如何使用比较器在Treemap中获取计数。

Question

我创建了一个Dog类，里面有String breed和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 : ";
}

在主方法中，我使用了比较器。

    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);
}

但我如何才能得到唯一对象的计数。我想得到如下的输出：Companion - Age : 10 - Count : 1 Herding - Age : 10 - Count : 2。

Answer 1

你可以使用一个流来按品种对狗进行分组，并最终在map(年龄)的map(breed)中按年龄对它们进行分组，就像这样。

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())));

然后你会得到这样的输出：

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

Answer 2

定义map为:

Map<DogName , Map<DogAge , Count>>

实现。

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());
}}

希望能帮到你 ，按照我的理解写了这个。