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Android Development for Beginners : Starter Gist for XML
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.example.android.practiceset2.MainActivity">
<TextView
android:id="@+id/display_text_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text=""
android:textSize="45sp" />
<TextView
android:id="@+id/display_text_view_2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text=""
android:textSize="45sp" />
<TextView
android:id="@+id/display_text_view_3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text=""
android:textSize="45sp" />
</LinearLayout>
@dominic-berlin

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commented Aug 9, 2016

Line 10 didn't work for me:
tools:context=".MainActivity">

I changed it the following:
tools:context="com.example.android.practiceset2.MainActivity">

@kaito007

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commented Aug 17, 2016

It worked for me.

@DGF86

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commented Aug 30, 2016

Funcionou para mim.

@manmeet-22

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commented Sep 21, 2016

In this Line 10 does not contain the package name-
`

tools:context=".MainActivity">`

Instead add this-
tools:context="com.androidexample.practiceset2.MainActivity">

@TulioIsaak

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commented Oct 15, 2016

Line 10 didn't work for me:
tools:context=".MainActivity">

I changed line 3 from AndroidManifest.xml file:
package="app.com.example.android.practiceset2">
to:
package="com.example.android.practiceset2">

than worked.

@ajayanku

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commented Nov 3, 2016

screenshot 43
its not working pls tell me?

@itechmode

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commented Dec 26, 2016

Get the same problem here.
I changed the line10. It was originally:
tools:context=".MainActivity"

I changed For:
I changed it the following:
tools:context="com.example.android.practiceset2.MainActivity"

The problem after that it was MainActivity get red. Any clues aboute this error?

@rdh47

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commented Jan 9, 2017

I had a similar issue, but it was because I did not create the project with the recommended domain. I corrected the domain at beginning of the main activity.java to the one I used, and it corrected the error.

@AnestisZisidis

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commented Jan 19, 2017

If it still doesn't work ,after the changes on tools:context= .
Just Rebuild your project (Build-->Clean Project,Build-->Rebuild Project).

@AbhishekCCT

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commented Jan 20, 2017

w.r.t. rdh47, there must be a spelling difference(of your project name) between the one in java code (first line) and the one in xml code (10th line). This was the case with me.

@Walizai

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commented Jan 30, 2017

Hello dear all,

I have the same problem with line number 10, it show .MyActivity with red color, I tried the solution above now show the praticeset2 with red color but it can run without any problem in Andriod Studio but in mobile says you application failed to start.

Any help!!!

@poltudora

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commented Feb 1, 2017

Same problems over here ? Have you figured out what is the issue ?

@gielmuda

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commented Feb 4, 2017

Do you gyus have the project name as Practice Set 2, or Practise Set 2?
The thing is it should be practice with two "C"s, not with a "c" and then "s".

@Prakhar32

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commented Feb 6, 2017

I am encountering following errors
Error:(8, 28) No resource found that matches the given name (at 'paddingBottom' with value '@dimen/activity_vertical_margin').
Error:(9, 26) No resource found that matches the given name (at 'paddingLeft' with value '@dimen/activity_horizontal_margin').
Error:(10, 27) No resource found that matches the given name (at 'paddingRight' with value '@dimen/activity_horizontal_margin').
Error:(11, 25) No resource found that matches the given name (at 'paddingTop' with value '@dimen/activity_vertical_margin').

What do I do?

@Fedor98

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commented Feb 10, 2017

@gielmude: It doesn't matter, it's just the name of a project you'll probably never publish to the public. So never mind if you misspelled.

@Prakhar32: in the Android view of the project, go to app -> res -> values. Right click on values, then "New" (right at the top of the pop-up menu), "Value resource file". In the "file name" space, type "dimens.xml" (without the quotation marks " ") and then simply "Ok".
Click on the newly created "dimens.xml" file. Remove all code in it and paste the following:
<?xml version="1.0" encoding="utf-8"?> <resources> <dimen name="activity_vertical_margin">8dp</dimen> <dimen name="activity_horizontal_margin">8dp</dimen> </resources>

This should work! :)

@Prakhar32

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commented Feb 11, 2017

Thanks Fedor98.
It worked like a charm.

@jcunanan05

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commented Mar 4, 2017

@Fedor98 thanks

@RidowanAhmed

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commented Mar 4, 2017

Why getting those red lines
image

@byronwarwick

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commented Mar 6, 2017

Thanks for that solution @Fedor98

@sumitPhD

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commented Mar 10, 2017

"package" in MainActivity.java and "tools:context" in activity_main.xml should be same and proper.
Just like below
git1
git2

@O-leChuck

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commented Mar 16, 2017

@Fedor98: Thanks a lot! Worked for me, too. Could maybe someone give me a short explanation of what this does or why the problem occured? I'd like to understand what was wrong :)

@lynden73

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commented Mar 18, 2017

@Fedor98: Thanks, that worked perfectly.

@DoulatRana

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commented Mar 18, 2017

@RidowanAhmed

Following are the instructions of @Fedor98. This instructions resolved the problem that you're facing.

