Lambda 表达式练习

目录

sorted() 四种排序

List 转 Map

map 映射

对象中 String 类型属性为空的字段赋值为 null

BiConsumer,>

T reduce(T identity, BinaryOperator accumulator)

allMatch(Predicate p)

groupingBy(Function f)

flatMap(Function f)

Optional.ofNullable(T t) 和 T orElse(T other)

---

先来写个基本类

public class LambdaTest {@Dataclass Student {private Integer id;private String name;private Integer age;public Student(Integer id, String name, Integer age) {this.id = id;this.name = name;this.age = age;}}public List<Student> getStudents() {List<Student> list = new ArrayList<>();list.add(new Student(100, "小明", 10));list.add(new Student(101, "小红", 11));list.add(new Student(102, "小李", 12));list.add(new Student(103, "小王", 13));list.add(new Student(104, "小张", 14));list.add(new Student(105, "小五", 15));list.add(new Student(106, "小华", 16));return list;}
}

sorted() 四种排序

@Test
public void sortedTest() {List<Student> students = this.getStudents();//正序：常规写法List<Student> collect = students.stream().sorted((a1, a2) -> Integer.compare(a1.getAge(), a2.getAge())).collect(Collectors.toList());collect.forEach(System.out::println);System.out.println();//正序：lambda 简写List<Student> collectComparator = students.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());collectComparator.forEach(System.out::println);System.out.println();//倒序：常规写法List<Student> collectReversed = students.stream().sorted((a1, a2) -> Integer.compare(a2.getAge(), a1.getAge())).collect(Collectors.toList());collectReversed.forEach(System.out::println);System.out.println();//正序：lambda 简写List<Student> collectComparatorReversed = students.stream().sorted(Comparator.comparing(Student::getAge).reversed()).collect(Collectors.toList());collectComparatorReversed.forEach(System.out::println);
}

List 转 Map

@Test
public void listToMapTest() {List<Student> students = this.getStudents();Map<Integer, Student> collect = students.stream().collect(Collectors.toMap(Student::getId, s -> s, (s1, s2) -> s1));collect.keySet().forEach(key -> System.out.println(String.format("key：%s，value：%s", key, collect.get(key))));
}

map 映射

 @Testpublic void mapTest() {List<Student> students = this.getStudents();
//        List<Student> students = Lists.newArrayList(new Student(null, "小炎姬", 5));Integer id = students.stream().map(Student::getId).max(Integer::compareTo).orElse(0);System.out.println(id);}

对象中 String 类型属性为空的字段赋值为 null

public static <T> void stringEmptyToNull(T t) {Class<?> clazz = t.getClass();Field[] fields = clazz.getDeclaredFields();Arrays.stream(fields).filter(f -> f.getType() == String.class).filter(f -> {try {f.setAccessible(true);String value = (String) f.get(t);return StringUtils.isEmpty(value);} catch (Exception ignore) {return false;}}).forEach(field -> {try {field.setAccessible(true);field.set(t, null);} catch (Exception ignore) {}});
}

BiConsumer<T, U>

<!-- lombok -->
<dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>1.18.12</version>
</dependency><!-- guava -->
<dependency><groupId>com.google.guava</groupId><artifactId>guava</artifactId><version>28.2-jre</version>
</dependency>

