No resource identifier found for attribute 'srcCompat' in package 'learnapp.android.example.com.learnapp'

Question

I am getting Error:(26) No resource identifier found for attribute 'srcCompat' in package 'learnapp.android.example.com.learnapp' when I try to run my app.I have just started learning android today.I have added a new activity and dint change anything in any xml files.

This is the exact error:

Documents/MyProjects/LearnApp/app/src/main/res/layout/activity_display_message.xml Error:(26) No resource identifier found for attribute 'srcCompat' in package 'learnapp.android.example.com.learnapp' 

Here is my activity_display_message.xml

<?xml version="1.0" encoding="utf-8"?> <LinearLayout 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:fitsSystemWindows="true"     tools:context="learnapp.android.example.com.learnapp.DisplayMessageActivity">      <android.support.design.widget.AppBarLayout         android:layout_width="match_parent"         android:layout_height="wrap_content"         android:theme="@style/AppTheme.AppBarOverlay">          <android.support.v7.widget.Toolbar             android:id="@+id/toolbar"             android:layout_width="match_parent"             android:layout_height="?attr/actionBarSize"             android:background="?attr/colorPrimary"             app:popupTheme="@style/AppTheme.PopupOverlay" />      </android.support.design.widget.AppBarLayout>      <include layout="@layout/content_display_message" />      <android.support.design.widget.FloatingActionButton         android:id="@+id/fab"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_gravity="bottom|end"         android:layout_margin="@dimen/fab_margin"         app:srcCompat="@android:drawable/ic_dialog_email" />  </LinearLayout> 

I have already read similar questions and saw that either the line

xmlns:app="http://schemas.android.com/apk/res-auto" 

or

app:srcCompat="@android:drawable/ic_dialog_email" /> 

was wrong in their case which is not in my case.

Incase it matters this activity's parent activity is MainActivity.java

Answer 1

I just changed

xmlns:app="http://schemas.android.com/apk/res-auto"  

to

xmlns:app="http://schemas.android.com/apk/lib/my_package_name" 

I saw the answer here