Java-Java集合流操作

List分组

List去重

1、单字段去重

2、多字段去重

List交集方法retainAll

List 分组去重获取最新记录

List 分组去重获取最新记录

修订记录版本是否发布

2020-01-25v1.0是

2021-03-19v1.1是

List分组

Map<String,List<ClassEntity>> = classEntities.stream().collect(Collectors.groupingBy(ClassEntity::getGrade));List去重

1、单字段去重

petList = petList.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(                () -> new TreeSet<>(Comparator.comparing(PetsPetInfo::getUserId))), ArrayList::new));2、多字段去重

java8去重(根据年级和专业,当年级和专业都相同的情况下看做是重复数据)

List<ClassEntity> distinctClass = classEntities.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(o -> o.getProfessionId() + ";" + o.getGrade()))), ArrayList::new));通过hashSet去重(如将classNames去重):该种去重是bean完全相同的时候算重复数据

List<String> classNameList = new ArrayList(new HashSet(classNames));List交集方法retainAll

public class Test {    public static void main(String[] args) {        List<String> list1 = new ArrayList<String>();        List<String> list2 = new ArrayList<String>();        List<String> list3 = new ArrayList<String>();        for (int i = 0; i < 20; i++) {            list1.add(i+"");            if(i%2 == 0) {                list2.add(i+"");            }            list3.add(i+"@");        }        // list1 与 list2 存在相同元素，list1集合只保留list2中存在的元素        list1.retainAll(list2);        if(list1.isEmpty()) {            System.out.println("不包含");        } else {            System.out.println("包含");        }        System.out.println(list1);        // list1 与 list3 不存在相同元素，list1集合变为空        list1.retainAll(list3);        if(list1.isEmpty()) {            System.out.println("不包含");        } else {            System.out.println("包含");        }        System.out.println(list1);    }}List 分组去重获取最新记录

public static void main(String[] args) throws InterruptedException {    List<VehicleOrder> vehicleOrderBqList = Lists.newArrayList();    VehicleOrder v1 = new VehicleOrder();    v1.setDealerCode("BQ0062");    v1.setCustomerPhone("13101030009");    v1.setReceivedTotalMoney(new BigDecimal(0));    v1.setUpdateTime(DateUtils.getNowLocalDateTime());    Thread.sleep(2000);    VehicleOrder v2 = new VehicleOrder();    v2.setDealerCode("BQ0062");    v2.setCustomerPhone("13101030009");    v2.setReceivedTotalMoney(new BigDecimal(0));    v2.setUpdateTime(DateUtils.getNowLocalDateTime());    Thread.sleep(2000);    VehicleOrder v3 = new VehicleOrder();    v3.setDealerCode("BQ0062");    v3.setCustomerPhone("13101030009");    v2.setReceivedTotalMoney(new BigDecimal(100));    v3.setUpdateTime(DateUtils.getNowLocalDateTime());    Thread.sleep(2000);    VehicleOrder v4 = new VehicleOrder();    v4.setDealerCode("BQ0062");    v4.setCustomerPhone("13101030009");    v4.setReceivedTotalMoney(new BigDecimal(500));    v4.setUpdateTime(DateUtils.getNowLocalDateTime());    Thread.sleep(2000);    vehicleOrderBqList.add(v1);    vehicleOrderBqList.add(v2);    vehicleOrderBqList.add(v3);    vehicleOrderBqList.add(v4);    //方式一：分组随机去重/*    List<VehicleOrder> distinctList = vehicleOrderBqList.parallelStream()        .collect(Collectors.collectingAndThen(Collectors.toCollection(            () -> new TreeSet<>(Comparator.comparing(en -> en.getDealerCode() + ";" + en.getCustomerPhone()))),            ArrayList::new));*/    //方式二：分组去重，根据UpdateTime获取最新记录(去重规则)    List<VehicleOrder> distinctList = Lists.newArrayList();    distinctList = vehicleOrderBqList.parallelStream()        .collect(Collectors.groupingBy(en -> en.getDealerCode() + ";" + en.getCustomerPhone())).values()        .parallelStream().map(list -> list.parallelStream().max(Comparator.nullsLast(Comparator.comparing(VehicleOrder::getUpdateTime))).orElse(null)        ).collect(Collectors.toList());   //方式三：分组去重，根据UpdateTime获取最新记录(去重规则)/*    vehicleOrderBqList.parallelStream()        .collect(Collectors.groupingBy(en -> en.getDealerCode() + ";" + en.getCustomerPhone())).forEach((k, v) -> {      VehicleOrder vvv = v.parallelStream()          .max(Comparator.nullsLast(Comparator.comparing(VehicleOrder::getUpdateTime))).orElse(null);      if (Optional.ofNullable(vvv).isPresent()) {        distinctList.add(vvv);      }      ;    });*/    System.out.println(distinctList);  }List 分组去重获取最新记录

// 若value为空，设置默认值""Map<String, String> map1 = list.stream().collect(                Collectors.toMap(i -> i.getName(), i-> Optional.ofNullable(i.getSex()).orElse(""), (v1, v2) -> v1));// 调用hashMap putAll方法， 注意key相同时，value会覆盖。Map<String, String> map2 = list.stream().collect(                HashMap::new, (m, i) -> m.put(i.getName(), i.getSex()), HashMap::putAll);