error: package android.support.v7.app does not exist

MainActivity.java

/**
 * IMPORTANT: Make sure you are using the correct package name. 
 * This example uses the package name:
 * package com.example.android.justjava
 * If you get an error when copying this code into Android studio, update it to match teh package name found
 * in the project's AndroidManifest.xml file.
 **/
 
 package com.example.android.justjava; 
 


import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;

/**
 * This app displays an order form to order coffee.
 */
public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    /**
     * This method is called when the order button is clicked.
     */
    public void submitOrder(View view) {
        display(1);
    }

    /**
     * This method displays the given quantity value on the screen.
     */
    private void display(int number) {
        TextView quantityTextView = (TextView) findViewById(R.id.quantity_text_view);
        quantityTextView.setText("" + number);
    }
}

Working through Udacity's course on Android development, in https://classroom.udacity.com/courses/ud836/lessons/4038208680/concepts/42701795700923 a code snippet had to be pasted in Android Studio.
Errors that occured are a result of:

• Not entering the package right: either use com.example.android.justjava or the one you created yourself (see Manifests > AndroidManifest.xml)

• Forgot about instructions regarding adding an ID to the textView object and/or adding android:onClick="submitOrder" to the button layout

These errors were corrected quite quickly, but in my case still didn't work... I kept getting the error message: error: package android.support.v7.app does not exist

The solution was found in the import statements:

instead of using: import android.support.v7.app.AppCompatActivity;
it has to be: import androidx.appcompat.app.AppCompatActivity;

Now it works fine!