Android Studio Help Java Error : Hi guys I have the following code and I have a Java error. Could someone please help me out?

Thanks

My activity_main.xml is:

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context="com.example.andriod.justjava.MainActivity">
    
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">
    
            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="QUANTITY"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintLeft_toLeftOf="parent"
                app:layout_constraintRight_toRightOf="parent"
                app:layout_constraintTop_toTopOf="parent" />
    
            <TextView
                android:id="@+id/quantity_text_view"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:padding="16dp"
                android:text="0"
                android:textColor="@android:color/black"
                android:textSize="16sp" />
    
            <Button
                android:id="@+id/button"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:onClick="submitOrder"
                android:text="Order" />
    
        </LinearLayout>
    
    </android.support.constraint.ConstraintLayout>

The red that is marking is the .MainActicity in     tools:context="com.example.andriod.justjava.MainActivity">
and I can't it is telling me ''Validates inside refernces inside Android XML file''.


While my 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;
    
    import com.example.andriod.justjava.R;
    
    /**
     * 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);
        }
    }

tells me that MainActivity from public class MainActivity extends AppCompatActivity {
''is not registed in the manifest Androidmainfest.xml file''

Culd someone please help me out since I'm assuming it's something pretty stupid and I'm a noob.

Thanks

