IMPORTANT: Remember to add your package name before you copy and paste this java code into the JustJava app's MainActivity.java file.

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);
    }
}

This is what worked for me:

/**

• 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 android.example.justjava

import android.os.Bundle
import android.view.View
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity

/**

• This app displays an order form to order coffee.
  */
  class MainActivity : AppCompatActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContentView(R.layout.activity_main)
  }

  /**

  • This method is called when the order button is clicked.
    */
    fun submitOrder(view: View?) {
    display(1)
    }

  /**

  • This method displays the given quantity value on the screen.
    */
    private fun display(number: Int) {
    val quantityTextView = findViewById(R.id.quantity_text_view) as TextView
    quantityTextView.text = "" +number
    }
    }