语法糖
语法糖(Syntactic sugar),也被译为糖衣语法,是由英国计算机科学家彼得·约翰·兰达(Peter J. Landin)发明的一个术语,指计算机语言中添加的某种语法,这种语法对语言的功能并没有影响,但是更方便程序员使用。通常来说使用语法糖能够增加程序的可读性,从而减少程序代码出错的机会。——摘抄自百度百科
本质上,JVM 并不支持语法糖,语法糖只存在于 编译期。当编译器将 .java 源文件编译成 .class 字节码文件时,会进行 解语法糖的操作,来还原最原始的基础语法结构。
我们所熟悉的编程语言中几乎都会包含语法糖,当然 JAVA 也不例外。JAVA 中的语法糖包含 条件编译、 断言、 switch 支持 String 与枚举、 可变参数、 自动装箱/拆箱、 枚举、 内部类、 泛型擦除、 增强for循环、 lambda表达式、 try-with-resources等等。今天我们先来了解下 枚举。
枚举类
JDK5 提供了一种新的特殊的类——枚举类,一般在类对象有限且固定的场景下使用,用来替代类中定义常量的方式。枚举相较于常量更加直观且类型安全。
枚举类的使用非常简单,用 enum 关键字来定义,多个枚举变量直接用逗号隔开。我们先来定义一个简单的枚举类 OrderStatus.java
<span class="hljs-keyword">public</span> <span class="hljs-keyword">enum</span> OrderStatus {<br>    <br>    NO_PAY, PAY, REFUNDING, REFUNDED, FAIL_REFUNDED, ;<br>}
在其他类中使用 enum 变量的时候,只需要【类名.变量名】就可以了,和使用静态变量一样。另外,枚举类型可以确保 JVM 中仅存在 一个常量实例,所以我们可以放心的使用” ==”来比较两个变量。
注意事项:
我们可以先通过 javac 命令或者 IDEA 的编译功能将 OrderStatus.java
编译为 OrderStatus.class
字节码文件,然后用 DJ Java Decompiler
反编译器对 .class 文件进行反编译。
如果需要 DJ Java Decompiler
反编译器的小伙伴可以 私信阿Q获取!
<span class="hljs-keyword">public</span> <span class="hljs-keyword">final</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">OrderStatus</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Enum</span><br></span>{<br><br>    <br>    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> OrderStatus[] values()<br>    {<br>        <span class="hljs-keyword">return</span> (OrderStatus[])$VALUES.clone();<br>    }<br><br>    <br>    <br>    <br>    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> OrderStatus <span class="hljs-title">valueOf</span><span class="hljs-params">(String name)</span><br>    </span>{<br>        <span class="hljs-keyword">return</span> (OrderStatus)Enum.valueOf(com/itcast/java/enumpack/OrderStatus, name);<br>    }<br><br>    <span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-title">OrderStatus</span><span class="hljs-params">(String s, <span class="hljs-keyword">int</span> i)</span><br>    </span>{<br>        <span class="hljs-keyword">super</span>(s, i);<br>    }<br><br>    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">final</span> OrderStatus NO_PAY;<br>    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">final</span> OrderStatus PAY;<br>    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">final</span> OrderStatus REFUNDING;<br>    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">final</span> OrderStatus REFUNDED;<br>    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">final</span> OrderStatus FAIL_REFUNDED;<br>    <span class="hljs-keyword">private</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">final</span> OrderStatus $VALUES[];<br><br>    <span class="hljs-keyword">static</span> <br>    {<br>        NO_PAY = <span class="hljs-keyword">new</span> OrderStatus(<span class="hljs-string">"NO_PAY"</span>, <span class="hljs-number">0</span>);<br>        PAY = <span class="hljs-keyword">new</span> OrderStatus(<span class="hljs-string">"PAY"</span>, <span class="hljs-number">1</span>);<br>        REFUNDING = <span class="hljs-keyword">new</span> OrderStatus(<span class="hljs-string">"REFUNDING"</span>, <span class="hljs-number">2</span>);<br>        REFUNDED = <span class="hljs-keyword">new</span> OrderStatus(<span class="hljs-string">"REFUNDED"</span>, <span class="hljs-number">3</span>);<br>        FAIL_REFUNDED = <span class="hljs-keyword">new</span> OrderStatus(<span class="hljs-string">"FAIL_REFUNDED"</span>, <span class="hljs-number">4</span>);<br>        $VALUES = (<span class="hljs-keyword">new</span> OrderStatus[] {<br>            NO_PAY, PAY, REFUNDING, REFUNDED, FAIL_REFUNDED<br>        });<br>    }<br>}
如源码所示:
- 编译器会自动帮我们创建一个 final 类型的类继承 Enum 类,所以枚举类不能被继承。
- 会自动生成 私有 构造方法,当然我们也可以定义构造方法,但必须是私有的,这样就不能在别处声明此类的对象了。
- 枚举项会被自动添加
public static final
修饰,并定义为OrderStatus
类型,并在 静态代码块 中被初始化。 - 并提供了 values() 和 valueOf(String name) 的静态方法。
我们定义的枚举变量实际上是编译器帮我们自动生成了构造函数。
所有枚举类都是 Enum 的子类,枚举类可以实现一个或多个接口。
Enum 是所有 Java 语言枚举类型的公共基类,实现了 Comparable 和 Serializable 接口。它包含 final 类型的 name 和 ordinal (此枚举常量的序号,从0开始)属性,下面我们来了解下它的方法
- protected Enum(String name, int ordinal);——构造方法;
- public String toString();——返回 name 字段,即枚举定义枚举变量的字符串;
- protected final Object clone();——抛出 CloneNotSupportedException 异常,保证枚举类永远不会被克隆;
- public final Class getDeclaringClass();——返回与此枚举常量的枚举类型对应的类对象;
- protected final void finalize();—— 枚举类不能有 finalize 方法;
- readObject(ObjectInputStream in);& readObjectNoData();—— 抛出InvalidObjectException 异常,防止默认反序列化;
枚举类也可以有抽象方法,但是枚举项必须重写该方法。
<br><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">interface</span> <span class="hljs-title">Order</span> </span>{<br>    <span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">printOrderStatus</span><span class="hljs-params">()</span></span>;<br>}<br><br><br><span class="hljs-keyword">public</span> <span class="hljs-keyword">enum</span> OrderStatus implements Order{<br>    NO_PAY(<span class="hljs-string">"未支付"</span>,<span class="hljs-number">0</span>){<br>        <br>        <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">printOrderStatus</span><span class="hljs-params">()</span> </span>{<br>            System.out.println(<span class="hljs-string">"未支付"</span>);<br>        }<br>    },<br>    PAY(<span class="hljs-string">"已支付"</span>,<span class="hljs-number">1</span>){<br>        <br>        <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">printOrderStatus</span><span class="hljs-params">()</span> </span>{<br>            System.out.println(<span class="hljs-string">"已支付"</span>);<br>        }<br>    },<br>    REFUNDING(<span class="hljs-string">"退款中"</span>,<span class="hljs-number">2</span>){<br>        <br>        <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">printOrderStatus</span><span class="hljs-params">()</span> </span>{<br>            System.out.println(<span class="hljs-string">"退款中"</span>);<br>        }<br>    },<br>    REFUNDED(<span class="hljs-string">"退款成功"</span>,<span class="hljs-number">3</span>){<br>        <br>        <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">printOrderStatus</span><span class="hljs-params">()</span> </span>{<br>            System.out.println(<span class="hljs-string">"退款成功"</span>);<br>        }<br>    },<br>    FAIL_REFUNDED(<span class="hljs-string">"退款失败"</span>,<span class="hljs-number">4</span>){<br>        <br>        <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">printOrderStatus</span><span class="hljs-params">()</span> </span>{<br>            System.out.println(<span class="hljs-string">"退款失败"</span>);<br>        }<br>    },<br>    ;<br><br>    <span class="hljs-keyword">private</span> <span class="hljs-keyword">final</span> String name;<br>    <span class="hljs-keyword">private</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">int</span> status;<br><br>    <span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-title">OrderStatus</span><span class="hljs-params">(String name,<span class="hljs-keyword">int</span> status)</span></span>{<br>        <span class="hljs-keyword">this</span>.name = name;<br>        <span class="hljs-keyword">this</span>.status = status;<br>    }<br>}
此时查看编译后的文件,会发现除了生成 OrderStatus.class 文件之外,还生成了多个 .class 文件:
它们是 OrderStatus.class 中生成的匿名内部类的文件。
状态转换
订单是电商项目中不可缺少的组成部分,而订单状态的转换也是我们经常讨论的问题。我们都知道订单状态的转换是有一定的逻辑性的,不可以随意转换。
例:你想购买某个商品,只是把它加入了购物车,此时应该是 未支付状态。如果来个请求想把它转换为 退款状态,那么系统应该抛出提示信息”状态转换失败,请先完成购买!”
