参考简述网址: java8 List 根据对象某个字段或多个字段去重、筛选、List转Map、排序、分组、统计计数等等

list简单方法示例如下：

一、先定义一个订单对象（Order）

public class Order {private Long id;private Long userId;private String num;private String type;private Float allAmt;private Float payAmt;private Integer orderNum;public Order(Long id, Long userId, String num, String type, Float allAmt, Float payAmt, Integer orderNum) {this.id = id;this.userId = userId;this.num = num;this.type = type;this.allAmt = allAmt;this.payAmt = payAmt;this.orderNum = orderNum;}// getting...  setting...
}

二、过滤筛选：

  List<Order> orders = Lists.newArrayList();// 筛选总金额大于1000的订单orders = orders.stream().filter(item -> item.getAllAmt() > 1000.00f).collect(Collectors.toList());

三、分组

  List<Order> orders = Lists.newArrayList();// 按照订单类型分组Map<String, List<Order>> orderGroupMap = orders.stream().collect(Collectors.groupingBy(Order::getType));

四、去重

去重并排序参考TreeSet: java TreeSet去重与排序入门

去重: Java中List集合对象去重及按属性去重

四、List 转 Map

  List<Order> orders = Lists.newArrayList();// 将订单集合转换成订单编号-应付金额 map，注意订单编号作为 key 不能重复，应先做去重处理Map<String, Float> numPayMap = orders.stream().collect(Collectors.toMap(Order::getNum, Order::getPayAmt));// 用 id 做 key 将 List 转成 MapMap<Long, Order> orderMap = orders.stream().collect(Collectors.toMap(Order::getId, item -> item));

五、排序

详细看比较器排序: Java Comparator使用指南

 List<Order> orders = Lists.newArrayList();// 按照订单总金额从高到低排序// 方式一orders.sort((o1, o2)-> o1.getAllAmt() == null ? 1 : (o2.getAllAmt() == null ? -1 : o2.getAllAmt().compareTo(o1.getAllAmt())));// 方式二orders.sort(Comparator.comparing(Order::getAllAmt, (o1, o2)-> o1 == null ? 1 : (o2 == null ? -1 : o2.compareTo(o1))));// 方式三 (allAmt 字段不能为 null， null 会导致排序失败)orders.sort(Comparator.comparing(Order::getAllAmt).reversed());// 先按照订单类型排序，再按照订单应付金额从高到低排序orders.sort(Comparator.comparing(Order::getType, (o1, o2)-> o1 == null ? 1 : (o2 == null ? -1 : o1.compareTo(o2))).thenComparing((o1, o2)-> o1.getPayAmt() == null ? 1 : (o2.getPayAmt() == null ? -1 : o2.getPayAmt().compareTo(o1.getPayAmt()))));

六、统计计数

  List<Order> orders = Lists.newArrayList();// 统计所有订单的总金额// 求和Double sum = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).sum();// 最大总金额OptionalDouble max = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).max();// 防止没有订单数据的处理Double maxAllAmt = max.isPresent() ? max.getAsDouble() : 0;// 最小总金额OptionalDouble min = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).min();// 平均总金额OptionalDouble average = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).average();

七、List 转 List (返回另一个对象)

//声明一个List集合 
List<Student> list = new ArrayList();list.add(new Student("1001", "小A"));list.add(new Student("1001", "小B"));list.add(new Student("1002", "小C"));list.add(new Student("1003", "小D"));//将List转换List   【不过滤，不去重】 
List<Teacher> teacherList = list.stream().map(stu -> {Teacher teacher = new Teacher();teacher.setNo(stu.getNo());teacher.setName(stu.getName());return teacher;}).collect(Collectors.toList());//将List转换List  【过滤，不去重】 
List<Teacher> teacherList2 =list.stream().filter( stu -> StringUtils.isNotBlank(stu.getName()) ).map(stu -> {Teacher teacher = new Teacher();teacher.setNo(stu.getNo());teacher.setName(stu.getName());return teacher;}).collect(Collectors.toList());

八、List 转 List (返回另一个对象) （多重集合，取里面集合，并合并)

//声明一个List集合
List<Student> list = new ArrayList();  list.add(new Student("1001", "小A"));  list.add(new Student("1001", "小B"));list.add(new Student("1002", "小C"));list.add(new Student("1003", "小D"));
List<Subject> subjectList = new ArrayList<>();Subject subject1 = new Subject();subject1.setId(1);subject1.setStudentList(studentList);subjectList.add(subject1);Subject subject2 = new Subject();subject1.setId(2);subject1.setStudentList(studentList);subjectList.add(subject2);Subject subject3 = new Subject();subject1.setId(3);subject1.setStudentList(studentList);subjectList.add(subject3);//将List转换List 【flatMap取集合中的集合 再合并】  (本案例：把所有科目下的报名学生，取出来，并合并。注意：学生没有去重)
List<Student> studentList =subjectList.stream().flatMap( item -> item.getStudentList().stream() ).collect(Collectors.toList());