Just JAVA - My First programme

ActivityMain.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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="10dp"
    android:paddingLeft="10dp"
    android:paddingRight="10dp"
    android:paddingTop="10dp"
    android:background="@drawable/coffee"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="16dp"
        android:text="Quantity"
        android:textSize="20dp"
        android:textStyle="bold"
        android:textColor="@color/colorPrimaryDark"
        android:textAllCaps="true" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <Button
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:onClick="decrement"
            android:textSize="20sp"
            android:text="-"
            android:textStyle="bold"/>

        <TextView
            android:id="@+id/quantity_text_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="8dp"
            android:layout_marginRight="8dp"
            android:text="0"
            android:textStyle="bold"
            android:textColor="@android:color/black"
            android:textSize="20sp" />

        <Button
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:onClick="increment"
            android:textStyle="bold"
            android:textSize="20sp"
            android:text="+" />
    </LinearLayout>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="16dp"
        android:layout_marginTop="16dp"
        android:text="price"
        android:textColor="@color/colorPrimaryDark"
        android:textSize="20dp"
        android:textStyle="bold"
        android:textAllCaps="true" />

    <TextView
        android:id="@+id/price_text_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="0$"
        android:textStyle="italic"
        android:textColor="@android:color/black"
        android:textSize="20sp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:onClick="submitOrder"
        android:padding="8dp"
        android:textSize="16dp"
        android:text="ORDER" />
    />
</LinearLayout>

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.mathsways.justjava;

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

/**
 * This app displays an order form to order coffee.
 */
public class MainActivity extends AppCompatActivity {
    int quantity = 0;

    @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 increment(View view) {
        quantity=quantity + 1;
        display(quantity);
    }
    /**
     * This method is called when the order button is clicked.
     */
    public void decrement(View view) {
        quantity=quantity - 1;
        display(quantity);
    }
    /**
     * This method is called when the order button is clicked.
     */
    public void submitOrder(View view) {
        int price = quantity*5;
        String priceMessage="You have " + quantity + " Cups of coffees \nThe total price is " + price + "$" + "\nTank you";
        displayMessage(priceMessage);
    }
    /**
     * 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 method displays the given price on the screen.
     */
    /**
     * This method displays the given price on the screen.
     */
    private void displayPrice(int number) {
        TextView priceTextView = (TextView) findViewById(R.id.price_text_view);
        priceTextView.setText(NumberFormat.getCurrencyInstance().format(number));
    }
    /**
     * This method displays the given text on the screen.
     */
    private void displayMessage(String message) {
        TextView priceTextView = (TextView) findViewById(R.id.price_text_view);
        priceTextView.setText(message);
    }
}