Using Comparator to sort ArrayList

Question

I have been working on a Scrabble assignment. I need to read words from a list, then read each char and assign a value, eventually assigning a total score to each word. That has been done! Phew. Now I need to use the Comparator to sort the words from greatest score to least. I have done a lot of reading and I'm still confused. I know that I could use the interface, but there's also using Comparator with a lambda expression, which is the direction that I think I want to go. I'm just not sure how. I need to compare the sumValue I have for each word, then print the words in decreasing order.

I created 2 loops to read the word (i), then the chars (j). I have printed to the screen the score of each word (sumValue) and its location (i) in my ArrayList. Now I need to use Comparator to compare the scores, and then reorder the location. I think my problem is that I feel like I'm not sorting the Arraylist. I'm sorting the scores, which are not in an ArrayList. Do I need to create a new ArrayList with scores attached to each word and sort that?

If you're working in Java, this might help: https://stackoverflow.com/questions/2839137/how-to-use-comparator-in-java-to-sort — joanis, Nov 06 '22 at 15:37
I'm working in Java. I added a couple of pics of what I have so far, with the middle cut out as all the if statements don't need to be shown. I am a beginning student. My code is cumbersome, I know. But I'm pretty proud of what I was able to do on my own. Just need guidance on how to use Comparator correctly. — ScarletCas, Nov 06 '22 at 15:49
[Don't post images of code](https://meta.stackoverflow.com/questions/285551/why-should-i-not-upload-images-of-code-data-errors) — Silvio Mayolo, Nov 06 '22 at 16:13
I have removed the pictures. I apologize. I am new to posting here and didn't realize this was an issue. I don't want to copy my code as I am in a programming class and working on assignments. I am trying to be vague enough that I get pointed in the right direction without anyone giving me coded answers as that is not what I'm looking for. I will try to figure out better ways to ask my questions. — ScarletCas, Nov 07 '22 at 01:04

score 1 · Answer 1 · edited Nov 13 '22 at 22:30

You can do it like so. The List interface has a sort() method that takes a Comparator.

The common practice for defining a Comparator is to make use of one of its static methods, like Comparator.comparing(Function<? super T,? extends U> keyExtractor). In this case, the KeyExtractor would be the Word.getValue method specified as a method reference (a lambda would also work).

But first, you should create a record or class to hold the word and value. That is so when you sort the records based on the value, the word and value will still be together as a unit.

record Word(String getWord, int getValue) {
     @Override
     public String toString() {
         return "%s -> %d".formatted(getWord, getValue);
     }
}

Here I have created a list of Word records. The values are arbitrary for demonstration.

List<Word> list = new ArrayList<>(
       List.of(new Word("the", 30), new Word("Hello", 20), new Word("GoodBye", 2)));

list.sort(Comparator.comparing(Word::getValue));
System.out.println(list);

prints

[GoodBye -> 2, Hello -> 20, the -> 30]

You can also sort in reverse order.

list.sort(Comparator.comparing(Word::getValue).reversed());
System.out.println(list);

prints

[GoodBye -> 2, Hello -> 20, the -> 30]

I would also suggest using a Map to hold the letters and their point value.

Map<String, Integer> letterMap = Map.of("A",1,"B",1,"C",1, ....);

Then you can access them like this.

int point = letterMap.get("A");

score 0 · Answer 2 · answered Nov 07 '22 at 11:12

So you have a score calculating method:

int score(String word) {
    ...
}

And a list of words:

Path path = Paths.get("C:/words.txt");
List<String> words = Files.readAllLines(path, Charset.defaultCharset());

words.sort(Comparator.comparingInt(this::score)
                     .thenComparing(Function.identity()));

This will first sort on the score, and then alphabetically for the same score.

The comparing/comparingInt/ComparingDouble/... functions indicate the function that will give one comparison key (from the word). For the comparison of the words them selve Function.identity() will use the word itself.

Using Comparator to sort ArrayList

2 Answers2