Java - Sort a list of objects that includes a special character

Basically i have an arraylist of objects like so:

[{name: 'A'}, {name: 'B'}, {name:'?'}]

I want to sort these so that the question mark is at the end like above.

But using the below code.

Collections.sort(myList);

This always results in the object with the question mark first, I think this is due to ASCII ordering? I think the correct way forward is to use a comparator function but i'm not sure how that would take shape with letters and special characters?

How would I implement this?

An alternative approach - one which sorts any punctuation to the end - can be achieved using a rules-based collator.

An example:

List<String> words = Arrays.asList(
        "?dog", "rat", "456", "123", "dog", "pig", "?cat", "!rat", "cat"
);
String englishRules = ("< a,A < b,B < c,C < d,D < e,E < f,F "
        + "< g,G < h,H < i,I < j,J < k,K < l,L "
        + "< m,M < n,N < o,O < p,P < q,Q < r,R "
        + "< s,S < t,T < u,U < v,V < w,W < x,X "
        + "< y,Y < z,Z < 0,1,2,3,4,5,6,7,8,9");

RuleBasedCollator rbc = new RuleBasedCollator(englishRules);
rbc.setStrength(Collator.PRIMARY);

Collections.sort(words, rbc);
words.forEach((word) -> {
    out.print(word + " ");
});

This outputs:

cat dog pig rat 123 456 !rat ?cat ?dog 

Points to note:

1. This specific example is limited to an English collation.

2. The general technique works because all non-mentioned characters are sorted to the end. So, not only are punctuation symbols sorted after English letters and digits - but so is every other character/symbol (e.g. those used by other scripts).

3. If you want a non-Unicode ordering of punctuation symbols, they need to be surrounded by single quotes in your rule string:

"... < 0,1,2,3,4,5,6,7,8,9 < '?' < '!'"

In Java 8, you may use a two-tiered custom comparator:

// given
List<YourObject> list;
list.sort((o1, o2) -> "?".equals(o1.getName()) ? 1 :
    ("?".equals(o2.getName()) ? -1 : o1.getName().compareTo(o2.getName())));

The sorting logic here is that if one or the other names be ?, then we always sort that ? last. If both names be ?, or if neither be ?, then we sort using the default lexicographical string sorting.

You can use some useful methods from java.util.Comparator to make your life easier and your code less error-prone than having to think about if-else or ternary operators:

class MyObj {
    private String name;

    MyObj(String name) {
        this.name = name;
    }

    String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "{name: '" + name + "'}";
    }
}

public class Demo {
    public static void main(String[] args) {
        List<MyObj> lst = Arrays.asList(new MyObj("B"), new MyObj("?"), new MyObj("A"));
        Comparator<String> questionMarksLast = Comparator
                .<String, Boolean>comparing("?"::equals)
                .thenComparing(Comparator.naturalOrder());

        lst.sort(Comparator.comparing(MyObj::getName, questionMarksLast));
        System.out.println(lst);  // prints [{name: 'A'}, {name: 'B'}, {name: '?'}]
    }
}