[android-developers] Cannot resolve symbol "r" on android studio

[Albert Muñoz]

I'm doing on UDACITY the android development for begginers course and in 
one exercice we must put this code o the activity_main.xml:


<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android";
    xmlns:tools="http://schemas.android.com/tools";
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.android.justjava.MainActivity">

    <TextView
        android:id="@+id/quantity"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Quantity"
        android:textAllCaps="true"/>
    <TextView
        android:id="@+id/number_quantity"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="0"
        android:textColor="#000000"
        android:textSize="16sp"
        android:paddingTop="16dp"
        android:paddingBottom="16dp"/>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="ORDER"
        android:OneClick="submitOrder"

        />
</LinearLayout>











and on the MainActivity.java this code:








/**

 * Add your package below. Package name can be found in the project's 
AndroidManifest.xml file.

 * This is the package name our example uses:

 *

 * 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);

    }

}







And on the MainActivity.java code the R is in red and i don't understand why???

I've tryed to clean the project, build the project and changing the code but i 
cannot resolve the problem.

Please HELP ME!!!!!

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