@Data
        class Student{
            public Integer age;
            public String name;
            public String sex;
        }
        List<Student> students = new ArrayList<>();
        Student student1 = new Student();
        student1.setAge(18);
        student1.setName("趙");
        student1.setSex("男");
        Student student2 = new Student();
        student2.setAge(19);
        student2.setName("錢(qián)");
        student2.setSex("女");
        Student student3 = new Student();
        student3.setAge(20);
        student3.setName("孫");
        student3.setSex("女");
        Collections.addAll(students,student1,student2,student3);
        //根據(jù)條件過(guò)濾 出所有的女同學(xué)
        List<Student> result = students.stream().filter(student -> student.getSex().equals("女")).collect(Collectors.toList());
        System.out.println(result);

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...
}

List<Order> orders = Lists.newArrayList();
??
// 篩選總金額大于1000的訂單
orders = orders.stream().filter(item -> item.getAllAmt() > 1000.00f).collect(Collectors.toList());

List<Order> orders = Lists.newArrayList();
??
// 按照訂單類(lèi)型分組
Map<String, List<Order>> orderGroupMap = orders.stream().collect(Collectors.groupingBy(Order::getType));

List<Order> orders = Lists.newArrayList();
??
// 按照訂單編號(hào)去重
orders = orders.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(()
? ? ? ? ? -> new TreeSet<>(Comparator.comparing(Order::getNum))), ArrayList::new));
?
// 按照訂單編號(hào)和類(lèi)型去重
orders = orders.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(()
? ? ? ? ? -> new TreeSet<>(Comparator.comparing(o -> o.getNum() + ";" + o.getType()))), ArrayList::new));

List<Order> orders = Lists.newArrayList();
??
// 將訂單集合轉(zhuǎn)換成訂單編號(hào)-應(yīng)付金額 map，注意訂單編號(hào)作為 key 不能重復(fù)，應(yīng)先做去重處理
Map<String, Float> numPayMap = orders.stream().collect(Collectors.toMap(Order::getNum, Order::getPayAmt));
?
// 用 id 做 key 將 List 轉(zhuǎn)成 Map
Map<Long, Order> orderMap = orders.stream().collect(Collectors.toMap(Order::getId, item -> item));

List<Order> orders = Lists.newArrayList();
??
? // 按照訂單總金額從高到低排序
? // 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 字段不能為 null， null 會(huì)導(dǎo)致排序失敗)
? orders.sort(Comparator.comparing(Order::getAllAmt).reversed());
?
? // 先按照訂單類(lèi)型排序，再按照訂單應(yīng)付金額從高到低排序
? 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()))));

List<Order> orders = Lists.newArrayList();
??
? // 統(tǒng)計(jì)所有訂單的總金額
? // 求和
? Double sum = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).sum();
?
? // 最大總金額
? OptionalDouble max = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).max();
? // 防止沒(méi)有訂單數(shù)據(jù)的處理
? Double maxAllAmt = max.isPresent() ? max.getAsDouble() : 0;
??
? // 最小總金額
? OptionalDouble min = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).min();
?
? // 平均總金額
? OptionalDouble average = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).average();