Lambda表达式例子

转 Lambda表达式例子

1.Java8 新特性介绍

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
Stream
Optional
Predicate
Function
Consumer
Filter
Map-Reduce
新的Date API

取id 列表

List<Integer> transactionsIds = transactions.parallelStream().
 filter(t -> t.getType() == Transaction.GROCERY).
 sorted(comparing(Transaction::getValue).reversed()).
 map(Transaction::getId).
 collect(toList());

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List
        for(String item:items){
            System.out.println(item);
        }
        //Java8 lambda遍历list
        items.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map
        for(Map.Entry<String,Integer> entry:items.entrySet()){
            System.out.println("key:" + entry.getKey() + " value:" + entry.getValue());
        }
        //Java8遍历map
        items.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Map
        Map<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)); 
    }
}

收集成实体本身map
代码如下：
accounts.stream().collect(Collectors.toMap(Account::getId, account -> account)); 
account -> account是一个返回本身的lambda表达式，其实还可以使用Function接口中的一个默认方法代替，使整个方法更简洁优雅：
accounts.stream().collect(Collectors.toMap(Account::getId, Function.identity())); 
重复key的情况
代码如下：
accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity())); 
这个方法可能报错（java.lang.IllegalStateException: Duplicate key），因为name是有可能重复的。toMap有个重载方法，可以传入一个合并的函数来解决key冲突问题：
accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2));
这里只是简单的使用后者覆盖前者来解决key重复问题。
指定具体收集的map
toMap还有另一个重载方法，可以指定一个Map的具体实现，来收集数据：
accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2, LinkedHashMap::new))

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 map
        String 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->String
        result =  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) {
        //anyMatch
        boolean isMatch = employeeList.stream().anyMatch(employee -> employee.getOffice().equals("London"));
        System.out.println(isMatch);
        //返回所有salary大于6000
        boolean 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Map
        Map<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Set
        Set<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 York
        OptionalDouble 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;
    }
    @Test
    public 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map
        Map<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Map
        Map<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map
        Map<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map
        Map<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());
    }
    @Test
    public void testEmployeeExample() {
        //anyMatch
        List<Employee> employeeList = generateData();
        boolean isMatch = employeeList.stream().anyMatch(employee -> employee.getOffice().equals("London"));
        System.out.println(isMatch);
        //allMatch
        boolean 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Map
        Map<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Set
        Set<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_);
    }
}