/*** @author vincent*/
public class LambdaTest {@Dataprivate static class Student {private Integer id;private String name;private Integer age;private Integer interestType;private List<Hobby> hobbies;private List<Fancy> fancies;private Subject subject;private Student(Integer id, String name, Integer age, Integer interestType, List<Hobby> hobbies, List<Fancy> fancies, Subject subject) {this.id = id;this.name = name;this.age = age;this.interestType = interestType;this.hobbies = hobbies;this.fancies = fancies;this.subject = subject;}}enum InterestType {HOBBY(1, "兴趣"),FANCY(2, "喜好");private Integer code;private String message;InterestType(Integer code, String message) {this.code = code;this.message = message;}public Integer getCode() {return code;}public static InterestType getInterestTypeByCode(int code) {return Arrays.stream(InterestType.values()).filter(interestType -> interestType.getCode() == code).findFirst().orElse(null);}}@Dataprivate static class Hobby {private String basketball;private String running;private String drinking;private Hobby(String basketball, String running, String drinking) {this.basketball = basketball;this.running = running;this.drinking = drinking;}}@Dataprivate static class Fancy {private String dance;private String takePhotos;private String meetGirls;private Fancy(String dance, String takePhotos, String meetGirls) {this.dance = dance;this.takePhotos = takePhotos;this.meetGirls = meetGirls;}}@Dataprivate static class Subject {private String english;private String chinese;private String mathematics;private Subject(String english, String chinese, String mathematics) {this.english = english;this.chinese = chinese;this.mathematics = mathematics;}}private List<Student> getStudent() {List<Student> list = Lists.newArrayList();list.add(new Student(100, "小明", 10, 1,Lists.newArrayList(new Hobby("篮球", "跑步", "喝酒"),new Hobby("篮球_1", "跑步_1", "喝酒_1"),new Hobby("篮球_2", "跑步_2", "喝酒_2"),new Hobby("篮球_3", "跑步_3", "喝酒_3")),Lists.newArrayList(new Fancy("街舞", "摄影", "泡妞"),new Fancy("街舞_1", "摄影_1", "泡妞_1"),new Fancy("街舞_2", "摄影_2", "泡妞_2"),new Fancy("街舞_3", "摄影_3", "泡妞_3")),new Subject("英语", "语文", "数学")));list.add(new Student(200, "小红", 10, 2,Lists.newArrayList(new Hobby("篮球", "跑步", "喝酒"),new Hobby("篮球_1", "跑步_1", "喝酒_1"),new Hobby("篮球_2", "跑步_2", "喝酒_2"),new Hobby("篮球_3", "跑步_3", "喝酒_3")),Lists.newArrayList(new Fancy("街舞", "摄影", "泡妞"),new Fancy("街舞_1", "摄影_1", "泡妞_1"),new Fancy("街舞_2", "摄影_2", "泡妞_2"),new Fancy("街舞_3", "摄影_3", "泡妞_3")),new Subject("英语", "语文", "数学")));return list;}@Dataprivate static class Person {private Integer pid;private String pname;private Integer page;private String interest;private String subject;}private final static BiConsumer<Person, Student> HOBBY = (person, student) -> {Optional.ofNullable(student.getHobbies()).flatMap(hobbies -> hobbies.stream().findFirst()).ifPresent(hobby -> person.setInterest(hobby.getDrinking()));Optional.ofNullable(student.subject).ifPresent(subject -> person.setSubject(subject.getEnglish()));};private final static BiConsumer<Person, Student> FANCY = (person, student) -> {Optional.ofNullable(student.getFancies()).flatMap(fancies -> fancies.stream().findFirst()).ifPresent(fancy -> person.setInterest(fancy.getDance()));Optional.ofNullable(student.subject).ifPresent(subject -> person.setSubject(subject.getMathematics()));};private final static ImmutableMap<InterestType, BiConsumer<Person, Student>> OF = ImmutableMap.of(InterestType.HOBBY, HOBBY,InterestType.FANCY, FANCY);/*** BiConsumer<T, U> 实例*/@Testpublic void t() {List<Student> studentList = getStudent();List<Object> collect = studentList.stream().map(student -> {Person person = new Person();Optional.ofNullable(student).ifPresent(stu -> {person.setPid(stu.getId());person.setPname(stu.getName());person.setPage(stu.getAge());Integer interestType = student.getInterestType();InterestType interestTypeByCode = InterestType.getInterestTypeByCode(interestType);person.setInterest(interestTypeByCode.message);OF.get(interestTypeByCode).accept(person, student);});return person;}).collect(Collectors.toList());System.out.println(collect);}
}

T reduce(T identity, BinaryOperator<T> accumulator)

<!-- commons-lang3 -->
<dependency><groupId>org.apache.commons</groupId><artifactId>commons-lang3</artifactId><version>3.9</version>
</dependency>