Go to Android view of the project (look to left side of the android studio), go to app -> res -> values. Right click on values, then "New" (right at the top of the pop-up menu), "Value resource file". In the "file name" space, type "dimens.xml" (without the quotation marks " ") and then simply "Ok".
Click on the newly created "dimens.xml" file. Remove all code in it and paste the following:

8dp 8dp

This should work! :)

@Mergen23

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commented Mar 23, 2017

important !!!!!

this 3 line must be same :

  • copy the package name in AndroidManifest.xml
    *paste in line 12 of activity_main.xml (example: tools:context="com.example.administrator.practiceset2.MainActivity")
    *paste in first line of MainActivity.java (example package com.example.administrator.practiceset2; )

it will solve the red MainActivity.

@kickupthefire

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commented Apr 8, 2017

@Fedor98 thanks for helping to resolve dimen, you're di man!

@neliamx4

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commented Apr 9, 2017

Thank you all guys! I would have big problems without your advice.

@nagyklaudiu

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commented Apr 11, 2017

after creating the dimens.xml file everything will be ok. Thank you Fedor98

@Wierdgeezer

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commented Apr 23, 2017

@Fedor98 - Thanks for the dimens.xml file tip. That saved me a lot of tears!

@O-leChuck - I could be wrong but it looks like the @Dimen elements are custom ones, not part of android studio's standard definitions. What @Fedor98's bit of code did was to create two variables named activity_vertical_margin and activity_horizontal_margin in a new resource called @Dimen and assign them both the value 8dp.

@watfordjc

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commented Apr 30, 2017

@Wierdgeezer - According to a Stack Overflow answer it is because we are starting from a ConstraintLayout Empty activity. Before Android Studio 2.3 the default activity created dimens.xml.

Up to this point I've just been deleting those lines, or replacing android.support.constraint.ConstraintLayout with LinearLayout/RelativeLayout, so thanks @Fedor98.

@inaborges

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commented May 11, 2017

Thanks @Fedor98 ! :)

@rohangho

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commented May 12, 2017

I think the line number 10 is a bit faulty.

@hoangtubevietnam

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commented May 14, 2017

thank for your advice @Fedor98

@tbanj

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commented May 14, 2017

@Fedor98

thanks for the comment on dimen.xml file,
after inserting the required xml code the error syntax which it was indicated in my activity_main.xml at the line 6 to 9 vanished.. it worked

@harneeblayze

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commented May 19, 2017

@Fedor98 you are saving lives... thanks a bunch!!!!!

@mystery1999

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commented May 20, 2017

@Fedor98...I had an error-no resource found at object @dimen/activity_vertical_margin
I've been able to resolve it but can I get an explanation on whyit occurred?

@Iszybee

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commented Jun 24, 2017

Thanks @Fedor98

@Vyshakh-Babu

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commented Jun 25, 2017

@Fedor98 Thanks bro

@Amn-Akash

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commented Jun 30, 2017

Thanks @Fedor90

@strategist-bal

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commented Jul 7, 2017

Thanks @Fedor98

@kenny57

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commented Jul 9, 2017

Thanks @Fedor98

@ershadak

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commented Jul 10, 2017

line 6 to 10 cannot resolve the values starting with @dimen/activity

@sandeepvenkata

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commented Jul 16, 2017

i'm geeting errors
just

@crisfrench

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commented Jul 23, 2017

Since the project doesn't have a dimen.xml file I commented out all the code [@dimen/...] and added [android:padding="8dp"]

As for tools:context , I used [tools:context=".MainActivity">]

More info on using dimension - https://developer.android.com/guide/topics/resources/more-resources.html#Dimension

@Guille55

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commented Aug 22, 2017

I had the same "@Dimen....." errors
Thanks to @Fedor98 for the solution!

@MokshMahajan

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commented Sep 24, 2017

Help me resolving this error
help

@ELHafizhHidayat

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commented Oct 2, 2017

thanks a lot @Fedor98, really helpfull.
@MokshMahajan just scroll up, and see @Fedor98 post

@ishaanwalter

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commented Oct 15, 2017

the package is missing, try adding the following code to the top of your Mainactivity.java
package com.example.android.justjava;
hope this would help you.

@FranzoJosefo

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commented Oct 16, 2017

screen shot 2017-10-15 at 11 26 55 pm

screen shot 2017-10-15 at 11 27 10 pm

Hi guys, any thoughts on why line 23 is failing on my main activity? for some reason it cannot resolve display_text_view but as you can see in the activity_main.xml file that id exists. Any thoughts?

Thanks in advance.

@kkchain

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commented Oct 28, 2017

Thanks @Fedor98, for sharing how to resolve the @Dimen problem at the xml

@rosberta

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commented Nov 9, 2017

Thank you @Fedor98!

@lixrg

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commented Nov 14, 2017

I got the @Dimen problem as well so I followed the instructions of @Fedor98 but now I'm still stuck. What did you guys write into the new dimens.xml to resolve the problem?? Would highly appreciate some help!

@cyberdeity

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commented Nov 16, 2017

@Fedor98 Thanks a lot, problem solved :)

@tgejason

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commented Nov 21, 2017

I got the @Dimen problem as well so I followed the instructions of @Fedor98 but now I'm still stuck. What did you guys write into the new dimens.xml to resolve the problem?? Would highly appreciate some help!

