1 package com.example.myapplication;
 3 import androidx.appcompat.app.AppCompatActivity;
 5 import android.os.Bundle;
 7 public class MainActivity extends AppCompatActivity {//第一个MainActivity
 9     @Override
10     protected void onCreate(Bundle savedInstanceState) {
11         super.onCreate(savedInstanceState);
12         setContentView(R.layout.activity_main);//其对应的布局文件layout是activity_main
13     }
14 }

 1 <?xml version="1.0" encoding="utf-8"?>
 2 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
 3     package="com.example.myapplication">
 4 
 5     <application
 6         android:allowBackup="true"
 7         android:icon="@mipmap/ic_launcher"
 8         android:label="@string/app_name"
 9         android:roundIcon="@mipmap/ic_launcher_round"
10         android:supportsRtl="true"
11         android:theme="@style/AppTheme">
12         <activity android:name=".MainActivity"><!--MainActivity在这里注册好了-->
13             <intent-filter>
14                 <action android:name="android.intent.action.MAIN" />
16                 <category android:name="android.intent.category.LAUNCHER" /><!--MainActivity是整个项目的启动LAUNCHER-->
17             </intent-filter>
18         </activity>
19     </application>
21 </manifest>

 1 <?xml version="1.0" encoding="utf-8"?>
 2 
 3 <!--把默认的改为LinearLayout-->
 4 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 5     xmlns:app="http://schemas.android.com/apk/res-auto"
 6     xmlns:tools="http://schemas.android.com/tools"
 7     android:layout_width="match_parent"
 8     android:layout_height="match_parent"
 9     tools:context=".MainActivity">
10 
11     <TextView
12         android:layout_width="wrap_content"
13         android:layout_height="wrap_content"
14         android:text="Hello World!"
15         app:layout_constraintBottom_toBottomOf="parent"
16         app:layout_constraintLeft_toLeftOf="parent"
17         app:layout_constraintRight_toRightOf="parent"
18         app:layout_constraintTop_toTopOf="parent" />
19 
20 </LinearLayout>