/*** @author qfxl*/
public class LambdaTest {@Dataprivate static class trip {private String departure;private String destination;private trip(String departure, String destination) {this.departure = departure;this.destination = destination;}}/*** reduce 规则：* 例子：1.上海 -> 北京*      2.北京 -> 上海*      3.天津 -> 西安*      4.拉萨 -> 灵芝*      5.灵芝 -> 兰州*      6.兰州 -> 西宁* 展示效果：上海-北京-上海,天津-西安,拉萨-灵芝-兰州-西宁*/private static final BinaryOperator<String> ACCUMULATOR = (v1, v2) -> {if (StringUtils.isEmpty(v1)) {return v2;}String[] item = StringUtils.split(v1, ",");String[] lastOfItem = StringUtils.split(item[item.length - 1], "-");String lastElement = lastOfItem[lastOfItem.length - 1];String[] nextItem = StringUtils.split(v2, "-");String startElement = nextItem[0];if (StringUtils.equals(lastElement, startElement)) {return v1 + "-" + nextItem[nextItem.length - 1];}return v1 + "," + v2;};@Testpublic void t2() {List<trip> list = Lists.newArrayList(new trip("上海", "北京"),new trip("北京", "上海"),new trip("天津", "西安"),new trip("拉萨", "灵芝"),new trip("灵芝", "兰州"),new trip("兰州", "西宁"));//[上海-北京-上海,天津-西安,拉萨-灵芝-兰州-西宁]String reduce = list.stream().map(t -> String.format("%s-%s", t.getDeparture(), t.getDestination())).reduce("", ACCUMULATOR);System.out.println(reduce);}
}

allMatch(Predicate p)

public static final String YYYYMMDD = "yyyyMMdd";
public static final String YYYY_MM_DD = "yyyy-MM-dd";
public static final String YYYY_MM_DD_HH_MM = "yyyy-MM-dd HH:mm";
public static final String YYYY_MM_DD_HH_MM_SS = "yyyy-MM-dd HH:mm:ss";
public static final String YYYY__MM__DD = "yyyy/MM/dd";
public static final String YYYY__MM__DD_HH_MM = "yyyy/MM/dd HH:mm";
public static final String YYYY__MM__DD_HH_MM_SS = "yyyy/MM/dd HH:mm:ss";@Test
public void t3() {@Dataclass DateValidate {private String date;private String dateFormat;public DateValidate(String date, String dateFormat) {this.date = date;this.dateFormat = dateFormat;}}List<DateValidate> list = Lists.newArrayList(new DateValidate("202086", YYYYMMDD),new DateValidate("2020086", YYYYMMDD),new DateValidate("2020806", YYYYMMDD),new DateValidate("20200806", YYYYMMDD),new DateValidate("20200806 19", null),new DateValidate("20200806 19:00", null),new DateValidate("20200806 19:00:00", null),new DateValidate("2020-8-06", YYYY_MM_DD),new DateValidate("2020-08-6", YYYY_MM_DD),new DateValidate("2020-08-06", YYYY_MM_DD),new DateValidate("2020-08-06 19", null),new DateValidate("2020-08-06 19:00", YYYY_MM_DD_HH_MM),new DateValidate("2020-08-06 19:00:00", YYYY_MM_DD_HH_MM_SS),new DateValidate("2020/8/06", YYYY__MM__DD),new DateValidate("2020/08/6", YYYY__MM__DD),new DateValidate("2020/08/06", YYYY__MM__DD),new DateValidate("2020/08/06 19", null),new DateValidate("2020/08/06 19:00", YYYY__MM__DD_HH_MM),new DateValidate("2020/08/06 19:00:00", YYYY__MM__DD_HH_MM_SS));list.forEach(item -> {boolean matchDateTime = matchDateTime(item.getDate());System.out.println(item.getDate() + " (" + item.getDateFormat() + ")：matchDateTime -> " + matchDateTime);});
}/*** 时间校验* @param dateTime* @return*/
public static boolean matchDateTime(String dateTime) {if (StringUtils.isEmpty(dateTime)) {return false;}String[] dt = dateTime.split("\\s+");if (dt.length == 1) {return dateMatch(dt[0], "/") || dateMatch(dt[0], "-");} else {String date = dt[0];String time = dt[1];return (dateMatch(date, "/") || dateMatch(date, "-")) && timeMatch(time, ":");}
}private static boolean timeMatch(String s, String split) {if (StringUtils.isEmpty(s)) {return true;}s = StringUtils.trim(s);String[] time = StringUtils.split(s, split);boolean isNumber = Arrays.stream(time).anyMatch(StringUtils::isNumeric);if (!isNumber) {return false;}if (time.length != 3) {return false;}if (time[0].length() > 2 || Integer.parseInt(time[0]) > 24) {return false;}if (time[1].length() > 2 || Integer.parseInt(time[1]) > 60) {return false;}if (time[2].length() > 2 || Integer.parseInt(time[2]) > 60) {return false;}return true;
}private static boolean dateMatch(String s, String spl) {if (StringUtils.isEmpty(s)) {return false;}s = StringUtils.trim(s);String[] date = StringUtils.split(s, spl);boolean isNumber = Arrays.stream(date).anyMatch(StringUtils::isNumeric);if (!isNumber) {return false;}if (date.length != 3) {return false;}if (date[0].length() != 4) {return false;}if (Integer.parseInt(date[1]) > 12) {return false;}if (Integer.parseInt(date[2]) > 31) {return false;}return true;
}

