切记：fragment一定要放在framlayout中，不然不会被替换完全（就是切换之后原来的fagment可能还会存在）
main.xml
<LinearLayout xmlns:andro
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#ffffff"
android:orientation="vertical">
<FrameLayout
android:>
<!--四个Button>
<LinearLayout/>
<LinearLayout/>
fragment_1.xml(灰色背景)
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:andro>
</LinearLayout>
fragment_2.xml(黑色背景)
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:andro>
</LinearLayout>
main.java
在Button点击事件里面：
FragmentManager fragmentManager = getFragmentManager();//
FragmentTransaction transaction = fragmentManager.beginTransaction();
transaction.replace(R.id.content, fragment_1);
transaction.commit();
彩蛋：在fragment中获得Context：view.getContext()