Error “package android.support.v7.app does not exist”
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First of all check if your project is using androidx or android support library. Check gradle.properties file:

android.useAndroidX=true

android.enableJetifier=true

If it contains the above lines, it is using androidx with an old code from some old tutorial.

In build.gradle (module:app)

Use

implementation 'androidx.appcompat:appcompat:1.0.0'
Instead of

compile 'com.android.support:appcompat-v7:28.0.0'
Also in MainActivity.java : Use

import androidx.appcompat.app.AppCompatActivity;
instead of :

import android.support.v7.app.AppCompatActivity;

свернуть