XML : android-studio errors: error: Cannot find symbol variable fab

when i run the below code i get this error and get it with multiple positions but with this code usually mean when i write code like this

  C:\Users\saad\Desktop\Sa2d\Android\JustJava\app\src\main\java\com\example\android\justjava\MainActivity.java  Error:(21, 76) error: cannot find symbol variable fab  Error:Execution failed for task ':app:compileDebugJavaWithJavac'.  > Compilation failed; see the compiler error output for details.    

this my code on "content_main.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: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=".MainActivity"      android:orientation="vertical"      tools:showIn="@layout/activity_main">       <TextView         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:text="Quantity"/>     <TextView         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:text="0"/>     <Button         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:text="Order"         android:id="@+id/button" />  </LinearLayout>`    

and this my code on "MainActivity.java" i am mark the error line with *

  `*FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);      fab.setOnClickListener(new View.OnClickListener() {          @Override          public void onClick(View view) {              Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)                      .setAction("Action", null).show();          }      });`    

what is the solution for this problem ???