迭代ArrayList以按对象属性计数

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class CountByProperty {
    public static void main(String[] args) {
        // 创建一个包含对象的ArrayList
        List<Person> personList = new ArrayList<>();
        personList.add(new Person("Alice", 25));
        personList.add(new Person("Bob", 30));
        personList.add(new Person("Charlie", 25));
        personList.add(new Person("David", 30));
        personList.add(new Person("Alice", 30));

        // 创建一个Map来存储属性计数结果
        Map<Integer, Integer> countMap = new HashMap<>();

        // 遍历ArrayList，按属性计数
        for (Person person : personList) {
            int age = person.getAge();
            if (countMap.containsKey(age)) {
                countMap.put(age, countMap.get(age) + 1);
            } else {
                countMap.put(age, 1);
            }
        }

        // 打印计数结果
        for (Map.Entry<Integer, Integer> entry : countMap.entrySet()) {
            System.out.println("Age " + entry.getKey() + ": " + entry.getValue());
        }
    }

    static class Person {
        private String name;
        private int age;

        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public int getAge() {
            return age;
        }
    }
}