Why I got "package R does not exist" error?

Question

I have a small project that I finished couple of months ago, and left it on my git account.

Today I have downloaded this project locally and tried to launch, but got such error

Error:(18, 19) error: package R does not exist

I tried clean and rebuild , but it doesn't helped

my log look like this

Information:Gradle tasks [:app:assembleDebug]
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java
    Error:(7, 39) error: cannot find symbol class R
Error:(18, 19) error: package R does not exist
    Error:(21, 79) error: package R does not exist
    Error:(22, 79) error: package R does not exist
    Error:(31, 17) error: package R does not exist
    Error:(176, 51) error: package R does not exist
    Error:(178, 53) error: package R does not exist
    Error:(179, 51) error: package R does not exist
    Error:(181, 53) error: package R does not exist
    Error:(182, 51) error: package R does not exist
    Error:(184, 53) error: package R does not exist
    Error:(185, 51) error: package R does not exist
    Error:(187, 53) error: package R does not exist
    Error:(189, 51) error: package R does not exist
    Error:(192, 62) error: package R does not exist
    Error:Execution failed for task ':app:compileDebugJavaWithJavac'.
    Compilation failed; see the compiler error output for details.
    Information:BUILD FAILED
    Information:Total time: 2.421 secs
    Information:98 errors
    Information:0 warnings
    Information:See complete output in console

my compiler error output look like this

/Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:7: error: cannot find symbol
    import com.example.aleksey.videoviewer.R;
    ^
    symbol:   class R
location: package com.example.aleksey.videoviewer
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:17: error: cannot find symbol
    import com.example.aleksey.videoviewer.R;
    ^
    symbol:   class R
location: package com.example.aleksey.videoviewer
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:22: error: cannot find symbol
    import com.example.aleksey.videoviewer.R;
    ^
    symbol:   class R
location: package com.example.aleksey.videoviewer
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentVideo.java:6: error: cannot find symbol
    import com.example.aleksey.videoviewer.R;
    ^
    symbol:   class R
location: package com.example.aleksey.videoviewer
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/adapters/AdapterList.java:15: error: cannot find symbol
    import com.example.aleksey.videoviewer.R;
    ^
    symbol:   class R
location: package com.example.aleksey.videoviewer
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivitySplash.java:12: error: cannot find symbol
    import com.example.aleksey.videoviewer.R;
    ^
    symbol:   class R
location: package com.example.aleksey.videoviewer
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:7: error: cannot find symbol
    import com.example.aleksey.videoviewer.R;
    ^
    symbol:   class R
    location: package com.example.aleksey.videoviewer
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:18: error: package R does not exist
    setTheme(R.style.AppTheme_Dark);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:21: error: package R does not exist
    Preference prefShareKey      = (Preference) findPreference(getString(R.string.pref_share_key));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:22: error: package R does not exist
    Preference prefRateReviewKey = (Preference) findPreference(getString(R.string.pref_rate_review_key));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:31: error: package R does not exist
    return R.xml.pref_about;
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:36: error: package R does not exist
    if(preference.getKey().equals(getString(R.string.pref_share_key))) {
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:40: error: package R does not exist
    getString(R.string.subject));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:41: error: package R does not exist
    shareIntent.putExtra(Intent.EXTRA_TEXT, getString(R.string.message) +
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:42: error: package R does not exist
    " " + getString(R.string.googleplay_url));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:43: error: package R does not exist
    startActivity(Intent.createChooser(shareIntent, getString(R.string.share_to)));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:44: error: package R does not exist
    }else if(preference.getKey().equals(getString(R.string.pref_rate_review_key))) {
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:47: error: package R does not exist
    getString(R.string.googleplay_url)));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:56: error: package R does not exist
    overridePendingTransition(R.anim.open_main, R.anim.close_next);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityAbout.java:56: error: package R does not exist
    overridePendingTransition(R.anim.open_main, R.anim.close_next);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:61: error: package R does not exist
    setContentView(R.layout.activity_home);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:63: error: package R does not exist
    layoutList = (FrameLayout) findViewById(R.id.fragment_container);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:64: error: package R does not exist
    toolbar = (Toolbar) findViewById(R.id.toolbar);

