1. Error Messages:
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: android.view.InflateException: Binary XML file line #9 in com.example.myapplication:layout/activity_main: Binary XML file line #9 in com.example.myapplication:layout/activity_main: Error inflating class fragment.
2. Locating the problem
Check the logs and find that the problem is located at: at com.example.myapplication.MainActivity.onCreate(MainActivity.java:11), click to jump to the line of code that says

  setContentView(R.layout.activity_main);

It was found that it was an XML file problem, so I carefully checked a piece of code I used fragment

<?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">

    <fragment
        android:name="com.example.myapplication.fragment.MyFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

It was found that the problem was caused by the fact that the fragment did not use the layout ID. after I added the ID to the fragment, the problem was successfully solved.

 <fragment
        android:id="@+id/my_fragment"
        android:name="com.example.myapplication.fragment.MyFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />