java统计数量

import java.util.ArrayList;
import java.util.List;

public class ListCountExample {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Alice");

        long count = names.stream().filter(name -> name.equals("Alice")).count();
        System.out.println("Alice count: " + count); // 输出: Alice count: 2
    }
}

import java.util.HashSet;
import java.util.Set;

public class SetCountExample {
    public static void main(String[] args) {
        Set<String> uniqueNames = new HashSet<>();
        uniqueNames.add("Alice");
        uniqueNames.add("Bob");
        uniqueNames.add("Alice"); // 不会被添加，因为Set不允许重复

        long count = uniqueNames.stream().filter(name -> name.equals("Alice")).count();
        System.out.println("Alice count: " + count); // 输出: Alice count: 1
    }
}

import java.util.HashMap;
import java.util.Map;

public class MapCountExample {
    public static void main(String[] args) {
        Map<String, Integer> nameCountMap = new HashMap<>();
        String[] names = {"Alice", "Bob", "Alice", "Charlie"};

        for (String name : names) {
            nameCountMap.put(name, nameCountMap.getOrDefault(name, 0) + 1);
        }

        System.out.println("Alice count: " + nameCountMap.get("Alice")); // 输出: Alice count: 2
    }
}

import java.util.concurrent.ConcurrentHashMap;
import java.util.Map;

public class ConcurrentMapCountExample {
    public static void main(String[] args) {
        Map<String, Integer> concurrentMap = new ConcurrentHashMap<>();
        // 假设这里有很多数据插入操作
        concurrentMap.put("Alice", concurrentMap.getOrDefault("Alice", 0) + 1);

        System.out.println("Alice count: " + concurrentMap.get("Alice"));
    }
}

if (name != null) {
    nameCountMap.put(name, nameCountMap.getOrDefault(name, 0) + 1);
}