How to set shape’s opacity?

In general you just have to define a slightly transparent color when creating the shape.

You can achieve that by setting the colors alpha channel.

#FF000000 will get you a solid black whereas #00000000 will get you a 100% transparent black (well it isn’t black anymore obviously).

The color scheme is like this #AARRGGBB there A stands for alpha channel, R stands for red, G for green and B for blue.

The same thing applies if you set the color in Java. There it will only look like 0xFF000000.

UPDATE

In your case you’d have to add a solid node. Like below.

<?xml version="1.0" encoding="UTF-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/shape_my">
    <stroke android:width="4dp" android:color="#636161" />
    <padding android:left="20dp"
        android:top="20dp"
        android:right="20dp"
        android:bottom="20dp" />
    <corners android:radius="24dp" />
    <solid android:color="#88000000" />
</shape>

The color here is a half transparent black.