@lixrg

this is what I found and what should be in 'dimens.xml'
< ?xml version="1.0" encoding="utf-8"? > <resources> <dimen name="activity_vertical_margin">8dp</dimen> <dimen name="activity_horizontal_margin">8dp</dimen> </resources>

@seni1

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commented Dec 2, 2017

@tgejason this is what I used thanx to @Fedor98 and it worked for me:

<?xml version="1.0" encoding="utf-8"?> <resources> <dimen name="activity_vertical_margin">8dp</dimen> <dimen name="activity_horizontal_margin">8dp</dimen> </resources>

... look out for spaces and other tiny misspellings... extra dot in the wrong place, messes the whole code... it's that kind of business m8!

@udacityandroid

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Owner Author

commented Dec 4, 2017

Thank you for all the feedback! And big shoutout to all the students taking the time to help others out.

Updates in activity_main.xml:

(1) In the LinearLayout, the tools:context has been updated to
tools:context="com.example.android.practiceset2.MainActivity"
which includes the full package name of the activity the layout is associated with

(2) In the LinearLayout, the following padding dimensions have been removed. Including them would require adding a separate dimens resource file (that was not initially included). For the purpose of this exercise, this was extraneous.

    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
@kr-vaz

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commented Jan 12, 2018

@FranzoJosefo perform a gradle sync that should solve the problem.

@jofego12

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commented Jan 13, 2018

Excuese me @udacityandroid I still having problems with "tools" and they're just solved if a put "MainActivity". If not, all remains like in this image.
practiceset2 xml

Also, I still having problems on MainActivity.java package it's called package com.example.android.practiceset2; and it's some problem with it statements (I'm attaching a second image for this issue).
practiceset2 java

Could you help me please? I tried all that your commented on this forum, unsueccesfuly.

@drszabodavid

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commented Jan 14, 2018

It works for me, thanks!

@Maxwe1

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commented Jan 31, 2018

All works for me, thanks a lot.

@Nogaeman

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commented Feb 2, 2018

Thanks
untitled

@ohi4fi

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commented Feb 3, 2018

works fine.

@tomnta

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commented Feb 5, 2018

If anyone is getting an error in your tools:context="com.example.tom.practiceset2.MainActivity where part or all of the text is showing up in red, please try this:

  1. In your Project Explorer (left "explorer-like" pane of Android Studio), navigate to: app > manifests > AndroidManifest.xml and double click that file to edit it:
    as3
  2. On line 3, copy the value of the package attribute: package="com.example.tom.practiceset2"
  3. On line 7 in your activity_main.xml, paste that value into: tools:context="PASTE VALUE HERE"
  4. On line 1 in your MainActivity.java, paste that value again into: package PASTE VALUE HERE
  5. On the menu, click BUILD > CLEAN PROJECT
  6. On the menu, click BUILD > REBUILD PROJECT
  7. Run the app

The reason for this error is because the company domain was left at its default value, which was likely different than the values in these gists. All we did in these steps was to make sure all the values for the company domain are in sync. The screen for this practice exercise should look like this:
app
Hope this makes sense and it helps!

@jubilee2

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commented Feb 8, 2018

Working well
image

@abdahma01

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commented Feb 13, 2018

Done

@jorge262

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commented Feb 17, 2018

@tomnta thank you1

@badrddinb

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commented Feb 21, 2018

DONE
image

@optmsshiv

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commented Apr 10, 2018

When I use:- tools:context="com.example.android.MainActivity" instead of tools:context="com.example.android.practiceset2.MainActivity" , my code run
practise
without any error.

@arhnita

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commented May 30, 2018

tools:context="com.example.android.practiceset2.MainActivity">
The error message i'm getting is "Typo in word "practiceset"
What do i do please

@lanrelinux

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commented Jun 3, 2018

for some weird reason the android studio misspelt the android.example.com as "andriod" recreate you project and correct it then it will work. I am using a Mac. I hope this helps someone.

@emanhamad

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commented Oct 1, 2018

it works well

@shankar-shiv

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commented Dec 23, 2018

it works fine!!!

@Saitama-Sensie

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commented Feb 11, 2019

for those having problems with package name, on the updated android studio the word android is left out of package name. try deleting android from package name

@Aadhitya01

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commented Jun 22, 2019

Worked for me.

@estudioporta19

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commented Aug 5, 2019

I close and open other projects. When I openned again ".MainActivity" wasn't red in "activity_main.xml" nor "AndroidManifest.xml". Although some other terms were in red in the MainActivity.java

Substituted:

import android.support.v7.app.AppCompatActivity;

for

import androidx.appcompat.app.AppCompatActivity;

and it runned in the phone.

Before the only thing that was in red was ".MainActivity", there were no errors when I run, but the app crashed on the phone.

@DutchSammmie

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commented Aug 18, 2019

Do you gyus have the project name as Practice Set 2, or Practise Set 2?
The thing is it should be practice with two "C"s, not with a "c" and then "s".

this must be it! Thanks...

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