Hashmap Sorting in ascending order

public class Main {
    public static void main(String[] args)  {

        Dog dog1 = new Dog();
        dog1.setName("Lab");
        dog1.setAge(3);

        Dog dog2 = new Dog();
        dog2.setName("Beagle");
        dog2.setAge(1);

        Dog dog3 = new Dog();
        dog3.setName("Golden");
        dog3.setAge(6);

        Dog dog4 = new Dog();
        dog4.setName("German");
        dog4.setAge(4);

        Dog dog5 = new Dog();
        dog5.setName("Husky");
        dog5.setAge(2);

        List<Dog> dogList = new ArrayList<>();
        dogList.add(dog1);
        dogList.add(dog2);
        dogList.add(dog3);
        dogList.add(dog4);
        dogList.add(dog5);
        Collections.sort(dogList);
        System.out.println(dogList);

        List<Cat> catList = new ArrayList<>();
        catList.add(new Cat("cat1", 2));
        catList.add(new Cat("cat2", 6));
        catList.add(new Cat("cat3", 7));
        catList.add(new Cat("cat4", 4));
        catList.add(new Cat("cat5", 3));
        Collections.sort(catList);
        System.out.println(catList);
        
        Map<Integer, Integer> ageMap = new HashMap<>();
  }
}

After sorting, I would like to insert the following key-value pairs in a HashMap - <Cat age, Dog age>.

Providing that both lists catList and dogList are properly sorted by age, the map can be populated with a simple loop.

for (int i = 0, n = Math.min(catList.size(), dogList.size()); i < n; i++) {
    Cat cat = catList.get(i);
    Dog dog = dogList.get(i);
    ageMap.put(cat.getAge(), dog.getAge());
}

Solution with iterators:

Iterator<Cat> itCat = catList.iterator();
Iterator<Dog> itDog = dogList.iterator();

while (itCat.hasNext() && itDog.hasNext()) {
    ageMap.put(itCat.next().getAge(), itDog.next().getAge());
}

Note: in order to maintain the order of entries in the ageMap it is better to replace HashMap with TreeMap which is sorted by keys, or LinkedHashMap which maintains order of insertion, but in our case the insertion will be ordered.

Map<Integer, Integer> ageMap = new TreeMap<>();

This is the ideal way to do this in java 8. Apologies for typo if any, in syntax.

List<Dog> dogList = 
list.sort(Comparator.comparing(Dog::getAge));
List<Cat> catList = 
list.sort(Comparator.comparing(Cat::getAge));
Map<Integer, Integer> ageMap = new HashMap<>();
int index= catList.size()-1;
if(dogList.size()==catList.size()){
dogList.forEach(dog -> {
ageMap.put(dog.getAge(), catList.get(index).getAge());
index--;
});
}