显示结果

202086 (yyyyMMdd)：matchDateTime -> false
2020086 (yyyyMMdd)：matchDateTime -> false
2020806 (yyyyMMdd)：matchDateTime -> false
20200806 (yyyyMMdd)：matchDateTime -> false
20200806 19 (null)：matchDateTime -> false
20200806 19:00 (null)：matchDateTime -> false
20200806 19:00:00 (null)：matchDateTime -> false
2020-8-06 (yyyy-MM-dd)：matchDateTime -> true
2020-08-6 (yyyy-MM-dd)：matchDateTime -> true
2020-08-06 (yyyy-MM-dd)：matchDateTime -> true
2020-08-06 19 (null)：matchDateTime -> false
2020-08-06 19:00 (yyyy-MM-dd HH:mm)：matchDateTime -> false
2020-08-06 19:00:00 (yyyy-MM-dd HH:mm:ss)：matchDateTime -> true
2020/8/06 (yyyy/MM/dd)：matchDateTime -> true
2020/08/6 (yyyy/MM/dd)：matchDateTime -> true
2020/08/06 (yyyy/MM/dd)：matchDateTime -> true
2020/08/06 19 (null)：matchDateTime -> false
2020/08/06 19:00 (yyyy/MM/dd HH:mm)：matchDateTime -> false
2020/08/06 19:00:00 (yyyy/MM/dd HH:mm:ss)：matchDateTime -> true

groupingBy(Function f)

@Test
public void t5() {@Dataclass Stu {private Integer id;private String name;private Long money;public Stu(Integer id, String name, Long money) {this.id = id;this.name = name;this.money = money;}}List<Stu> list = Lists.newArrayList(new Stu(1, "小明", 100L),new Stu(1, "小红", 200L),new Stu(2, "小黄", 200L),new Stu(2, "小紫", 200L));Map<Integer, List<Stu>> collect = list.stream().collect(Collectors.groupingBy(Stu::getId));System.out.println(JSON.toJSONString(collect, SerializerFeature.PrettyFormat));
}

显示结果

{1:[{"id":1,"money":100,"name":"小明"},{"id":1,"money":200,"name":"小红"}],2:[{"id":2,"money":200,"name":"小黄"},{"id":2,"money":200,"name":"小紫"}]
}

flatMap(Function f)

@Data
private static class TravelInfo {private String trip;private String hotelName;private List<Order> orders;public TravelInfo(String trip, String hotelName, List<Order> orders) {this.trip = trip;this.hotelName = hotelName;this.orders = orders;}
}@Data
private static class Order {private Long orderId;private List<Travellers> travellers;public Order(Long orderId, List<Travellers> travellers) {this.orderId = orderId;this.travellers = travellers;}
}@Data
private static class Travellers {private String userName;private String email;public Travellers() {}public Travellers(String userName, String email) {this.userName = userName;this.email = email;}
}/*** flatMap(Function f)：扁平化*/
@Test
public void t5() {TravelInfo travelInfo = new TravelInfo("三人行", "天上人间",Lists.newArrayList(new Order(123456789L, Lists.newArrayList(new Travellers("zhangSanFirst", "[e-mail]"),new Travellers("zhangSanSecond", "[e-mail]"))),new Order(987654321L, Lists.newArrayList(new Travellers("liSiFirst", "[e-mail]"),new Travellers("liSiSecond", "[e-mail]"))),new Order(987654322L, Lists.newArrayList(new Travellers("wangWu", "[e-mail]"))),new Order(987654323L, Lists.newArrayList()),new Order(987654323L, null)));System.out.println(JSON.toJSONString(travelInfo, SerializerFeature.PrettyFormat));System.out.println();List<String> email = travelInfo.getOrders().stream().filter(Objects::nonNull).map(Order::getTravellers).filter(CollectionUtils::isNotEmpty).flatMap(Collection::stream).filter(Objects::nonNull).map(Travellers::getEmail).collect(Collectors.toList());System.out.println(email);
}

