如何使用比较器在 Treemap 中获取计数

Question

I have created a Dog class with String breed and int ageinmonths.

String breed;
int ageInMonths;
public String getBreed() {
    return breed;
}
public void setBreed(String breed) {
    this.breed = breed;
}
public int getAgeInMonths() {
    return ageInMonths;
}
public void setAgeInMonths(int ageInMonths) {
    this.ageInMonths = ageInMonths;
}
public Dog(String breed, int ageInMonths) {
    this.breed = breed;
    this.ageInMonths = ageInMonths;
}

public String toString() {
    return this.getBreed()+" - Age : "+
    this.getAgeInMonths()+" - Count : ";
}

In main method i have used comparator.

    public static void main(String[] args) {
    Dog dog1 = new Dog("Companion", 10);
    Dog dog2 = new Dog("Herding", 4);
    Dog dog3 = new Dog("Terrier", 15);
    Dog dog4 = new Dog("Companion", 7);
    Dog dog5 = new Dog("Terrier", 15);
    Dog dog6 = new Dog("Terrier", 9);
    Dog dog7 = new Dog("Herding", 10);
    Dog dog8 = new Dog("Herding", 10);

    TreeMap<Integer, Dog> dogDetails = new TreeMap<Integer, Dog>();
    dogDetails.put(1, dog1);
    dogDetails.put(2, dog2);
    dogDetails.put(3, dog3);
    dogDetails.put(4, dog4);
    dogDetails.put(5, dog5);
    dogDetails.put(6, dog6);
    dogDetails.put(7, dog7);
    dogDetails.put(8, dog8);
    System.out.println("The dog details are given below : "+dogDetails);


}
@Override
public int compare(Dog dog1, Dog dog2) {
    return dog1.breed.compareTo(dog2.breed);
}

But how can i get the count of the unique objects. I wanted output like mentioned below: Companion - Age: 10 - Count: 1 Herding - Age: 10 - Count: 2

Answer 1

you could use a stream to group the dogs by breed and eventually group them by age in a map (breed) of a map(age) like this:

Map<String, TreeMap<Integer, Long>> collect = Stream.of(dog1, dog2, dog3, dog4, dog5, dog6, dog7, dog8)
   .collect(groupingBy(Dog::getBreed,
               groupingBy(Dog::getAgeInMonths, TreeMap::new, counting())));

then you get an output like:

{Companion={7=1, 10=1}, Terrier={9=1, 15=2}, Herding={4=1, 10=2}}

Answer 2

Define map as:

Map<DogName , Map<DogAge , Count>>

Implementation:

Map<String , Map<Integer , Integer> > map = new HashMap<>();

// When inputting for object dog1 : You can write this as a function
Map<Integer , Integer> innerMap = map.get(dog1);
if(innerMap != null) {
  innerMap.put(dog1.getAge , innerMap.getOrDefault(dog1.getAge , 1));
} 
else if(innerMap == null) {
  innerMap = new HashMap<>();
  innerMap.put(dog1.getAge() , 1);
}
map.put(dog1.getBreed() , innerMap);

// For Printing the output : 
for(Map.Entry<String , Map<Integer , Integer>> entry : map.entrySet()) {
 for(Map.Entry<Integer , Integer> innerEntry : entry.getValue().entrySet()) {
   System.out.println(entry.getKey()  + " - Age :  " + innerEntry.getKey() + " - count : " + innerEntry.getValue());
}}

Hope this helps, wrote this as per my understanding.