        mCurrentUser = ParseUser.getCurrentUser();

        setProgressBarIndeterminateVisibility(true);

        ParseQuery<ParseUser> query = ParseUser.getQuery();
        query.findInBackground(new FindCallback<ParseUser>() {
            public void done(List<ParseUser> users, ParseException e) {
                if (e == null) {
                    mUsers = users;
                } else {
                    // Something went wrong.
                }
            }
        });

        //This give you a list of  users. Then to access one user you need to know the index the user is  at.

        ParseUser user = mUsers.get(5); 

        //then you can access that user's information

        user.getString("name");
        user.getNumber("age"); 
        user.getString("headline");
    }
}

下面是xml的代码

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

</LinearLayout>

如有任何帮助，我们将不胜感激。提前致谢。

更新的代码:收到错误对于行

mUsers = new List<ParseUser>();

我收到以下消息:
“无法实例化类型 List ”

线条

 user.getString("name");
 user.getNumber("age"); 
 user.getString("headline");

我收到以下错误:
“无法解析用户”

提前致谢，下面是完整的更新代码。

public class MatchingActivity extends Activity {

    protected ParseRelation<ParseUser> mFriendsRelation;
    protected ParseUser mCurrentUser;   
    protected List<ParseUser> mUsers;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        setContentView(R.layout.matching);
        // Show the Up button in the action bar.
         //create list variable
        mUsers = new List<ParseUser>(); 

    }

    @Override
    protected void onResume() {
        super.onResume();

        mCurrentUser = ParseUser.getCurrentUser();

        setProgressBarIndeterminateVisibility(true);

        ParseQuery<ParseUser> query = ParseUser.getQuery();
        query.findInBackground(new FindCallback<ParseUser>() {
            public void done(List<ParseUser> users, ParseException e) {
                if (e == null) {

                    //add all the users to your list variable 
                    mUsers.addAll(users); 

                } else {
                    // Something went wrong.
                }
            }
        });

        //check the size of your list to see how big it is before accessing it
        final int size = mUsers.size(); 

        //or use a loop to loop through each one
        for(ParseUser mParseUser : mUsers) {
            //skip over the current user
            if(mParseUser == ParseUser.getCurrentUser())
                continue; 

            user.getString("name");
            user.getNumber("age"); 
            user.getString("headline");
        }

    }
}

Answer 1

我认为你的问题出在下面这行代码中

  ParseUser user = mUsers.get(5); 

mUsers.get(5) 可能为空，或者不存在。您的代码中没有任何内容可以保证列表的大小至少为 5。

试试这个

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
    setContentView(R.layout.matching);

    //create list variable
    mUsers = new List<ParseUser>(); 
}


@Override
protected void onResume() {

    ...

    ParseQuery<ParseUser> query = ParseUser.getQuery();
    query.findInBackground(new FindCallback<ParseUser>() 
    {

        public void done(List<ParseUser> users, ParseException e) 
        {
            if (e == null) 
            {
                //add all the users to your list variable 
                mUsers.addAll(users); 

                //check the size of your list to see how big it is before accessing it
                final int size = mUsers.size(); 

                //or use a loop to loop through each one
                for(ParseUser mParseUser : mUsers)
                {
                    //skip over the current user
                    if(mParseUser == ParseUser.getCurrentUser())
                           continue; 

                    //use the correct type when getting
                     final String mName = user.getString("name");
                     final int mNumber = user.getNumber("age"); 
                     final String mHeadLine = user.getString("headline");
                 }


            } 

           else 
           {
                // Something went wrong.
           }

        }

    });
}