Let’s take a look at the following case:
//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);</student></student>
1. Reasonable use of map operations
In actual development, reasonable use of map’s own methods can solve many problems
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); } }</arrearagedeal></arrearagedeal>
2. Use guava’s Multimap
Multimap<string> mulMap = ArrayListMultimap.create(); for (Student stu : list) { mulMap.put(stu.getGrade,stu); }</string>
3. Use the new features of jdk8 – don’t reject new things
After all, java14 has come out, and you still need to know more about the new features of java8
//一行就可以解决 Map<string>> collect = list.stream().collect(Collectors.groupingBy(ArrearageDeal::getGrade));</string>
From the perspective of code volume among the above three, java8 was the simplest. However, in actual development, both 2 and 3 are good choices based on specific scenarios.
Java8 Multiple Field Group Statistics
// 分组统计 Map<string> 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());</record></string>
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