【Java8】Lambda 表达式

面向对象编程是对数据进行抽象；函数式编程是对行为进行抽象。

Lambda 特点

面向对象编程是对数据进行抽象；函数式编程是对行为进行抽象。 核心思想：使用不可变值和函数，函数对一个值进行处理，映射成另一个值。

1. 内部可以使用静态、非静态和局部变量
2. 在 Java 中又称为闭包或匿名函数
3. 有个限制，那就是只能引用 final 或 final 局部变量

Lambda 使用以及 Stream 下的接口

stream & parallelStream

parallelStream 其实就是一个并行执行的流，它通过默认的 ForkJoinPool，可能提高你的多线程任务的速度。

stream 中常用方法

1. stream(), parallelStream()
2. filter()
3. findFirst()
4. sorted()
5. forEach()
6. map(), reduce()
7. flatMap()
8. collect(Collectors.toList())
9. distinct(),limit()
10. count()
11. min(), max()

示例

findFirst()

List<User> userList = Arrays.asList(new User("zhangsan","张三","123456","男","123@qq.com","")
                ,new User("zhangsan1","张三","123456","女","123@qq.com",""));

        Optional<User> first = userList.stream().filter(user ->
                StringUtils.endsWithIgnoreCase("张三", user.getUserName())).findFirst();
        System.out.println(first);

输出：

Optional[User[id=null, loginName='zhangsan', userName='张三', password='123456', sex='男', email='123@qq.com', address='']]

sorted()

按照年龄排序

        List<User> userList = Arrays.asList(new User("zhangsan", "张三", "123456", "男", "123@qq.com", "", 23)
                , new User("zhangsan1", "张三", "123456", "女", "123@qq.com", "", 19));
        List<User> collect = userList.stream().sorted(Comparator.comparing(User::getAge)).collect(Collectors.toList());
        System.out.println(collect);

输出：

[User[id=null, loginName='zhangsan1', userName='张三', password='123456', sex='女', email='123@qq.com', address='', age=19], User[id=null, loginName='zhangsan', userName='张三', password='123456', sex='男', email='123@qq.com', address='', age=23]]

distinct()

List<User> userList = Arrays.asList(new User("zhangsan", "张三", "123456", "男", "123@qq.com", "", 23)
                , new User("zhangsan1", "张三", "123456", "女", "123@qq.com", "", 19));
        List<String> collect = userList.stream().map(User::getLoginName).distinct().collect(Collectors.toList());
        System.out.println(collect);

输出：

[zhangsan, zhangsan1]

count()

List<User> userList = Arrays.asList(new User("zhangsan", "张三", "123456", "男", "123@qq.com", "", 23)
                , new User("zhangsan1", "张三", "123456", "女", "123@qq.com", "", 19));
        long count = userList.stream().filter(user -> user.getAge() > 19).count();
        System.out.println(count);

输出：

1

macth

anyMatch

        List<User> userList = Arrays.asList(new User("zhangsan", "张三", "123456", "男", "123@qq.com", "", 23)
                , new User("zhangsan1", "张三", "123456", "女", "123@qq.com", "", 19));
        boolean b = userList.stream().anyMatch(user -> user.getAge() == 19);
        System.out.println(b);

输出：

true

allmatch

List<User> userList = Arrays.asList(new User("zhangsan", "张三", "123456", "男", "123@qq.com", "", 23)
                , new User("zhangsan1", "张三", "123456", "女", "123@qq.com", "", 19));
        boolean b = userList.stream().allMatch(user -> user.getAge() == 19);
        System.out.println(b);

输出：

false

四大内置函数接口

消费型接口

Consumer<T> void accept (T t) 有参数，无返回值的抽象方法；

供给型接口

Supplier <T> T get () 无参有返回值的抽象方法；

断定型接口

Predicate boolean test (T t): 有参，但是返回值类型是固定的 boolean

函数型接口

Function<T,R> R apply (T t) 有参有返回值的抽象方法；