大家看一下下面的案例：
//下面是初始化的数据
List<Student> list = new ArrayList<Student>();
Student student1 = new Student("李四1", "女", "一班");
Student student2 = new Student("李四2", "女", "一班");
Student student3 = new Student("李四3", "女", "一班");
Student student4 = new Student("李四4", "男", "一班");
Student student5 = new Student("李四5", "男", "一班");
Student student6 = new Student("李四6", "男", "二班");
Student student7 = new Student("李四7", "男", "二班");
Student student8 = new Student("李四8", "男", "二班");
Student student9 = new Student("李四9", "男", "二班");
list.add(student1);
list.add(student2);
list.add(student3);
list.add(student4);
list.add(student5);
list.add(student6);
list.add(student7);
list.add(student8);
list.add(student9);1.合理利用map操作
在实际开发中合理的利用map自带的方法，能解决很多问题
for (Student stu : list) {
if (!map.containsKey(stu.getProvinceCode())) {
ArrayList<ArrearageDeal> al = new ArrayList<ArrearageDeal>();
map.put(stu.getProvinceCode(), al.add(stu));
} else {
map.get(stu.getProvinceCode()).add(stu);
}
}2.利用guava的Multimap
Multimap<String, Student> mulMap = ArrayListMultimap.create();
for (Student stu : list) {
mulMap.put(stu.getGrade,stu);
}3.使用jdk8新特性–不要排斥新东西
毕竟java14都出来了，java8的新特性还是需要多了解
//一行就可以解决 Map<String, List<Student >> collect = list.stream().collect(Collectors.groupingBy(ArrearageDeal::getGrade));
上面三种当时从代码量上来看，java8的最简洁。但是实际开发中结合具体场景来说2、3两种都是不错的选择。
Java8 多个字段分组统计
// 分组统计
Map<String, Long> countMap = records.stream().collect(Collectors.groupingBy(o -> o.getProductType() + "_" + o.getCountry(), Collectors.counting()));
List<Record> countRecords = countMap.keySet().stream().map(key -> {
String[] temp = key.split("_");
String productType = temp[0];
String country = temp[1];
Record record = new Record();
record.set("device_type", productType);
record.set("location", country;
record.set("count", countMap.get(key).intValue());
return record;
}).collect(Collectors.toList()); 