显示结果

{"hotelName":"天上人间","orders":[{"orderId":123456789,"travellers":[{"email":"[e-mail]","userName":"zhangSanFirst"},{"email":"[e-mail]","userName":"zhangSanSecond"}]},{"orderId":987654321,"travellers":[{"email":"[e-mail]","userName":"liSiFirst"},{"email":"[e-mail]","userName":"liSiSecond"}]},{"orderId":987654322,"travellers":[{"email":"[e-mail]","userName":"wangWu"}]},{"orderId":987654323,"travellers":[]},{"orderId":987654323}],"trip":"三人行"
}[[e-mail], [e-mail], [e-mail], [e-mail], [e-mail]]

Optional.ofNullable(T t) 和 T orElse(T other)

@Data
private class PaymentDto {private Long id;private String liqAccountName;/*** 来款业务：COMPANY-企业来款；OFFLINE-订单线下收款*/private String paymentMode;private List<ClaimDto> claimDto;private List<OrderClaimDto> orderClaimDto;public PaymentDto(Long id, String liqAccountName, String paymentMode, List<ClaimDto> claimDto, List<OrderClaimDto> orderClaimDto) {this.id = id;this.liqAccountName = liqAccountName;this.paymentMode = paymentMode;this.claimDto = claimDto;this.orderClaimDto = orderClaimDto;}
}@Data
private static class ClaimDto {private Long paymentId;private String partnerName;private Integer amount;public ClaimDto(Long paymentId, String partnerName, Integer amount) {this.paymentId = paymentId;this.partnerName = partnerName;this.amount = amount;}
}@Data
private static class OrderClaimDto {private Long paymentId;private Integer amount;public OrderClaimDto(Long paymentId, Integer amount) {this.paymentId = paymentId;this.amount = amount;}
}@Data
private static class PaymentApiDto {private Long id;private String liqAccountName;private String paymentMode;private Long paymentId;private String partnerName;private Integer amount;
}private List<PaymentDto> getPaymentDto() {List<PaymentDto> list = Lists.newArrayList();list.add(new PaymentDto(123456789L, "收款账户_COMPANY", "COMPANY",Lists.newArrayList(new ClaimDto(123456789L, "企业名称1", 999),new ClaimDto(123456789L, "企业名称2", 888),new ClaimDto(123456789L, "企业名称3", 777)),Lists.newArrayList(new OrderClaimDto(123456789L, 666),new OrderClaimDto(123456789L, 555))));list.add(new PaymentDto(987654321L, "收款账户_OFFLINE", "OFFLINE",Lists.newArrayList(new ClaimDto(987654321L, "企业名称1", 999),new ClaimDto(987654321L, "企业名称2", 888)),Lists.newArrayList(new OrderClaimDto(987654321L, 666),new OrderClaimDto(987654321L, 555))));list.add(new PaymentDto(888888888L, "收款账户", null, null, null));return list;
}/*** Optional.ofNullable(T t) 和 T orElse(T other)*/
@Test
public void t6() {List<PaymentDto> paymentDtos = getPaymentDto();paymentDtos.forEach(System.out::println);System.out.println();/*** 根据 paymentMode 把 claimDto、orderClaimDto 集合数据查分为单条数据*/List<PaymentApiDto> collect = paymentDtos.stream().map(paymentDto -> {PaymentApiDto apiDto = new PaymentApiDto();apiDto.setId(paymentDto.getId());apiDto.setLiqAccountName(paymentDto.getLiqAccountName());apiDto.setPaymentMode(paymentDto.getPaymentMode());ImmutableList<PaymentApiDto> defaultList = ImmutableList.of(apiDto);if (StringUtils.equals(paymentDto.getPaymentMode(), "COMPANY")) {return Optional.ofNullable(paymentDto.getClaimDto()).map(claimDtos -> claimDtos.stream().map(claimDto -> {PaymentApiDto paymentApiDto = new PaymentApiDto();paymentApiDto.setId(paymentDto.getId());paymentApiDto.setLiqAccountName(paymentDto.getLiqAccountName());paymentApiDto.setPaymentMode(paymentDto.getPaymentMode());paymentApiDto.setPaymentId(claimDto.getPaymentId());paymentApiDto.setPartnerName(claimDto.getPartnerName());paymentApiDto.setAmount(claimDto.getAmount());return paymentApiDto;}).collect(Collectors.toList())).orElse(defaultList);}if (StringUtils.equals(paymentDto.getPaymentMode(), "OFFLINE")) {return Optional.ofNullable(paymentDto.getOrderClaimDto()).map(orderClaimDtos -> orderClaimDtos.stream().map(orderClaimDto -> {PaymentApiDto paymentApiDto = new PaymentApiDto();paymentApiDto.setId(paymentDto.getId());paymentApiDto.setLiqAccountName(paymentDto.getLiqAccountName());paymentApiDto.setPaymentMode(paymentDto.getPaymentMode());paymentApiDto.setAmount(orderClaimDto.getAmount());return paymentApiDto;}).collect(Collectors.toList())).orElse(defaultList);}return defaultList;}).flatMap(Collection::stream).collect(Collectors.toList());collect.forEach(System.out::println);
}

