练习1 Just Java

<?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);
        }
    }
}