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 ???