Java的Optional实战

小鱼儿 2021-08-28 22:53 261阅读 0赞

**一 引入Optional前的问题**

1 代码

/**
    * Copyright (C), 2020-2020, 软件公司
    * FileName: WithoutOptionalDemo
    * Author:   cakin
    * Date:     2020/4/24
    * Description: 没有 Optional 会有什么问题
    */
    package optionaldemo;
    
    import lombok.extern.slf4j.Slf4j;
    
    /**
    * @ClassName: WithoutOptionalDemo
    * @Description: 没有 Optional 会有什么问题
    * @Date: 2020/4/24
    * @Author: cakin
    */
    @Slf4j
    public class WithoutOptionalDemo {
        class Member {
            /**
             * 名字
             */
            private String name;
    
    
            public String getName() {
                return name;
            }
    
            public void setName(String name) {
                this.name = name;
            }
        }
    
        public static void main(String[] args) {
            Member mem = getMemberByIdFromDB();
            /**
             * 问题在这里，需要判断是否为空，否则会引起NullPointerException问题
             */
            if (mem != null) {
                log.info(mem.getName());
            }
        }
    
        public static Member getMemberByIdFromDB() {
            // 当前 ID 的会员不存在
            return null;
        }
    }

**二 引入Optional解决的问题**

1 代码

/**
    * Copyright (C), 2020-2020, 软件公司
    * FileName: OptionalDemo
    * Author:   cakin
    * Date:     2020/4/24
    * Description: Optional 解决NullPointerException这个问题
    */
    package optionaldemo;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.Optional;
    
    /**
    * @ClassName: OptionalDemo
    * @Description: Optional 解决NullPointerException这个问题
    * @Date: 2020/4/24
    * @Author: cakin
    */
    @Slf4j
    public class OptionalDemo {
        public static void main( String[] args ) {
            Optional<Member> optional = getMemberByIdFromDB();
            optional.ifPresent(mem -> {
                log.info("name: " + mem.getName());
            });
        }
    
        public static Optional<Member> getMemberByIdFromDB() {
            boolean hasName = true;
            if (hasName) {
                return Optional.of(new Member("cakin"));
            }
            return Optional.empty();
        }
    }
    
    class Member {
        /**
         * 名字
         */
        private String name;
    
    
        public String getName() {
            return name;
        }
    
        Member( String name ) {
            this.name = name;
        }
    }

2 测试

20:05:58.115 [main] INFO optionaldemo.OptionalDemo - name: cakin

**三 Optional实战**

1 代码

/**
    * Copyright (C), 2020-2020, 软件公司
    * FileName: OptionalTest
    * Author:   cakin
    * Date:     2020/4/24
    * Description: Optional 用法
    */
    
    
    package optionaldemo;
    
    
    import common.Constant;
    import lombok.extern.slf4j.Slf4j;
    
    
    import java.util.NoSuchElementException;
    import java.util.Optional;
    import java.util.function.Predicate;
    
    
    /**
    * @ClassName: OptionalTest
    * @Description: Optional 用法
    * @Date: 2020/4/24
    * @Author: cakin
    */
    @Slf4j
    public class OptionalTest {
        public static void main( String[] args ) {
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
            /**
             * 创建 Optional 对象
             */
            Optional<String> empty = Optional.empty();
            log.info("empty is " + empty);
    
    
            Optional<String> opt = Optional.of("cakin");
            log.info("opt is " + opt);
    
    
            String name = null;
            try {
                Optional<String> optnull = Optional.of(name);
            } catch (NullPointerException e) {
                log.info("catch NullPointerException");
            }
    
    
            String name1 = null;
            Optional<String> optOrNull = Optional.ofNullable(name1);
            log.info("optOrNull is" + optOrNull);
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             * 判断值是否存在
             */
            Optional<String> opt1 = Optional.of("cakin");
            log.info("opt1 is " + opt1.isPresent());
    
    
            Optional<String> optOrNull1 = Optional.ofNullable(null);
            log.info("optOrNull1 is " + optOrNull1.isPresent());
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             * 非空表达式
             */
            Optional<String> optOrNull2 = Optional.ofNullable("cakin");
            if (optOrNull2.isPresent()) {
                log.info("optOrNull2 length is " + optOrNull2.get().length());
            }
    
    
            Optional<String> opt2 = Optional.of("cakin");
            opt2.ifPresent(str -> log.info("str length is " + str.length()));
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             * orElse() :用于返回包裹在 Optional 对象中的值，如果该值不为 null，则返回；否则返回默认值。
             * 该方法的参数类型和值得类型一致。
             */
            String nullName = null;
            String name2 = Optional.ofNullable(nullName).orElse("cakin");
            log.info("name2 is " + name2);
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             * orElseGet() :与 orElse() 方法类似，但参数类型不同。
             * 如果 Optional 对象中的值为 null，则执行参数中的函数。
             */
            String nullName1 = null;
            String name3 = Optional.ofNullable(nullName1).orElseGet(() -> "cakin");
            log.info("name3 is " + name3);
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             * orElse() 方法和 orElseGet() 方法分别调用 getDefaultValue() 方法返回默认值。
             * 两者输出结果一致
             * 类名 :: 方法名是 Java 8 引入的语法，方法名后面是没有 () 的，表明该方法并不一定会被调用。
             */
            String name4 = null;
            log.info("orElse");
            String name5 = Optional.ofNullable(name4).orElse(getDefaultValue());
            log.info("name5 is " + name5);
    
