JDK1.8新特征之Optional

1.基本API的使用

of

of	为非null的值创建一个optional

 Optional<String> s = Optional.of("2222");
        String s1 = s.get();
        System.out.println(s1);

2222

ifNullable

ifNullable	为指定的值创建一个Optional,如果指定的值为null,则返回一个空的Optional

public class TestBean {
    public static void main(String[] args) {
        User user = null;
        Optional<User> s = Optional.ofNullable(user);
        User user1 = s.get();  // 还不是不能get的但是他会调用empty() 返回一个空的optional
    }
}

异常 空指针

isPrensent

isPrensent	如果值存在返回true,不存在返回false

public class TestBean {
    public static void main(String[] args) {
        User user = null;
        Optional<User> s = Optional.ofNullable(user);
        boolean present = s.isPresent();
        System.out.println(present);
    }
}

false

get

get	如果有值则返回,没有值抛出NoSuchElementException异常

public class TestBean {
    public static void main(String[] args) {
        User user = null;
        Optional<User> s = Optional.ofNullable(user);
        User user1 = s.get();  // 还不是不能get的但是他会调用empty() 返回一个空的optional
    }
}

异常 空指针

ifPresent

ifPresent	如果有值,则为其调用consumer接口,否则不做处理

public class TestBean {
    public static void main(String[] args) {
        User user = new User();
        Optional<User> s = Optional.ofNullable(user);
        s.ifPresent((value)-> System.out.println(value));
    }
}

orElse

orElse	有值则返回没有值,则返回指定值

public class TestBean {
    public static void main(String[] args) {
        User user =null;
        Optional<User> s = Optional.ofNullable(user);
        User user1 = s.orElse(new User());
    }
}

orElseGet

orElseGet	他和orElse的区别在于,orElse将传入的值作为默认值,而他可以接受supplier接口实现默认值

public class TestBean {
    public static void main(String[] args) {
        User user =null;
        Optional<User> s = Optional.ofNullable(user);
        User user1 = s.orElseGet(()->{
            User user2 = new User();
            user2.setName("wwww");
            return user2;
        });
    }
}

orElseThrow

orElseThrow	有值返回,没有值抛出supplier接口创建的异常

public class TestBean {
    public static void main(String[] args) {
        User user =null;
        Optional<User> s = Optional.ofNullable(user);
        try {
            User user1 = s.orElseThrow(()->{
             return  new Exception();
            });
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

map

map	有值,对其执行调用mapping函数得到返回值,如果返回值不为null,则创建包含mapping返回值的optional作为map的返回值,否则返回空的optional

public class TestBean {
    public static void main(String[] args) {
        User user = new User();
        user.setName("wwww");
        Optional<User> s = Optional.ofNullable(user);
        Optional<String> s1 = s.map((up) -> up.getName().toUpperCase());
        String s2 = s1.get();
        System.out.println(s2);
    }
}

WWW

flatMap

flatMap	他map类似,区别在于他调用结束的时候,不会对optional做一个封装

 User user = new User();
        user.setName("wwww");
        Optional<User> s = Optional.ofNullable(user);
        //需要我们自己的去封装一个Optional
        Optional<String> s1 = s.flatMap((up) -> Optional.of(up.getName().toUpperCase()));

filter

filter	有值,返回满足条件的optional,否则返回空的optional

public class TestBean {
    public static void main(String[] args) {
        User user = new User();
        user.setName("wwww");
        Optional<User> s = Optional.ofNullable(user);
        Optional<User> www = s.filter(up -> up.getName().equals("www"));
    }
}