Sorting Strings using Collections.sort?

Question

I'm attempting to sort two strings using the collection.sort() method, but I'm having issues understanding the logic of the implementation. Here is what I have so far. Are there any issues with my implementation? Note: I want to sort them alphabetically: "Apple" > "Orange"

Collections.sort(mailbox.getMessages() , (String a, String b) -> {
    if (String.valueOf(a.charAt(0)) > String.valueOf(b.charAt(0))) {
        return -1;
    }
    else if (String.valueOf(a.charAt(0)) <
        String.valueOf(b.charAt(0))) {
        return 1;
    }
    else {
        return 0;
    }
});

Yes there are issues. Try compiling it and see what it says, — Paul Boddington
– Paul Boddington, Commented Apr 12, 2015 at 20:45
Only issue I could see is if either a or b has a possibility of being null. You should also only perform 1 comparison between characters and store that result. — Obicere
– Obicere, Commented Apr 12, 2015 at 20:46
@Obicere: and the fact that it only sorts on the first characters. In general a string is sorted lexicographical. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Apr 12, 2015 at 20:47
Why not use the plain Collections.sort() method? That will sorts string lexicographically anyway. See docs.oracle.com/javase/7/docs/api/java/lang/… — DNA
– DNA, Commented Apr 12, 2015 at 20:47
I'm getting a "bad operand type error" after compilation. I thought that the int value of the characters could be compared using ">" , "<" etc. — Shan
– Shan, Commented Apr 12, 2015 at 20:47

willeM_ Van Onsem · Accepted Answer · 2015-04-12 20:52:09Z

5

String implements a Comparable<String> which is implemented as a lexicographical comparison, in other words, by default "Apple".compareTo("Orange") < 0. So the default is sufficient.

Now Collections.sort has a variant that takes this comparator into account, you can thus simply use:

Collections.sort(mailbox.getMessages());

About your own implementation:

You shouldn't use String.valueof to cast back to a string: you can compare chars with the <, but you can't use this operator on Strings. Furthermore your implementation is not recursive: if the two first characters are equal, that doesn't mean the String's are equal per se, for instance "Apple" and "Ambiguous". So you would have to implement a more complex comparator.

edited Apr 12, 2015 at 20:52

answered Apr 12, 2015 at 20:49

willeM_ Van Onsem

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1 Comment

Shan Over a year ago

Thanks! I misread the API and I didn't see that compareTo() was lexographic

Jean-François Savard · Accepted Answer · 2015-04-12 20:57:32Z

3

You can't compare String with the symbol >. You can simply do :

Collections.sort(mailbox.getMessages(), (String a, String b) -> {
     return Character.compare(a.charAt(0), b.charAt(0));
});

Note that this will sort according only to first character. If you want to sort on the entire string value lexographically then you can simply use Collections.sort(mailbox.getMessages()) as String already implements Comparable.

edited Apr 12, 2015 at 20:57

answered Apr 12, 2015 at 20:47

Jean-François Savard

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1 Comment

Obicere Over a year ago

Why not just Character.compare(a.charAt(0), b.charAt(0)) then?

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