Sort an arrayList with multiple objects

Question

I made a list of Animals as follows :

        ArrayList<Animal> animals = new ArrayList<Animal>();
        animals.add(new Animal(1, "animal1", 50, "10 Janvier 2016", "Noir", 4, true));
        animals.add(new Animal(2, "animal2", 50, "10 Janvier 2016", "Noir", 4, true));
        animals.add(new Animal(3, "animal3", 50, "10 Janvier 2016", "Noir", 4, true));
        animals.add(new Animal(4, "animal4", 50, "10 Janvier 2016", "Noir", 4, true));
        animals.add(new Animal(5, "animal5", 50, "10 Janvier 2016", "Noir", 4, true));

I want to sort my list of animals in ArrayList by their ID. From what i've seen i have to use a comparator.

This is what i created so far...

public class ComparatorAnimal implements Comparator<Animal> {

    public int compare(Animal animals.get(0), Animal animals.get(1) {
        return animals.get(0).idAnimal - animals.get(1).idAnimal;
    }

The Comperator interface actually tells you the signature of the method that you do have to implement. If you don´t have a method with this signature inside your class then it would not compile, which i guess is the case right now. Well and you can´t name your variables like animals.get(0) — SomeJavaGuy
– SomeJavaGuy, Commented Jan 11, 2016 at 14:37
This type of questions have been asked and answered multiple times. It's a duplicate question. — OPK
– OPK, Commented Jan 11, 2016 at 15:19

Gianni B. · Accepted Answer · 2016-01-11 14:44:20Z

8

public class ComparatorAnimal implements Comparator<Animal> {

public int compare(Animal animals.get(0), Animal animals.get(1) {
    return animals.get(0).idAnimal - animals.get(1).idAnimal;
}

The method signature is wrong: you are not comparing two lists of Animal but two Animal object. Then you are comparing two id, you don't need to subtract it. Just use the same method from the Integer class.

Change your method like this:

public class ComparatorAnimal implements Comparator<Animal> {

public int compare(Animal o1, Animal o2) {
    return Integer.compare(o1.idAnimal, o2.idAnimal);
}

Now you have to use a sorted collection (like TreeMap instead of ArrayList) or invoke Collections.sort(yourList, yourComparator)

edited Jan 11, 2016 at 14:44

answered Jan 11, 2016 at 14:30

Gianni B.

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Mohammed Aouf Zouag · Accepted Answer · 2016-01-11 14:35:25Z

2

Change

public int compare(Animal animals.get(0), Animal animals.get(1) {
    return animals.get(0).idAnimal - animals.get(1).idAnimal;
}

to

public int compare(Animal animal1, Animal animal2 {
    if(animal1.idAnimal > animal2.idAnimal)
        return 1;
    else if(animal1.idAnimal < animal2.idAnimal)
        return -1;

    return 0;
}

& then use

Collections.sort(animals, new ComparatorAnimal());

edited Jan 11, 2016 at 14:35

answered Jan 11, 2016 at 14:30

Mohammed Aouf Zouag

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