What is printed when the following code is executed?

public class CustomReverse {
    public static String customRev(String s) {
        if(s.length() <= 1) return s;
        return s.charAt(0) + customRev(s.substring(1)) + s.charAt(0);
    }
    public static void main(String[] args) {
        System.out.println(customRev("ab"));
    }
}