How do I sort an Arraylist so its collection is sorted by a list of values

Question

I have an Arraylist which contains recipies and each recipe contains a list of ingredients. The recipies is shown in a UI list of an Android app and the useres wants to sort the list based on the ingredients they like most.

For example this is the list of ingredients they like most

1 - Cheese
2 - Potato
3 - Tuna
4 - Chicken

So the list of recipies shall show first all the ingredients that contains cheese and then potato etc. How do I archive this with java stream()?

Here is how my model classes is at the moment

public class Recipe {

    String[] ingredients;
    private String description;
    private String title;


    public String[] getIngredients() {
        return ingredients;
    }

    public void setIngredients(String[] ingredients) {
        this.ingredients = ingredients;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

.

public class Ingredient {

    String title;

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

Why do you want to use Java 8 Streams? Just use `Collections.sort(list);`. — dan1st, May 04 '20 at 20:18
I mean you can use `Stream#sorted` too in order to get a sorted stream. — dan1st, May 04 '20 at 20:19
[This](https://stackoverflow.com/a/18130019/6650475) might help, but see concerns about performance. — Andrew S, May 04 '20 at 20:25
@dan1st I see that Streams has a many features for these kind of cases — , May 04 '20 at 21:01
@Muddz - Can a dish have more than one of their favorite ingredients ? Ex. Cheese & Chicken ? Is Cheese & Chicken < Cheese & Potato ? — MasterJoe, May 05 '20 at 05:04

0xh3xa · Answer 1 · 2020-05-05T13:17:18.707

You can create comparatorPotato and comparatorCheese then sort based on them

  static class Recipie {
        List<String> list;

        public Recipie(List<String> list) {
            this.list = list;
        }

        @Override
        public String toString() {
            return list.toString();
        }
    }

    public static void main(String[] args) {
               List<Recipie> recipies = new ArrayList<>();
        recipies.add(new Recipie(Arrays.asList("Cheese", "Potato", "Onions")));
        recipies.add(new Recipie(Arrays.asList("Tuna", "Potato", "Chicken")));
        recipies.add(new Recipie(Arrays.asList("Potato", "Cheese")));
        recipies.add(new Recipie(Arrays.asList("Chicken")));
        recipies.add(new Recipie(Arrays.asList("Chicken", "Potato")));
        recipies.add(new Recipie(Arrays.asList("Cheese", "Tomato")));

        List<Recipie> result = recipies.stream().sorted(Comparator.comparing(r -> !r.list.contains("Potato")))
                .sorted(Comparator.comparing(r -> !r.list.contains("Cheese"))).collect(Collectors.toList());

        System.out.println(result);
    }

, output

[[Cheese, Potato, Onions], [Potato, Cheese], [Cheese, Tomato], [Tuna, Potato, Chicken], [Chicken, Potato], [Chicken]]

You should check your logical conditions. `first && !second || first && !second` is redundant, as both alternatives are identical. Further, `first && second || first && !second` contains `second` and `!second` in both alternatives while having `first && …` in common, so the result is just `first`, regardless of `second`’s content. You should have noted in your result that lists containing `"Cheese"` *and* `"Potato"` are not at the beginning. Don’t implement such comparators manually, just use, e.g. `Comparator.comparing(r -> !r.list.contains("Potato"))`. — Holger, May 05 '20 at 09:51

score 0 · Answer 2 · answered May 04 '20 at 23:15

Your Recipe class is not using your Recipe class, so your model can be modified to use the Ingredient class.

public class Recipe {
    List<Ingredient> ingredients;
    ...
}

You can create your own Comparator to sort if a Recipe has some ingredient.

public class ContainsIngredientComparator implements Comparator<Recipe> {

    private Ingredient ingredient;

    public ContainsIngredientComparator(Ingredient ingredient) {
        this.ingredient = ingredient;
    }   

    @Override
    public int compare(Recipe r1, Recipe r2) {
        if (r1.getIngredients().contains(ingredient)) {
            if (r2.getIngredients().contains(ingredient)) {
                return 0;
            } else {
                return -1;
            }
        } else if (r2.getIngredients().contains(ingredient)) {
            return 1;
        } else {
            return 0;
        }
    }
}

Now, you can create a Comparator for each ingredient and chain all the comparators. You can use the Stream API to map the favorites ingredients to comparators and use thenComparing to chain the comparators:

Optional<ContainsIngredientComparator> favoritesComparator = favorites.stream()
        .map(ContainsIngredientComparator::new)
        .reduce((c1, c2) -> (ContainsIngredientComparator) c1.thenComparing(c2));

You can use the result Comparator to sort the list:

List<Recipe> recipeList = // ... get the recipeList
if (favoritesComparator.isPresent()) {
    recipeList.sort(favoritesComparator.get());
}

Or just `Comparator.comparing(r -> !r.getIngredients().contains(ingredient))` — Holger, May 05 '20 at 09:59

How do I sort an Arraylist so its collection is sorted by a list of values

2 Answers2