问题现象：recyclerview 中的顶部数据被appbar遮盖，如下图红框所示

解决方法：

在activity_main.xml里面CoordinatorLayout布局下 即与AppBarLayout布局并列下加上 如下代码

 <!--app:layout_behavior="@string/appbar_scrolling_view_behavior"-->

  <!--在RecyclerView或者其他支持嵌套滚动的view中如果添加以上属性将和AppBarLayout绑定-->

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 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="com.iwm.qa.mdm.view.MainActivity">


<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>


<android.support.v7.widget.RecyclerView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/toolbar"
        android:id="@+id/recycler_view"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        app:layout_behavior="@string/appbar_scrolling_view_behavior" />


<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" />


</android.support.design.widget.CoordinatorLayout>

When you are using CoordinatorLayout and AppBarLayout, you are setting up for that coordinated scrolling where the toolbar pushes out of the way first. But in order to get that, you need to give the view below the toolbar the appbar scrolling view behavior. This not only sets up the coordinated scroll, but tells the CoordinatorLayout to layout the lower view so that it appears beneath the toolbar.

If you don't want the coordinated toolbar scrolling, replace CoordinatorLayout with a vertical LinearLayout.

附参考博客：

https://stackoverflow.com/questions/38601099/appbarlayout-overlaps-recyclerview