      /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:70: error: package R does not exist
    channelNames = getResources().getStringArray(R.array.channel_names);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:71: error: package R does not exist
    channelId = getResources().getStringArray(R.array.channel_id);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:72: error: package R does not exist
    videoTypes = getResources().getStringArray(R.array.video_types);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:87: error: package R does not exist
    .withHeaderBackground(R.drawable.main_icon)
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:127: error: package R does not exist
    .replace(R.id.fragment_container, fragment)
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:133: error: package R does not exist
    overridePendingTransition(R.anim.open_next, R.anim.close_main);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:133: error: package R does not exist
    overridePendingTransition(R.anim.open_next, R.anim.close_main);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:100: error: package R does not exist
    .withName(getString(R.string.about))
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:156: error: package R does not exist
    .replace(R.id.fragment_container, fragment)
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:163: error: package R does not exist
    Fragment f = getSupportFragmentManager().findFragmentById(R.id.fragment_container);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:186: error: package R does not exist
    String.format(getString(R.string.error_player),
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:208: error: package R does not exist
    getMenuInflater().inflate(R.menu.activity_home, menu);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:215: error: package R does not exist
    case R.id.menuAbout:
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:219: error: package R does not exist
    overridePendingTransition(R.anim.open_next, R.anim.close_main);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:219: error: package R does not exist
    overridePendingTransition(R.anim.open_next, R.anim.close_main);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/activities/ActivityHome.java:278: error: package R does not exist
    (FragmentVideo) getFragmentManager().findFragmentById(R.id.video_fragment_container);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:78: error: package R does not exist
    View view = inflater.inflate(R.layout.fragment_video_list, container, false);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:86: error: package R does not exist
    view.findViewById(R.id.ultimate_recycler_view);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:87: error: package R does not exist
    mLblNoResult                = (TextView) view.findViewById(R.id.lblNoResult);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:88: error: package R does not exist
    mLytRetry                   = (LinearLayout) view.findViewById(R.id.lytRetry);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:89: error: package R does not exist
    mPrgLoading                 = (CircleProgressBar) view.findViewById(R.id.prgLoading);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:90: error: package R does not exist
    AppCompatButton btnRetry    = (AppCompatButton) view.findViewById(R.id.raisedRetry);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:93: error: package R does not exist
    mPrgLoading.setColorSchemeResources(R.color.accent_color);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:112: error: package R does not exist
    .inflate(R.layout.progressbar, null));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:120: error: package R does not exist
    .inflate(R.layout.progressbar, null));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:189: error: package R does not exist
    Utils.PARAM_KEY_YOUTUBE + getResources().getString(R.string.youtube_apikey) + "&" +
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:198: error: package R does not exist
    Utils.PARAM_KEY_YOUTUBE + getResources().getString(R.string.youtube_apikey) + "&" +
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:315: error: package R does not exist
    msgSnackBar = getResources().getString(R.string.no_internet_connection);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:317: error: package R does not exist
    msgSnackBar = getResources().getString(R.string.response_error);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:350: error: package R does not exist
    Utils.PARAM_KEY_YOUTUBE+getResources().getString(R.string.youtube_apikey)+"&"+
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:426: error: package R does not exist
    msgSnackBar = getResources().getString(R.string.no_internet_connection);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:428: error: package R does not exist
    msgSnackBar = getResources().getString(R.string.response_error);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentChannelVideo.java:502: error: package R does not exist
    case R.id.raisedRetry:
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/fragments/FragmentVideo.java:21: error: package R does not exist
    initialize(getResources().getString(R.string.youtube_apikey), this);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/adapters/AdapterList.java:45: error: package R does not exist
    View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.adapter_video_list, parent, false);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/adapters/AdapterList.java:74: error: package R does not exist
    R.mipmap.empty_photo, R.mipmap.empty_photo));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/adapters/AdapterList.java:74: error: package R does not exist
    R.mipmap.empty_photo, R.mipmap.empty_photo));
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/adapters/AdapterList.java:109: error: package R does not exist
    txtTitle = (TextView) v.findViewById(R.id.txtTitle);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/adapters/AdapterList.java:110: error: package R does not exist
    txtDuration = (TextView) v.findViewById(R.id.txtDuration);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/adapters/AdapterList.java:111: error: package R does not exist
    txtPublished = (TextView) v.findViewById(R.id.txtPublishedAt);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/adapters/AdapterList.java:112: error: package R does not exist
    imgThumbnail = (ImageView) v.findViewById(R.id.imgThumbnail);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:141: error: package R does not exist
    timeAgo = dim + " " + ctx.getResources().getString(R.string.date_util_unit_minutes);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:143: error: package R does not exist
    timeAgo = ctx.getResources().getString(R.string.date_util_prefix_about) + " " +
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:144: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_hour);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:147: error: package R does not exist
    timeAgo = ctx.getResources().getString(R.string.date_util_prefix_about) + " " +
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:149: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_hour);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:151: error: package R does not exist
    timeAgo = ctx.getResources().getString(R.string.date_util_prefix_about) + " " +
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:153: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_hours);

