How to use Collections.sort() in Java?

Question

I got an object Recipe that implements Comparable<Recipe> :

public int compareTo(Recipe otherRecipe) {
    return this.inputRecipeName.compareTo(otherRecipe.inputRecipeName);
}

I've done that so I'm able to sort the List alphabetically in the following method:

public static Collection<Recipe> getRecipes(){
    List<Recipe> recipes = new ArrayList<Recipe>(RECIPE_MAP.values());
    Collections.sort(recipes);
    return recipes;
}

But now, in a different method, lets call it getRecipesSort() , I want to sort the same list but numerically, comparing a variable that contains their ID. To make things worse, the ID field is of the type String .

How do I use Collections.sort() to perform the sorts in Java?

Answer 1

Use this method Collections.sort(List,Comparator) . Implement a Comparator and pass it to Collections.sort().

class RecipeCompare implements Comparator<Recipe> {

    @Override
    public int compare(Recipe o1, Recipe o2) {
        // write comparison logic here like below , it's just a sample
        return o1.getID().compareTo(o2.getID());
    }
}

Then use the Comparator as

Collections.sort(recipes,new RecipeCompare());

Answer 2

The answer given by NINCOMPOOP can be made simpler using Lambda Expressions:

Collections.sort(recipes, (Recipe r1, Recipe r2) ->
r1.getID().compareTo(r2.getID()));

Also introduced after Java 8 is the comparator construction methods in the Comparator interface. Using these, one can further reduce this to ¹ :

recipes.sort(comparingInt(Recipe::getId));

---

¹ Bloch, J. Effective Java (3 ^rd Edition). 2018. Item 42, p. 194.

Answer 3

Create a comparator which accepts the compare mode in its constructor and pass different modes for different scenarios based on your requirement

public class RecipeComparator implements Comparator<Recipe> {

public static final int COMPARE_BY_ID = 0;
public static final int COMPARE_BY_NAME = 1;

private int compare_mode = COMPARE_BY_NAME;

public RecipeComparator() {
}

public RecipeComparator(int compare_mode) {
    this.compare_mode = compare_mode;
}

@Override
public int compare(Recipe o1, Recipe o2) {
    switch (compare_mode) {
    case COMPARE_BY_ID:
        return o1.getId().compareTo(o2.getId());
    default:
        return o1.getInputRecipeName().compareTo(o2.getInputRecipeName());
    }
}

}

Actually for numbers you need to handle them separately check below

public static void main(String[] args) {
    String string1 = "1";
    String string2 = "2";
    String string11 = "11";

    System.out.println(string1.compareTo(string2)); 
    System.out.println(string2.compareTo(string11));// expected -1 returns 1
   // to compare numbers you actually need to do something like this

    int number2 = Integer.valueOf(string1);
    int number11 = Integer.valueOf(string11);

    int compareTo = number2 > number11 ? 1 : (number2 < number11 ? -1 : 0) ;
    System.out.println(compareTo);// prints -1
}

Answer 4

Use the method that accepts a Comparator when you want to sort in something other than natural order.

Collections.sort(List, Comparator)

Answer 5

Sort the unsorted hashmap in ascending order.

// Sorting the list based on values
Collections.sort(list, new Comparator<Entry<String, Integer>>() {
public int compare(Entry<String, Integer> o1, Entry<String, Integer> o2) 
{
                return o2.getValue().compareTo(o1.getValue());
        }
    });

    // Maintaining insertion order with the help of LinkedList
    Map<String, Integer> sortedMap = new LinkedHashMap<String, Integer>();
    for (Entry<String, Integer> entry : list) {
        sortedMap.put(entry.getKey(), entry.getValue());
    }