<?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=".MainActivity">

    <LinearLayout

        android:layout_width="match_parent"

        android:layout_height="match_parent"

        android:padding="16dp"

        android:orientation="vertical">

        <TextView

            android:id="@+id/quantityText_text_view"

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:text="Quantity"

            android:textColor="@android:color/darker_gray"/>

        <LinearLayout

            android:layout_width="match_parent"

            android:layout_height="wrap_content">

            <Button

                android:layout_width="48dp"

                android:layout_height="wrap_content"

                android:text="+"

                android:onClick="increment" />

            <TextView

                android:id="@+id/quantity_text_view"

                android:layout_width="wrap_content"

                android:layout_height="wrap_content"

                android:text="0"

                android:textSize="16sp"

                android:paddingLeft="8dp"

                android:paddingRight="8dp"

                android:textColor="@android:color/black" />

            <Button

                android:layout_width="48dp"

                android:layout_height="wrap_content"

                android:text="-"

                android:onClick="decrement"/>

        </LinearLayout>

        <TextView

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:text="Price"

            android:textColor="@android:color/darker_gray"/>

        <TextView

            android:id="@+id/price_text_view"

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:textColor="@android:color/black"

            android:textSize="16sp"

            android:text="$0"/>

        <Button

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:text="ORDER"

            android:onClick="submitOrder"

            tools:ignore="OnClick" />

    </LinearLayout>

</android.support.constraint.ConstraintLayout>

package com.learn.nianhao.justjava;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.view.View;

import android.widget.TextView;

import org.w3c.dom.Text;

public class MainActivity extends AppCompatActivity {

    int number =0;

    int price = 5;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        display(number);

        displayPrice(number*price);

    }

    /**

     * This method is called when the order button is clicked.

     */

    public void submitOrder(View view) {

        display(number);

        displayPrice(number*price);

    }

    /**

     * 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);

    }

    private void displayPrice(int price){

        TextView priveTextView = (TextView) findViewById(R.id.price_text_view);

        priveTextView.setText("$"+price);

    }

    public void increment(View view) {

        number+=1;

        display(number);

        displayPrice(number*price);

    }

    public void decrement(View view) {

        if(number>0){

            number-=1;

            display(number);

            displayPrice(number*price);

        }

    }

}