    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:158: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_day);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:161: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_days);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:164: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_daya);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:167: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_day);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:169: error: package R does not exist
    timeAgo = ctx.getResources().getString(R.string.date_util_prefix_about) + " "+
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:170: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_month);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:178: error: package R does not exist
    timeAgo = ctx.getResources().getString(R.string.date_util_prefix_about) + " "+
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:182: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_year);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:184: error: package R does not exist
    timeAgo = ctx.getResources().getString(R.string.date_util_prefix_almost) + " 2 " +
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:185: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_years);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:187: error: package R does not exist
    timeAgo = ctx.getResources().getString(R.string.date_util_prefix_about) + " " +
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:189: error: package R does not exist
    ctx.getResources().getString(R.string.date_util_unit_years);
    ^
    /Users/admin/UserData/androidProjects/YoutubeClient/app/src/main/java/com/example/aleksey/videoviewer/activities/utils/Utils.java:192: error: package R does not exist
    return timeAgo + " " + ctx.getResources().getString(R.string.date_util_suffix);
    ^
    Note: Some input files use or override a deprecated API.
    Note: Recompile with -Xlint:deprecation for details.
    97 errors

    :app:compileDebugJavaWithJavac FAILED

    FAILURE: Build failed with an exception.

    * What went wrong:
    Execution failed for task ':app:compileDebugJavaWithJavac'.
    > Compilation failed; see the compiler error output for details.

    * Try:
    Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

    BUILD FAILED

    Total time: 3.564 secs
    Executing tasks: [:app:generateDebugSources, :app:generateDebugAndroidTestSources,

according to this output

Note: Some input files use or override a deprecated API.

API that I used in this project was deprecated, but how I can know which one and how I can fix it?

Also I have such warning

Invalid VCS root mapping
                The directory /Users/admin is registered as a Git root, but no Git repositories were found there.

but I think it is doesn't impact directly to R class error.

What am I doing wrong?

Answer 1

Late to the party, but this just happened to me in Android Studio 3.0 after adding a new activity to my project. Turns out the activity had the wrong package name (d'oh! Not sure if that was my fault or AS's). Correcting the package name at the top of the java file and moving it to the correct folder solved the problem.

Answer 2

Once it happened to me as well. The reason was I copied a java file from another project and pasted that in project. Then I started getting the error 'R does not exist'. The reason behind that was I copied a java file from different project and in that project R file is available in different package. What I did I just imported the R file that belongs to my package. And done. You need to make sure if you are copying any activity file in you project please update R's package according to your project package. NOTE: One more thing you can do you just go to the area where all the imports statements available(on the top your java file). Remove import statement for R's file and then import that again. It will import your package R file.Hope it will help you out.