Java8 lambda 的使用

摘要: 1.Java8 新特性介绍 2.项目中应用

1.Java8 新特性介绍

原文链接 https://my.oschina.net/chenxiaobian/blog/704421，https://www.cnblogs.com/hmdrzql/p/6354010.html

现在我们有一个需求：给一个user组成的list 按照user的年龄排序。实现不难，代码如下：

UserCompare是一个实现了Comprator的类

这种方法由于sort方法的第二个参数是Comparator 所以你要写一个实现类（我这里是UserCompare类），并且override该接口的实现方法。

java8提供了lambda来简化，有了lambda程序员从此不加班呀～

刚才那个Comparator的实现类以及内部若干代码就都省了，代之以lambda表达式。

另外，IntelliJ会提示你改成更好的写法

实现类里有多少代码，你就省了多少行代码。

高兴了半天，到底是什么原理呢？其实是java8新提供的语法糖。

我们打开java1.8 Comparator的定义，发现了类定义上面多了一个@FunctionalInterface

对，1.8多了一个概念就是FunctionalInterface，这个Comparator就是一个FunctionalInterface

有了这个注解，原来使用实现类的地方就可以代之以lambda表达式。

写java的同学对java8肯定知道那么java8到底有哪些特性呢，总结如下：

Lambda表达式函数式接口StreamOptionalPredicateFunctionConsumerFilterMap-Reduce新的Date API

最核心的当然是函数式编程了，写代码非常简单，请看下面详细例子介绍

2.Java8 lambda使用总结－结合实例介绍

很多同学一开始接触Java8可能对Java8 Lambda表达式有点陌生，下面我将结合实例介绍Java8的使用并与Java7进行比较：

基础类

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Person {private int id;private String name;private String address;}

1.List操作

public class ExampleList {private static List<String> items = new ArrayList<>();static {items.add("A");items.add("BC");items.add("C");items.add("BD");items.add("E");}public static void main(String[] args) {//Java8之前操作Listfor(String item:items){System.out.println(item);}//Java8 lambda遍历listitems.forEach(c-> System.out.println(c));items.forEach(item->{if("C".equals(item)){System.out.println(item);}});System.out.println("--------");//先过滤items.stream().filter(s->s.contains("B")).forEach(c1-> System.out.println(c1));}
}

2.Map操作

public class ExampleMap {private static Map<String, Integer> items = new HashMap<>();static {items.put("A", 10);items.put("B", 20);items.put("C", 30);items.put("D", 40);items.put("E", 50);items.put("F", 60);}public static void main(String[] args) {//Java8之前遍历是这样遍历mapfor(Map.Entry<String,Integer> entry:items.entrySet()){System.out.println("key:" + entry.getKey() + " value:" + entry.getValue());}//Java8遍历mapitems.forEach((key,value)-> System.out.println("key:" + key + " value:" + value));}}

3.Groupingby操作

/*** *Java8 Collectors.groupingBy and Collectors.mapping example*/
public class ExampleMapping {private static List<Person> personList = Lists.newArrayList();static {personList.add(Person.builder().id(10).address("apple").address("shanghai").build());personList.add(Person.builder().id(12).address("apple").address("wuhan").build());personList.add(Person.builder().id(16).address("apple").address("nanjing").build());}public static void main(String[] args) {//分组Map<String, List<Person>> collect = personList.stream().collect(Collectors.groupingBy(c -> c.getAddress()));System.out.println(collect);}
}

4.List转换为Map

public class ExampleListConvertMap {private static List<Person> personList = Lists.newArrayList();static{personList.add(Person.builder().id(20).name("zhangsan").address("shanghai").build());personList.add(Person.builder().id(30).name("lisi").address("nanjing").build());}public static void main(String[] args) {//Java8 List转换MapMap<Integer,Person> map_ = personList.stream().collect(Collectors.toMap((key->key.getId()),(value->value)));map_.forEach((key,value)-> System.out.println(key + ":" + value));Map<Integer, Person> mappedMovies = personList.stream().collect(  Collectors.toMap(Person::getRank, Person::getData)); }
}

5.FilterMap操作

public class ExampleFilterMap {private static Map<Integer,String> map_ = Maps.newHashMap();static{map_.put(1, "linode.com");map_.put(2, "heroku.com");map_.put(3, "digitalocean.com");map_.put(4, "aws.amazon.com");}public static void main(String[] args) {//before java iterator mapString result = null;for(Map.Entry<Integer,String> entry:map_.entrySet()){if("heroku.com".equals(entry.getValue())){result = entry.getValue();}}System.out.println("Before Java 8 :" + result);//Java8 Map->Stream->Filter->Stringresult =  map_.entrySet().stream().filter(map->"heroku.com".equals(map.getValue())).map(map->map.getValue()).collect(Collectors.joining());System.out.println("Java 8 :" + result);Map<Integer,String> collect =  map_.entrySet().stream().filter(c->c.getKey()==2).collect(Collectors.toMap(p->p.getKey(),p->p.getValue()));System.out.println(collect);}
}

