簡單。生活。享受

●實作練習：xml

<?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.student.lab0201.MainActivity">
    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        tools:layout_editor_absoluteY="0dp"
        tools:layout_editor_absoluteX="0dp"
        tools:ignore="MissingConstraints">
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Quantity"
            android:textAllCaps="true"
            android:textSize="25sp"
            android:paddingBottom="16dp"
            tools:ignore="HardcodedText" />
        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content">
            <Button
                android:id="@+id/add"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="+"
                android:textSize="25sp"
                android:layout_marginBottom="16dp"
                android:layout_marginRight="16dp"
                android:onClick="add"
                tools:ignore="HardcodedText,RtlHardcoded" />
            <TextView
                android:id="@+id/quantity"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="0"
                android:textSize="25sp"
                android:textColor="@android:color/black"
                android:layout_marginBottom="16dp"
                tools:ignore="HardcodedText" />
            <Button
                android:id="@+id/subtract"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="-"
                android:textSize="25sp"
                android:layout_marginBottom="16dp"
                android:layout_marginLeft="16dp"
                android:onClick="subtract"
                tools:ignore="HardcodedText,RtlHardcoded" />
        </LinearLayout>
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="price"
            android:textAllCaps="true"
            android:textSize="25sp"
            tools:ignore="HardcodedText" />
        <TextView
            android:id="@+id/price"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text=""
            android:textSize="25sp"
            android:textColor="@android:color/black"
            android:background="@android:color/holo_orange_light"/>
        <Button
            android:id="@+id/send"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="ok"
            android:textSize="25sp"
            android:onClick="send"
            tools:ignore="HardcodedText" />

    </LinearLayout>
</android.support.constraint.ConstraintLayout>

●實作練習：java code

package com.example.student.lab0201;

import android.os.Build;
import android.os.Bundle;
import android.support.annotation.RequiresApi;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;

import java.text.NumberFormat;


public class MainActivity extends AppCompatActivity {
    TextView quantity;
    TextView price;
    //一般命名變數時，前面加個m代表整個類別可以使用的變數，設定初始值
    private int mQuantity=0;
    private int mPrice=5;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        quantity=(TextView)findViewById(R.id.quantity);
        price=(TextView)findViewById(R.id.price);
    }
    @RequiresApi(api = Build.VERSION_CODES.N)
    public void send(View v){
        displaytotalPirce();

    }
    public void displayquantity(){
        String ntoString=String.valueOf(mQuantity);
        quantity.setText(ntoString);
    }
    public void add(View v){
        mQuantity++;
        displayquantity();
        price.setText("");
    }
    public void subtract(View v){
        if(mQuantity>0){
            mQuantity--;
            displayquantity();
        }
        price.setText("");

    }
    @RequiresApi(api = Build.VERSION_CODES.N)
    public void displaytotalPirce(){
        int totalP=mQuantity*mPrice;
        //貨幣格式化=>可以出現$符號
        //Attention：You should import "java.text.NumberFormat" here, or this app will crash.
        String pay=NumberFormat.getCurrencyInstance().format(totalP);
        String msg=pay+(mQuantity==0?"\nFree":"\nThank you!");
        price.setText(msg);

    }
}

*程式參考(gitHub)：加減計算的練習與實作