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Get Ids Dynamically using findViewById() in Android using Java

In my XML layout I have some TextView with ids like slot0, slot1...slot15.
Is there any way to generate the corresponding Id dynamically in java like the following?
findViewById(R.id.*customStringForId*)
then access each of TextView using a for loop?
I am currently unable to use findViewById(R.id.*customStringForId*) because I can't find it in the XML.

Thats a bad practice for access component from your xml
You need set manual for id with findViewById for tell java class if in your xml there existing textview with id which already you set and give you access for do whatever like implement onclick event, settext, etc.
If you cant find your id, you need check if setContentView in your java point to your xml.

there are some ways to solve your problem but you should write your layout in the question to let us know how you design the layout.
But for example if you have a list of TextViews inside layout like the following:
<LinearLayout
android:id="#+id/layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="#+id/slot0"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="example" />
<TextView
android:id="#+id/slot1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="example" />
<TextView
android:id="#+id/slot2"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="example" />
</LinearLayout>
you can access the TextView Dynamically via the layout like the following:
public TextView getTextView(int index){
return ((LinearLayout) findViewById(R.id.layout)).getChildAt(index)
}

How to fix this > attribute app:constraintBottom_toBottomOf not found

Android newbie here.
I am building and AR app and now I'm planning to add a User Interface. I am trying to add a constraint layout to app:layout_constraintBottom_toBottomOf="parent", however Im getting the error that this constraint was not found
These are my codes and error messages:
All the lines of code I add in my activity_ux.xml file that begin with "app"
e.g. app:layout_constraintBottom_toTopOf do not seem to work, I just showed you an example here with one of them just to make it easier to debug I think.
I tried searching online, one of the solutions was that I should install "ConstraintLayout for Android" and "Solver for ConstraintLayout" in my SDK Tools, but I already had this installed.
<FrameLayout
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"
xmlns:app="http://schemas.android.com/apk/res-auto"
tools:context="com.google.ar.sceneform.Overlei.Overlei">
<fragment android:name="com.google.ar.sceneform.ux.ArFragment"
android:id="#+id/ux_fragment"
android:layout_width="match_parent"
android:layout_height="match_parent" />
<LinearLayout
android:id="#+id/gallery_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintBottom_toBottomOf="parent"
>
</LinearLayout>
</FrameLayout>
I expected this to just work, I don't know where I can start looking if an in-built function(such as this) does not work.

Thanks for the suggestions guys. I have managed to deal with the error by adding this line
implementation 'com.android.support.constraint:constraint-layout:1.1.3'
into my build.gradle(module.app) under dependencies
Again, thanks for everything

You should use app:layout_constraintBottom_toBottomOf only in ConstraintLayout. You can try to use inside of FrameLayout
then android:gravity = "bottom" in your LinearLayout.

Wrap you FrameLayout in a ConstraintLayout or change the FrameLayout to ConstraintLayout

Android Studio cannot find symbol variable

I installed Android Studio about two days ago, I also installed the most updated version of Java and JDK 1.8 (I think it was Java SE 8u101). I am able to edit code and I'm following a tutorial from the official Android Studio website. (Here: https://developer.android.com/training/basics/firstapp/building-ui.html)
However, upon hitting run I've gotten the two following errors:
Error:(24, 57) error: cannot find symbol variable activity_display_message
Code for this:
ViewGroup layout = (ViewGroup) findViewById(R.id.activity_display_message);
Error:(33, 25) error: cannot find symbol variable EXTRA_MESSAGE
Code for this:
intent.putExtra(EXTRA_MESSAGE, message);
What might I be doing wrong? The emulator pops up and I can get to other apps on the phone but I just can't get my app to load. Should I have installed Java SE 8u102 instead?
I also got a second error message:
Error:Execution failed for task ':app:compileDebugJavaWithJavac'. > Compilation failed; see the compiler error output for details.
Any help is appreciated.
Below is my code for the first xml file, "activity_main.xml":
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent">
<EditText android:id="#+id/edit_message"
android:layout_weight="1"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:hint="#string/edit_message" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/button_send"
android:onClick="sendMessage" />
</LinearLayout>
Below is my code for "activity_display_message.xml"
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context="com.example.appno1.DisplayMessageActivity">
</RelativeLayout>

Add this to your MainActivity:
public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
You might have missed this line, which can be found in the "Build an Intent" part.
Refer this for more, from the original page.

I think you might lose the last note in the tutorial.
so you just add android:id attribute to the activity_display_message.xml file and it will be ok!oh actually you should add into the layout element which is the valid location.

My advice is, try to clean the project and build it again. If you want to access newly added resources you need to build the project. Cleaning will help to remove all previously autogenerated resources from project and building will help to create them again + new ones.

You should add EXTRA_MESSAGE as static final, like:
private static final String EXTRA_MESSAGE = "message";

Regarding this error,
ViewGroup layout = (ViewGroup) findViewById(R.id.activity_display_message);
check that the file res > layout > activity_display_message.xml as the id specified as android:id="#+id/activity_display_message"

problems with R - ANDROID

when i create a layout called " main.xml ":
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:padding="10dip"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<Button
android:id="#+id/button"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="#string/button" />
<Spinner
android:id="#+id/spinner"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
/>
</LinearLayout>
when i go to the main to create a button
" button = (Button) findViewById(R.id.button); "
i have an error , i cant find the button in R.id
and same for the spinner
the id dont exist in R.id
help me plz

You're probably importing the wrong version of R. There are two versions which exist: the static R class which exists in android.R and then there is also yourPackageName.R. If you import the former none of your Ids will be found.

Make sure you don't have any files, pictures, etc in your drawable or assets folder with capital letters, spaces, or dashes. Just use plane old lowercase. Also, try to clean your project. Go to project > clean. Then restart eclipse. Hope this helps. Also eclipse won't reconize any changes in XML files without saving first. Make sure to save after you add a button, ect.

Try to verify that there is no errors in your XML files,than try to clean the project.

TextView Not Accessible

I am new to Android.I am making a random generator program which on button click generates a number and display it on textview. I am unable to access textview when i type R.id. i don't get the name of the textview but i get the name of the other components. My layout code is:-
<?xml version="1.0" encoding="utf-8"?>
<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="horizontal">
<EditText android:id="#+id/edit_message"
android:layout_weight="1"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:hint="#string/edit_message" />
<TextView android:id="#+id/txt02" android:layout_width="fill_parent"
android:layout_height="wrap_content" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/button_send"
android:onClick="sendMessage" />
</LinearLayout>
Java code is:-
public void sendMessage(View view) {
// Do something in response to button
EditText editmessage = (EditText)findViewById(R.id.edit_message);
String message = editmessage.getText().toString();
Random r=new Random();
int i1=(r.nextInt(80) +65);
message += "\n " + i1;
// Create the text view
TextView textView = (TextView) findViewById(R.id.); // not able to access R.id.txt02
}

The problem most likely is that your R.java file hasn't been updated with the EditText you have added. In eclipse, you can force an update to do this. Try Project->Clean as mentioned by Michal Z. in the comments.
You can also set the option to build automatically in eclipse by checking the Project->Build Automatically option. This will ensure that eclipse will automatically build your project and update your R.java file every time you save your project.

If cleaning your project doesn't help, I'd check your source files to make sure there isn't a line saying "import android.R" in your imports because that will import the standard Android R object (which doesn't include any of your declared resources). Eclipse does this sometimes when you cut and paste code and it can be frustrating, but deleting the import and cleaning / building your project should fix it.