[log] Java8 optional，一次操作，多次练习

绝地灬酷狼 2021-09-18 10:32 89阅读 0赞

> [https://www.jianshu.com/p/00fa8597d0c7][https_www.jianshu.com_p_00fa8597d0c7]  
> [https://www.cnblogs.com/zhangboyu/p/7580262.html][https_www.cnblogs.com_zhangboyu_p_7580262.html]

光看不行，得动手

package com.dudu.java8.read;
    
    import com.dudu.User;
    import org.springframework.web.servlet.handler.UserRoleAuthorizationInterceptor;
    
    import javax.swing.text.html.Option;
    import java.util.Optional;
    
    /**
     * 用optional 取代null
     *
     * @create: 20190412
     * @author: liu
     **/
    
    public class Read10 {
        /**
         * Optional  只是对类简单封装
         * 变量不存在时，缺省的值会被建模成一个空的optional对象
         * 由empty 返回
         * <p>
         * 帮助更好的设计出合适的api，看到方法签名，就能了解它是否接收一个option的值
         *
         * @create: 2019-04-12
         * @author: liu
         **/
    
        public static void main(String[] args) {
            Optional<Object> empty = Optional.empty(); // 声明一个空的optional
    
            User user = new User();
            Optional<User> user1 = Optional.of(user);
            // 依据一个非空值创建一个optional对象
            // 如果是个null，就会立即抛出一个NullPointerException
    
    
            user.setId(123L);
            user.setUsername("liu");
            Optional<User> user2 = Optional.ofNullable(user);
            // 可以接收null的optional
    
            Optional<String> s = user2.map(User::getUsername);
            System.out.println(s.get());
    
    
            Optional<User> user3 = Optional.ofNullable(new User());
            Long aLong = user3.map(User::getId).orElse(111L);  // 如果为空就设置一个默认值
            System.out.println(aLong);
    
    
            // flatMap 将一个两层的optional对象合并在一起
            //   对一个空的optional进行操作不会发生任何改变，返回值也是个空的optional对象
            //   调用链上的任何一个方法返回一个空的optional，那么就过就为空
    
    
            /**
             * 默认方法
             *
             * get 如果变量存在就返回，否则抛出nosuchelementexception
             *
             * orElse  对象不包含值时，提供一个默认值
             *
             * orElseGet 是延迟调用版，Supplier方法只有optional不为空时才进行调用
             *
             * orElseThrow  optional为空时 抛出一个异常
             *
             * ifPresent  变量值存在的时候执行操作
             *
             *  @create: 2019-04-12
             *  @author: liu
             **/
            user3.get().setUsername("hua");
    
    
            // Optional<Long> id1 = Optional.of(user3.get().getId());
            // 传入的值为空时会报错
    
            
            Long id = Optional.ofNullable(user3.get().getId()).orElseGet(() -> user3.get().getId());
            System.out.println(id);
    
            user3.get().setId(222L);
            Long id2 = Optional.ofNullable(user3.get().getId()).orElseGet(() -> user3.get().getId());
            System.out.println("set : " + id2.toString());
    
            // id 存在就输出
            Optional.ofNullable(user3.get().getId()).ifPresent((x) -> System.out.println("ifPresent : " + x));
    
    
            //email 不存在就不执行操作
            Optional.ofNullable(user3.get().getEmail()).ifPresent((x) -> System.out.println(x));
    
    
            Optional<User> user4 = Optional.ofNullable(new User());
    
    
            // Optional.ofNullable(user4.get().getId()).orElseThrow(RuntimeException::new);
            //    没有id   就会抛出一个运行时异常，也可以是自定义的异常
    
            //  包含值也会返回true
            System.out.println(" isPresent : " +   Optional.ofNullable(user3.get().getId()).isPresent());
    
    
            // 对user3的id进行过滤 ， 如果=222 就打印user3
            user3.filter( x -> "222".equals(user3.get().getId().toString())).ifPresent(
                    x-> System.out.println(x)
            );
        }
    
    }

动手之后，就要经常使用。

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[https_www.jianshu.com_p_00fa8597d0c7]: https://www.jianshu.com/p/00fa8597d0c7
[https_www.cnblogs.com_zhangboyu_p_7580262.html]: https://www.cnblogs.com/zhangboyu/p/7580262.html
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