[android] Android Studio: Gradle: error: cannot find symbol variable

I was working on my app and everything was normal until I tried to display image in java.

I ran the app once and it ran normally, the picture was displayed. After that it asked me to import some libraries and I imported them. After that I got errors for my activities.

Errors like:

Gradle: error: cannot find symbol variable activity_main
Gradle: error: cannot find symbol variable button1
Gradle: error: cannot find symbol variable button2
Gradle: error: cannot find symbol variable textView
Gradle: error: cannot find symbol variable secondActivity

In MainActivity I have imported these libraries:

import android.R;
import android.content.Intent;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.widget.Button;

and in secondActivity these:

import android.R;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

Does anyone know how to fix this?

EDIT: I deleted import android.R; and now it works normally.

If you are using a String build config field in your project, this might be the case:

buildConfigField "String", "source", "play"

If you declare your String like above it will cause the error to happen. The fix is to change it to:

buildConfigField "String", "source", "\"play\""

Open project in android studio click file and click Invalidate Caches/Restart

Another alternative to @TouchBoarder's answer above is that you may also have two layout files with the same name but for different api versions. You should delete the older my_file.xml file

my_file.xml
my_file.xml(v21)

If you are using multiple flavors?

-make sure the resource file is not declared/added both in only one of the flavors and in main.

Example: a_layout_file.xml file containing the symbol variable(s)

src:

flavor1/res/layout/(no file)

flavor2/res/layout/a_layout_file.xml

main/res/layout/a_layout_file.xml

This setup will give the error: cannot find symbol variable, this is because the resource file can only be in both flavors or only in the main.

Make sure you have MainActivity and .ScanActivity into your AndroidManifest.xml file:

<activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".ScanActivity">

    </activity>

make sure that the imported R is not from another module. I had moved a class from a module to the main project, and the R was the one from the module.

Make sure your variables are in scope for the method that is referencing it. For example I had defined a textview locally in a method in the class and was referencing it in another method.

I moved the textview definition outside the method right below the class definition so the other method could access the definition, which resolved the problem.