What is the use of the res/values/public.xml file on Android

The fileres/values/public.xmlis used to assign fixed resource IDs to Android resources.

Consider these set of string resources inres/values/strings.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="string1">String 1</string>
    <string name="string3">String 3</string>
</resources>

The Android Asset Packaging Tool (aapt) might assign the following resource IDs for these resources when the app is compiled:

public final class R {
    // ...
    public static final class string {
        public static final int string1=0x7f040000;
        public static final int string3=0x7f040001;
    }
}

Now,change the set of string resources to

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="string1">String 1</string>
    <string name="string2">String 2</string>
    <string name="string3">String 3</string>
</resources>

and you'll notice that the resource ID for@string/string3has changed:

public final class R {
    // ...
    public static final class string {
        public static final int string1=0x7f040000;
        public static final int string2=0x7f040001;
        public static final int string3=0x7f040002; // New ID! Was 0x7f040001
    }
}

To prevent this,you can useres/values/public.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <public type="string" name="string3" id="0x7f040001" />
</resources>

which will result in the resource IDs being assigned as follows:

public final class R {
    // ...
    public static final class string {
        public static final int string1=0x7f040000;
        public static final int string2=0x7f040002;
        public static final int string3=0x7f040001; // Resource ID from public.xml
    }
}

Applications rarely have any use forres/values/public.xmlsince the resource IDs assigned to resources does not matter. When they change,the entire application is rebuilt anyway so any references in Java code to resources by resource ID will be updated.

The most significant user ofres/values/public.xmlis the Android platform itself. Applications built against old versions of Android assumes that certain resource have a certain resource ID. For example,the Android resource@android:style/Thememust always have the resource ID 0x01030005 for the platform to be backwards compatible with apps built against old versions of the platform.

If you are curious about more details on how resource IDs are assigned,please refer to this answer:How does the mapping between android resources and resources ID work?