今天狀況不好,
等一下又有事要忙,
先將程式碼貼出來,
明天再做講解.

activity_main.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=".MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <TextView
            android:id="@+id/TextView"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true"
            android:text="@string/bmi_hello" />

        <TextView
            android:id="@+id/textView2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/bmi_height" />

        <EditText
            android:id="@+id/height"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:ems="10"
            android:inputType="numberDecimal" />

        <TextView
            android:id="@+id/textView3"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/bmi_weight" />

        <EditText
            android:id="@+id/weight"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:ems="10"
            android:inputType="numberDecimal" />

        <Button
            android:id="@+id/button"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/bmi_btn" />

        <TextView
            android:id="@+id/result"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textAppearance="?android:textAppearanceLarge"/>

        <TextView
            android:id="@+id/suggest"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textAppearance="?android:textAppearanceLarge"/>
    </LinearLayout>
</android.support.constraint.ConstraintLayout>

MainActivity.java

package com.example.user.mybmi;

import android.content.res.Configuration;
import android.content.res.Resources;
import android.os.LocaleList;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import java.text.DecimalFormat;
import java.util.Locale;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button button = (Button)findViewById(R.id.button);
        button.setOnClickListener(calcBMI);
    }

    private View.OnClickListener calcBMI = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            DecimalFormat nf = new DecimalFormat("0.00");
            EditText fieldheight = (EditText)findViewById(R.id.height);
            EditText fieldweight = (EditText)findViewById(R.id.weight);
            //身高
            double height = Double.parseDouble(fieldheight.getText().toString())/100;
            //體重
            double weight = Double.parseDouble(fieldweight.getText().toString());
            //計算出BMI值
            double BMI = weight / (height*height);

            //結果
            TextView result = (TextView)findViewById(R.id.result);
            result.setText(getText(R.string.bmi_result)
                    + nf.format(BMI));

            //建議
            TextView fieldsuggest = (TextView)findViewById(R.id.suggest);
            if(BMI > 25) //太重了
                fieldsuggest.setText(R.string.advice_heavy);
            else if(BMI < 20) //太輕了
                fieldsuggest.setText(R.string.advice_light);
            else //剛剛好
                fieldsuggest.setText(R.string.advice_average);
        }
    };
}

strings.xml

<resources>
    <string name="app_name">MyBMI</string>
    <string name="bmi_hello">哈囉!BMI</string>
    <string name="bmi_height">身高 (cm)</string>
    <string name="bmi_weight">體重 (kg)</string>
    <string name="bmi_btn">計算BMI值</string>
    <string name="bmi_result">你的BMI值是</string>
    <string name="advice_light">你該多吃點</string>
    <string name="advice_average">體型很棒喔!</string>
    <string name="advice_heavy">你該節食了</string>
</resources>

以下是執行結果

這個數值...絕對不是我!