Java8新特性，List分组排序
1.分组（多字段分组）
Map<String,List<Fruit>> fruitMap = fruitList.stream().collect(Collectors.groupingBy(Fruit::getName));Map<String, List<CalbPrOutputSum>> map = outputSums.stream().collect(Collectors.groupingBy(t -> t.getModelNum()));Map<String, List<WorkPlanVo>> collect = value.stream().collect(Collectors.groupingBy(a -> MessageFormat.format("{0}#{1}#{2}#{3}#{4}", a.getWorkshop(),a.getProjectCode(), a.getProdLineCode(), a.getModelCode(), a.getOutputOp())));
2.使用年龄进行升序排序
List<StudentInfo> studentsSortName = studentList.stream().sorted(paring(StudentInfo::getAge)).collect(Collectors.toList());
3.使用年龄进行降序排序（使用reversed()方法）:
List<StudentInfo> studentsSortName = studentList.stream().sorted(paring(StudentInfo::getAge).reversed()).collect(Collectors.toList());
4. 使用年龄进行降序排序，年龄相同再使用身高升序排序
List<StudentInfo> studentsSortName = studentList.stream().sorted(paring(StudentInfo::getAge).reversed().thenComparing(StudentInfo::getHeight)).collect(Collectors.toList());
5.过滤
List<Employee> employees = new ArrayList<Employee>();employees.addAll(Arrays.asList(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10));Optional<Employee> first = employees.stream().filter(t -> "M".equals(t.getGender())).findFirst();List<Employee> collect = employees.stream().filter(t -> "M".equals(t.getGender())).collect(Collectors.toList());List<Employee> collect1 = employees.stream().filter(t -> "M".equals(t.getGender())).filter(t -> "Ricky".equals(t.getFirstName())).collect(Collectors.toList());
5.在使用Collectors.groupingBy分组时,如果分组的字段中有值为null,会抛出空指针异常
example:
class Stu{String age;String sex;public Stu(String age, String sex) {this.age = age;this.sex = sex;}}@Testvoid test1(){List<Stu> objects = new ArrayList<>();objects = Arrays.asList(new Stu("18",null),new Stu("19","男"),new Stu("17","女"));Map<String, List<Stu>> collect = objects.stream().collect(Collectors.groupingBy(x -> x.getSex()));System.out.println(collect);}输出：java.lang.NullPointerException: element cannot be mapped to a null key
解决办法：
@Testvoid test1(){List<Stu> objects = new ArrayList<>();objects = Arrays.asList(new Stu("18",null),new Stu("19","男"),new Stu("17","女"));Map<Optional<String>, List<Stu>> collect = objects.stream().collect(Collectors.groupingBy(x -> Optional.ofNullable(x.getSex())));System.out.println(collect);ArrayList<Stu> finalList = new ArrayList<Stu>();for (Map.Entry<Optional<String>, List<Stu>> optionalListEntry : collect.entrySet()) {//Optional<String> key = optionalListEntry.getKey();//key.isPresent()//分组的键不为空if (optionalListEntry.getKey().isPresent()){finalList.addAll(optionalListEntry.getValue());}}System.out.println(finalList);}输出：{Optional.empty=[Stu{age='18', sex='null'}], Optional[女]=[Stu{age='17', sex='女'}], Optional[男]=[Stu{age='19', sex='男'}]}[Stu{age='17', sex='女'}, Stu{age='19', sex='男'}]
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