首先，在res下面新建一个文件夹drawable，在drawable下面新建三个xml文件：shape_corner_down.xml、shape_corner_up.xml和shape_corner.xml，分别是下面两个角是圆角边框，上面两个角是圆角边框，四个角全部是圆角边框。

shape_corner_down.xml：

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<?xml version=
"1.0" 
encoding=
"utf-8"
?>
<shape xmlns:android=
""
>
    
<solid android:color=
"#0099CC" 
/>
    
<corners android:bottomRightRadius=
"20dp" 
        
android:bottomLeftRadius=
"20dp"
/>
    
<stroke android:width=
"1dp" 
android:color=
"#000000"
/>
</shape>

shape_corner_up.xml：

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<?xml version=
"1.0" 
encoding=
"utf-8"
?>
<shape xmlns:android=
""
>
    
<solid android:color=
"#CCCC99" 
/>   
    
<corners android:topLeftRadius=
"20dp" 
        
android:topRightRadius=
"20dp"  
/>
    
<stroke android:width=
"1dp" 
android:color=
"#000000" 
/>
</shape>

shape_corner.xml：

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<?xml version=
"1.0" 
encoding=
"utf-8"
?>
<shape xmlns:android=
""
>
    
<solid android:color=
"#99CCFF" 
/>   
    
<corners android:topLeftRadius=
"20dp"
        
android:topRightRadius=
"20dp" 
        
android:bottomRightRadius=
"20dp" 
        
android:bottomLeftRadius=
"20dp"
/>
    
<stroke android:width=
"1dp" 
android:color=
"#000000" 
/>
</shape>

<solid android:color>设置了背景颜色。android:topLeftRadius、android:topRightRadius、android:bottomLeftRadius、android:bottomRightRadius分别是左上角、右上角、左下角、右下角的半径值，设置了半径值，相应的角就是圆角，半径值越大，圆角就越大。<stroke>设置边界属性，如边界的宽度、颜色等。

在activity_main.xml上面放置三个LinearLayout，分别设置android:background属性为shape_corner_up.xml、shape_corner_down.xml和shape_corner.xml，运行结果如下：