关于Java8 Stream的简单实用记录

Java8 stream的使用
1、集合对象取某个属性形成集合
List<String> knows = examPaperList.stream().map(TestExam::getKnowPoint).collect(Collectors.toList());
2、统计上述String的个数形成Map
Map<String, Long> mapKnow = knows.stream().collect(groupingBy(p -> p,Collectors.counting()));
3、集合对象去重累加形成Map
Map<String, Double> resultFull = examPaperList.stream().collect(Collectors.toMap( TestExam::getKnowPoint, TestExam::getFullScore,(v1,v2)->{ return v1+v2;} ));
4、Map遍历新方法(遍历上述map):
resultFull.foreach((k,v)->{Systeam.out.println("key:"+k+"value:"+v)});
5、把集合中按照某个属性分成多个集合
List<SubjectToStudent> testSubjectList = wangYueTestMapper.queryTestSubjectOther(projectId,subject);
testSubjectList.stream().collect(Collectors.groupingBy(SubjectToStudent::getSubject,Collectors.toList()))
.forEach((subjects,fooListByAge)->{
groupList.add(fooListByAge);
});
6、集合差集
list=list1.stream().filter(t-> !list2.contains(t)).collect(Collectors.toList());
7、集合交集
List<Integer> list=list1.stream().filter(t->list2.contains(t)).collect(Collectors.toList());
8、统计集合重复元素出现次数，并且去重返回hashmap
Map<String, Long> map = list.stream().
collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
System.out.println(map);
9、由于hashmap无序，所以在排序放入LinkedHashMap里(key升序)
Map<String, Long> sortMap = new LinkedHashMap<>();
map.entrySet().stream().sorted(Map.Entry.comparingByKey()).
forEachOrdered(e -> sortMap.put(e.getKey(), e.getValue()));
System.out.println(sortMap);
10、获取排序后map的key集合
List<String> keys = new LinkedList<>();
sortMap.entrySet().stream().forEachOrdered(e -> keys.add(e.getKey()));
System.out.println(keys);
11、获取排序后map的value集合
List<Long> values = new LinkedList<>();
sortMap.entrySet().stream().forEachOrdered(e -> values.add(e.getValue()));
System.out.println(values);
12、集合某属性求和
Double sum = investorList.stream().mapToDouble(n -> CommonUtils.isNumeric(n.getInvestMoney()) ?
Double.parseDouble(n.getInvestMoney()) : 0.00).summaryStatistics().getSum();
13、集合按属性降序输出（升序去掉reversed()方法）
List<TestListOut> newList = unique.stream().sorted(Comparator.comparing(TestListOut::getPublishTime).reversed())
.collect(Collectors.toList());