如图，在没有输入文字前，会显示提醒文字。

引用的是文本文件（strings.xml）中的对应文字

activity_main.xml对应的红色代码：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="examples.ouc.com.myapplication2.MainActivity"
    tools:showIn="@layout/activity_main">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        android:id="@+id/textView" />

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/etUser"
        android:hint="@string/hint_etUser"
        android:layout_below="@+id/textView"
        android:layout_alignParentEnd="true" />
    <EditText
        android:id="@+id/etPassword"
        android:hint="@string/hint_etPassword"
        android:inputType="textPassword"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/etUser"
        android:layout_alignParentEnd="true" />
</RelativeLayout>

strings.xml中对应的代码为：

 1 <resources>
 2     <string name="app_name">My Application2</string>
 3     <string name="action_settings">Settings</string>
 4     <string name="hint_etUser">User Name</string>
 5     <string name="hint_etPassword">Password</string>
 6     <string name="hint_btSubmit">Submit</string>
 7     <string name="text_btClear">Clear</string>
 8     <string name="hint_etPhone">Phone Number</string>
 9     <string name="hint_etAge">Age</string>
10     <string name="msg_ClearAllFields">clear all the fields</string>
11 </resources>
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