6.Optional操作可以防止NullPointException

Optional<String> optional = Optional.of("hello");
System.out.println(optional.isPresent());//true
System.out.println(optional.get());//hello
System.out.println(optional.orElse("false"));
optional.ifPresent((s)-> System.out.println(s.charAt(0)));//h

7.给出一个详细的例子

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Employee {private String name;private int salary;private String office;
}public class ExampleEmployee {private static List<Employee> employeeList = Lists.newArrayList();static{employeeList.add(Employee.builder().name("Matt").salary(5000).office("New York").build());employeeList.add(Employee.builder().name("Steve").salary(6000).office("London").build());employeeList.add(Employee.builder().name("Carrie").salary(20000).office("New York").build());employeeList.add(Employee.builder().name("Peter").salary(7000).office("New York").build());employeeList.add(Employee.builder().name("Pat").salary(8000).office("London").build());employeeList.add(Employee.builder().name("Tammy").salary(29000).office("Shanghai").build());}public static void main(String[] args) {//anyMatchboolean isMatch = employeeList.stream().anyMatch(employee -> employee.getOffice().equals("London"));System.out.println(isMatch);//返回所有salary大于6000boolean matched = employeeList.stream().allMatch(employee -> employee.getSalary()>4000);System.out.println(matched);//找出工资最高Optional<Employee> hightestSalary = employeeList.stream().max((e1,e2)->Integer.compare(e1.getSalary(),e2.getSalary()));System.out.println(hightestSalary);//返回姓名列表List<String> names = employeeList.stream().map(employee -> employee.getName()).collect(Collectors.toList());System.out.println(names);//List转换成MapMap<String,Employee> employeeMap = employeeList.stream().collect(Collectors.toMap((key->key.getName()),(value->value)));employeeMap.forEach((key,value)-> System.out.println(key + "=" + value));//统计办公室是New York的个数long officeCount = employeeList.stream().filter(employee -> employee.getOffice().equals("Shanghai")).count();System.out.println(officeCount);//List转换为SetSet<String> officeSet = employeeList.stream().map(employee -> employee.getOffice()).distinct().collect(Collectors.toSet());System.out.println(officeSet);//查找办公室地点是New York的员工Optional<Employee> allMatchedEmployees = employeeList.stream().filter(employee -> employee.getOffice().equals("New York")).findAny();System.out.println(allMatchedEmployees);//按照工资的降序来列出员工信息List<Employee> sortEmployeeList =  employeeList.stream().sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary())).collect(Collectors.toList());//按照名字的升序列出员工信息List<Employee> sortEmployeeByName = employeeList.stream().sorted((e1,e2)->e1.getName().compareTo(e2.getName())).collect(Collectors.toList());System.out.println(sortEmployeeList);System.out.println("按照名字的升序列出员工信息:" + sortEmployeeByName);//获取工资最高的前2条员工信息List<Employee> top2EmployeeList= employeeList.stream().sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary())).limit(2).collect(Collectors.toList());System.out.println(top2EmployeeList);//获取平均工资OptionalDouble averageSalary = employeeList.stream().mapToInt(employee->employee.getSalary()).average();System.out.println("平均工资:" + averageSalary);//查找New YorkOptionalDouble averageSalaryByOffice = employeeList.stream().filter(employee -> employee.getOffice().equals("New York")).mapToInt(employee->employee.getSalary()).average();System.out.println("New York办公室平均工资:" + averageSalaryByOffice);}
}