    
            log.info("orElseGet");
            String name6 = Optional.ofNullable(name4).orElseGet(OptionalTest::getDefaultValue);
            log.info("name6 is " + name6);
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             *  Optional 对象的值不为 null,orElse() 方法和 orElseGet() 方法输出结果不一样
             *  orElseGet() 没有去调用 getDefaultValue()，性能更优
             */
            String name7 = "cakin";
            log.info("orElse");
            String name8 = Optional.ofNullable(name7).orElse(getDefaultValue());
            log.info("name8 is " + name8);
    
    
            log.info("orElseGet");
            String name9 = Optional.ofNullable(name7).orElseGet(OptionalTest::getDefaultValue);
            log.info("name9 is " + name9);
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             * 获取值
             * get() 方法才是最正宗的获取 Optional 对象值的方法，但很遗憾，该方法是有缺陷的，
             * 因为假如 Optional 对象的值为 null，该方法会抛出 NoSuchElementException 异常。
             * 建议 orElseGet() 方法获取 Optional 对象的值。
             */
            String name10 = null;
            Optional<String> optOrNull10 = Optional.ofNullable(name10);
            try {
                log.info(optOrNull10.get());
            } catch (NoSuchElementException e) {
                log.info("catch NoSuchElementException");
            }
            log.info(optOrNull10.orElseGet(() -> name10));
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             * 过滤值 filter方法实现
             */
            String password = "12345";
            Optional<String> opt3 = Optional.ofNullable(password);
            // 由于 password 的长度为 5 ，所以程序输出的结果为 false。
            log.info("pass:" + opt3.filter(pwd -> pwd.length() > Constant.NUM6).isPresent());
    
    
            Predicate<String> len6 = pwd -> pwd.length() > Constant.NUM6;
            Predicate<String> len10 = pwd -> pwd.length() < Constant.NUM10;
            Predicate<String> containA = pwd -> pwd.contains("A");
    
    
            password = "1234567A";
            Optional<String> opt4 = Optional.ofNullable(password);
            boolean result = opt4.filter(len6.and(len10).and(containA)).isPresent();
            // 密码变成了 7 位，在 6 到 10 位之间,并且包括A，所以通过了
            log.info("pass:" + result);
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
    
    
            /**
             * 转换值
             */
            // 名字转换为长度
            nameLength();
            // 验证密码
            validPassword();
            log.info("++++++++++++++++++++++++++++++++++++++++++++++");
        }
    
    
        /**
         * 功能描述：验证密码
         *
         * @author cakin
         * @date 2020/4/24
         */
        private static void validPassword() {
            String password = "pAssWord";
            Optional<String> opt = Optional.ofNullable(password);
    
    
            Predicate<String> len6 = pwd -> pwd.length() > Constant.NUM6;
            Predicate<String> len10 = pwd -> pwd.length() < Constant.NUM10;
            Predicate<String> eq = pwd -> pwd.equals("password");
    
    
            boolean result = opt.map(String::toLowerCase).filter(len6.and(len10).and(eq)).isPresent();
            log.info("pass:" + result);
        }
    
    
        /**
         * 功能描述：将name转成长度
         *
         * @author cakin
         * @date 2020/4/24
         */
        private static void nameLength() {
            String name = "cakin";
            Optional<String> nameOptional = Optional.of(name);
            Optional<Integer> intOpt = nameOptional
                    .map(String::length);
            log.info("name length is " + intOpt.orElse(0));
        }
    
    
        public static String getDefaultValue() {
            log.info("getDefaultValue");
            return "cakin";
        }
    }

2 测试

21:37:18.303 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.309 [main] INFO optionaldemo.OptionalTest - empty is Optional.empty
    21:37:18.311 [main] INFO optionaldemo.OptionalTest - opt is Optional[cakin]
    21:37:18.311 [main] INFO optionaldemo.OptionalTest - catch NullPointerException
    21:37:18.311 [main] INFO optionaldemo.OptionalTest - optOrNull isOptional.empty
    21:37:18.311 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.311 [main] INFO optionaldemo.OptionalTest - opt1 is true
    21:37:18.311 [main] INFO optionaldemo.OptionalTest - optOrNull1 is false
    21:37:18.311 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.311 [main] INFO optionaldemo.OptionalTest - optOrNull2 length is 5
    21:37:18.375 [main] INFO optionaldemo.OptionalTest - str length is 5
    21:37:18.375 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.375 [main] INFO optionaldemo.OptionalTest - name2 is cakin
    21:37:18.375 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - name3 is cakin
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - orElse
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - getDefaultValue
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - name5 is cakin
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - orElseGet
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - getDefaultValue
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - name6 is cakin
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - orElse
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - getDefaultValue
    21:37:18.376 [main] INFO optionaldemo.OptionalTest - name8 is cakin
    21:37:18.377 [main] INFO optionaldemo.OptionalTest - orElseGet
    21:37:18.377 [main] INFO optionaldemo.OptionalTest - name9 is cakin
    21:37:18.377 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.377 [main] INFO optionaldemo.OptionalTest - catch NoSuchElementException
    21:37:18.378 [main] INFO optionaldemo.OptionalTest - null
    21:37:18.379 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.383 [main] INFO optionaldemo.OptionalTest - pass:false
    21:37:18.389 [main] INFO optionaldemo.OptionalTest - pass:true
    21:37:18.389 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    21:37:18.391 [main] INFO optionaldemo.OptionalTest - name length is 5
    21:37:18.393 [main] INFO optionaldemo.OptionalTest - pass:true
    21:37:18.393 [main] INFO optionaldemo.OptionalTest - ++++++++++++++++++++++++++++++++++++++++++++++
    
    Process finished with exit code 0

**四 参考**

[https://blog.csdn.net/qing\_gee/article/details/104767082][https_blog.csdn.net_qing_gee_article_details_104767082]

[https_blog.csdn.net_qing_gee_article_details_104767082]: https://blog.csdn.net/qing_gee/article/details/104767082