显示结果

LambdaTest.PaymentDto(id=123456789, liqAccountName=收款账户_COMPANY, paymentMode=COMPANY, claimDto=[LambdaTest.ClaimDto(paymentId=123456789, partnerName=企业名称1, amount=999), LambdaTest.ClaimDto(paymentId=123456789, partnerName=企业名称2, amount=888), LambdaTest.ClaimDto(paymentId=123456789, partnerName=企业名称3, amount=777)], orderClaimDto=[LambdaTest.OrderClaimDto(paymentId=123456789, amount=666), LambdaTest.OrderClaimDto(paymentId=123456789, amount=555)])
LambdaTest.PaymentDto(id=987654321, liqAccountName=收款账户_OFFLINE, paymentMode=OFFLINE, claimDto=[LambdaTest.ClaimDto(paymentId=987654321, partnerName=企业名称1, amount=999), LambdaTest.ClaimDto(paymentId=987654321, partnerName=企业名称2, amount=888)], orderClaimDto=[LambdaTest.OrderClaimDto(paymentId=987654321, amount=666), LambdaTest.OrderClaimDto(paymentId=987654321, amount=555)])
LambdaTest.PaymentDto(id=888888888, liqAccountName=收款账户, paymentMode=null, claimDto=null, orderClaimDto=null)LambdaTest.PaymentApiDto(id=123456789, liqAccountName=收款账户_COMPANY, paymentMode=COMPANY, paymentId=123456789, partnerName=企业名称1, amount=999)
LambdaTest.PaymentApiDto(id=123456789, liqAccountName=收款账户_COMPANY, paymentMode=COMPANY, paymentId=123456789, partnerName=企业名称2, amount=888)
LambdaTest.PaymentApiDto(id=123456789, liqAccountName=收款账户_COMPANY, paymentMode=COMPANY, paymentId=123456789, partnerName=企业名称3, amount=777)
LambdaTest.PaymentApiDto(id=987654321, liqAccountName=收款账户_OFFLINE, paymentMode=OFFLINE, paymentId=null, partnerName=null, amount=666)
LambdaTest.PaymentApiDto(id=987654321, liqAccountName=收款账户_OFFLINE, paymentMode=OFFLINE, paymentId=null, partnerName=null, amount=555)
LambdaTest.PaymentApiDto(id=888888888, liqAccountName=收款账户, paymentMode=null, paymentId=null, partnerName=null, amount=null)