8.Java8常见操作


public class EmployeeTest {public static List<Employee> generateData() {return Arrays.asList(new Employee("Matt", 5000, "New York"),new Employee("Steve", 6000, "London"),new Employee("Carrie", 10000, "New York"),new Employee("Peter", 7000, "New York"),new Employee("Alec", 6000, "London"),new Employee("Sarah", 8000, "London"),new Employee("Rebecca", 4000, "New York"),new Employee("Pat", 20000, "New York"),new Employee("Tammy", 9000, "New York"),new Employee("Fred", 15000, "Tokyo"));}public static Map<String, Integer> generateMapData() {Map<String, Integer> items = Maps.newHashMap();items.put("A", 10);items.put("B", 20);items.put("C", 30);items.put("D", 40);items.put("E", 50);items.put("F", 60);return items;}@Testpublic void testEmployee() {List<Employee> results = generateData();//打印出名字是Steve的员工信息results.forEach(c -> {if (c.getName().equals("Steve")) {System.out.println(c);}});System.out.println("---------");//找出年薪超过6000的员工results.stream().filter(c -> c.getSalary() >= 60000).forEach(c -> System.out.println(c));System.out.println("--->>>>>>----");//java8遍历mapMap<String, Integer> map_ = generateMapData();map_.forEach((key, value) -> System.out.println("key:" + key + "," + "value:" + value));System.out.println("---->>>>分组>>>-----");//java8 分组操作Map<String, List<Employee>> groupMap = results.stream().collect(Collectors.groupingBy(c -> c.getOffice()));System.out.println(groupMap);System.out.println("---->>>>List转化为Map>>>----");//List转化MapMap<String, Object> map = results.stream().collect(Collectors.toMap(Employee::getName, Employee::getOffice));map.forEach((key, value) -> System.out.println("key:" + key + "," + "value:" + value));System.out.println("---->>>>>>>----");Map<Integer, Employee> employeeMap = results.stream().collect(Collectors.toMap((key -> key.getSalary()), (value -> value)));employeeMap.forEach((key, value) -> System.out.println(key + "," + value));System.out.println("---->>遍历map>>>----");//打印出值大于30的mapMap<String, Integer> resultMap = map_.entrySet().stream().filter(c -> c.getValue() > 30).collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));resultMap.forEach((key, value) -> System.out.println(key + "=" + value));System.out.println(">>>>>>>>>>>>>>>");//打印key=D的mapMap<String, Integer> mapResults = map_.entrySet().stream().filter(c -> c.getKey().equals("D")).collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));mapResults.forEach((key, value) -> System.out.println(key + ">>>>" + value));System.out.println(">>>>>>>Optional>>>>>>>");Optional<String> optional = Optional.of("hello");System.out.println(optional.isPresent());}@Testpublic void testEmployeeExample() {//anyMatchList<Employee> employeeList = generateData();boolean isMatch = employeeList.stream().anyMatch(employee -> employee.getOffice().equals("London"));System.out.println(isMatch);//allMatchboolean matched = employeeList.stream().allMatch(employee -> employee.getOffice().equals("London"));System.out.println(matched);//找出工资最高的Optional<Employee> employeeOptional = employeeList.stream().max((e1,e2)->Integer.compare(e1.getSalary(),e2.getSalary()));System.out.println(employeeOptional);//找出工资最少的Optional<Employee> employee = employeeList.stream().min((e1,e2)->Integer.compare(e1.getSalary(),e2.getSalary()));System.out.println(employee);//返回姓名列表List<String> names = employeeList.stream().map(c->c.getName()).collect(Collectors.toList());System.out.println(names);System.out.println(">>>>>>>>>>>");//List转化MapMap<String,Employee> employeeMap = employeeList.stream().collect(Collectors.toMap((key->key.getName()),(value->value)));employeeMap.forEach((key,value)-> System.out.println(key + "=" + value));//统计办公室是New York的个数long officeCount =  employeeList.stream().filter(c->c.getOffice().equals("New York")).count();System.out.println(officeCount);long salaryCount = employeeList.stream().filter(c->c.getSalary()>60000).count();System.out.println(salaryCount);//List转化为SetSet<String> officeSet = employeeList.stream().map(c->c.getOffice()).distinct().collect(Collectors.toSet());System.out.println(officeSet);Set<Integer> salarySet = employeeList.stream().map(c->c.getSalary()).distinct().collect(Collectors.toSet());System.out.println(salarySet);//查找办公室地点是New York的员工Optional<Employee> optionals = employeeList.stream().filter(c->c.getOffice().equals("New York")).findAny();System.out.println(optionals);System.out.println(">>>>>工资降序排序>>>>>");//按照工资的降序来列出员工信息List<Employee> sortSalaryEmployeeList = employeeList.stream().sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary())).collect(Collectors.toList());System.out.println(sortSalaryEmployeeList);System.out.println(">>>>>姓名升序排序>>>>>");List<Employee> sortNameEmployeeList = employeeList.stream().sorted((e1,e2)->e1.getName().compareTo(e2.getName())).collect(Collectors.toList());System.out.println(sortNameEmployeeList);System.out.println(">>>>获取工资最高的前2条员工信息");List<Employee> dispaly2EmployeeList = employeeList.stream().sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary())).limit(2).collect(Collectors.toList());System.out.println(dispaly2EmployeeList);System.out.println(">>>>获取平均工资");OptionalDouble averageSalary = employeeList.stream().mapToInt(c->c.getSalary()).average();System.out.println(averageSalary);System.out.println(">>>>获取工作地点的平均工资");OptionalDouble optionalDouble = employeeList.stream().filter(c->c.getOffice().equals("New York")).mapToInt(c->c.getSalary()).average();System.out.println(optionalDouble);System.out.println(">>>>>>Java8 Optional用法>>>>>>");Optional<String> stringOptional = Optional.of("test");System.out.println(stringOptional.get());Optional<String> isOptional = Optional.ofNullable("hello");System.out.println(isOptional.isPresent());System.out.println(isOptional.get());System.out.println(isOptional.orElse("0"));System.out.println(">>>>>>>>>>>>");//Optional<String> optionalVal = Optional.of(null);// System.out.println(optionalVal);Optional<String> optional = Optional.ofNullable("optional");System.out.println(optional);System.out.println(optional.isPresent());System.out.println(optional.get());System.out.println(optional.orElse("haha"));System.out.println(">>>>>>>>>>>>");Optional<Employee> employeeOptional_ = employeeList.stream().filter(c->c.getOffice().equals("New York")).findFirst();System.out.println(employeeOptional_);}