平时工作中,很多时候都会用到对数据进行分组操作,例如一个学生对象,有班级、名字、性别、分数等,需要按班级分组统计,该怎么操作呢?一个合理的算法可以提升不少效率。
大家看一下下面的案例:
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//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
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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
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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
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//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
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// 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()); |
以上为个人经验,希望能给大家一个参考,也希望大家多多支持服务器之家。
原文链接:https://blog.csdn.net/qq_19642249/article/details/103826466