接下来我们就用 枚举来完成一下订单状态转换的限制。
枚举类定义:
<span class="hljs-keyword">public</span> <span class="hljs-keyword">enum</span> OrderStatus{<br>    NO_PAY(<span class="hljs-string">"未支付"</span>,<span class="hljs-number">0</span>){<br>        <br>        <span class="hljs-function"><span class="hljs-keyword">public</span> Boolean <span class="hljs-title">canChange</span><span class="hljs-params">(OrderStatus orderStatus)</span> </span>{<br>            <span class="hljs-keyword">switch</span> (orderStatus){<br>                <span class="hljs-keyword">case</span> PAY:<br>                    <span class="hljs-keyword">return</span> <span class="hljs-keyword">true</span>;<br>                <span class="hljs-keyword">default</span>:<br>                    <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>;<br>            }<br>        }<br>    },<br>    PAY(<span class="hljs-string">"已支付"</span>,<span class="hljs-number">1</span>){<br>        <br>        <span class="hljs-function"><span class="hljs-keyword">public</span> Boolean <span class="hljs-title">canChange</span><span class="hljs-params">(OrderStatus orderStatus)</span> </span>{<br>            <br>            <span class="hljs-keyword">switch</span> (orderStatus){<br>                <span class="hljs-keyword">case</span> REFUNDING:<br>                    <span class="hljs-keyword">return</span> <span class="hljs-keyword">true</span>;<br>                <span class="hljs-keyword">default</span>:<br>                    <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>;<br>            }<br>        }<br>    },<br>    REFUNDING(<span class="hljs-string">"退款中"</span>,<span class="hljs-number">2</span>){<br>        <br>        <span class="hljs-function"><span class="hljs-keyword">public</span> Boolean <span class="hljs-title">canChange</span><span class="hljs-params">(OrderStatus orderStatus)</span> </span>{<br>            <span class="hljs-keyword">switch</span> (orderStatus){<br>                <span class="hljs-keyword">case</span> REFUNDED:<br>                <span class="hljs-keyword">case</span> FAIL_REFUNDED:<br>                    <span class="hljs-keyword">return</span> <span class="hljs-keyword">true</span>;<br>                <span class="hljs-keyword">default</span>:<br>                    <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>;<br>            }<br>        }<br>    },<br>    REFUNDED(<span class="hljs-string">"退款成功"</span>,<span class="hljs-number">3</span>),<br>    FAIL_REFUNDED(<span class="hljs-string">"退款失败"</span>,<span class="hljs-number">4</span>),<br>    ;<br><br>    <span class="hljs-keyword">private</span> <span class="hljs-keyword">final</span> String name;<br>    <span class="hljs-keyword">private</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">int</span> status;<br><br>    <span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-title">OrderStatus</span><span class="hljs-params">(String name,<span class="hljs-keyword">int</span> status)</span></span>{<br>        <span class="hljs-keyword">this</span>.name = name;<br>        <span class="hljs-keyword">this</span>.status = status;<br>    }<br><br>    <br>    <span class="hljs-function"><span class="hljs-keyword">public</span> Boolean <span class="hljs-title">canChange</span><span class="hljs-params">(OrderStatus orderStatus)</span></span>{<br>        <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>;<br>    }<br>}
调用方法:
<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">EnumTest</span> </span>{<br><br>    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span> </span>{<br>        Boolean aBoolean = OrderStatus.NO_PAY.canChange(OrderStatus.PAY);<br>        String statusStr = aBoolean?<span class="hljs-string">"可以"</span>:<span class="hljs-string">"不可以"</span>;<br>        System.out.println(<span class="hljs-string">"是否可以完成状态转换:"</span>+ statusStr);<br><br>        Boolean flag = OrderStatus.REFUNDED.canChange(OrderStatus.FAIL_REFUNDED);<br>        String flagStr = flag?<span class="hljs-string">"可以"</span>:<span class="hljs-string">"不可以"</span>;<br>        System.out.println(<span class="hljs-string">"是否可以完成状态转换:"</span>+ flagStr);<br>    }<br>}
返回结果:
这样我们就用枚举类实现了订单状态转换的限制。此例子只是为状态转换提供一种 思路,具体的流程还需要根据自己系统中的业务来具体处理。
如果你有不同的意见或者更好的 idea
,欢迎联系阿Q,添加阿Q可以加入技术交流群参与讨论呦!
Original: https://www.cnblogs.com/aqsaycode/p/15406956.html
Author: 阿Q说代码
Title: 语法糖甜不甜?巧用枚举实现“状态”转换限制
原创文章受到原创版权保护。转载请注明出处:https://www.johngo689.com/570008/
转载文章受原作者版权保护。转载请注明原作者出处!