爲什麼建議你使用枚舉？

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"枚舉是 JDK 1.5 新增的數據類型，使用枚舉我們可以很好的描述一些特定的業務場景，比如一年中的春、夏、秋、冬，還有每週的週一到周天，還有各種顏色，以及可以用它來描述一些狀態信息，比如錯誤碼等。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"枚舉類型不止存在在 Java 語言中，在其它語言中也都能找到它的身影，例如 C# 和 Python 等，但我發現在實際的項目中使用枚舉的人很少，所以本文就來聊一聊枚舉的相關內容，好讓朋友們對枚舉有一個大概的印象，這樣在編程時起碼還能想到有“枚舉”這樣一個類型。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"枚舉的 7 種使用方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"很多人不使用枚舉的一個重要的原因是對枚舉不夠熟悉，那麼我們就先從枚舉的 7 種使用方法說起。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"用法一：常量"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在 JDK 1.5 之前，我們定義常量都是 "},{"type":"codeinline","content":[{"type":"text","text":"public static final..."}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" ，但有了枚舉，我們就可以把這些常量定義成一個枚舉類了，實現代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public enum ColorEnum { \n RED, GREEN, BLANK, YELLOW \n} \n"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"用法二：switch"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"將枚舉用在 switch 判斷中，使得代碼可讀性更高了，實現代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"enum ColorEnum {\n GREEN, YELLOW, RED\n}\npublic class ColorTest {\n ColorEnum color = ColorEnum.RED;\n\n public void change() {\n switch (color) {\n case RED:\n color = ColorEnum.GREEN;\n break;\n case YELLOW:\n color = ColorEnum.RED;\n break;\n case GREEN:\n color = ColorEnum.YELLOW;\n break;\n }\n }\n}\n"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"用法三：枚舉中增加方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們可以在枚舉中增加一些方法，讓枚舉具備更多的特性，實現代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class EnumTest {\n public static void main(String[] args) {\n ErrorCodeEnum errorCode = ErrorCodeEnum.SUCCESS;\n System.out.println(\"狀態碼：\" + errorCode.code() + \n \" 狀態信息：\" + errorCode.msg());\n }\n}\n\nenum ErrorCodeEnum {\n SUCCESS(1000, \"success\"),\n PARAM_ERROR(1001, \"parameter error\"),\n SYS_ERROR(1003, \"system error\"),\n NAMESPACE_NOT_FOUND(2001, \"namespace not found\"),\n NODE_NOT_EXIST(3002, \"node not exist\"),\n NODE_ALREADY_EXIST(3003, \"node already exist\"),\n UNKNOWN_ERROR(9999, \"unknown error\");\n\n private int code;\n private String msg;\n\n ErrorCodeEnum(int code, String msg) {\n this.code = code;\n this.msg = msg;\n }\n\n public int code() {\n return code;\n }\n\n public String msg() {\n return msg;\n }\n\n public static ErrorCodeEnum getErrorCode(int code) {\n for (ErrorCodeEnum it : ErrorCodeEnum.values()) {\n if (it.code() == code) {\n return it;\n }\n }\n return UNKNOWN_ERROR;\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上程序的執行結果爲："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"狀態碼：1000 狀態信息：success"}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"用法四：覆蓋枚舉方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們可以覆蓋一些枚舉中的方法用於實現自己的業務，比如我們可以覆蓋 "},{"type":"codeinline","content":[{"type":"text","text":"toString()"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 方法，實現代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class EnumTest {\n public static void main(String[] args) {\n ColorEnum colorEnum = ColorEnum.RED;\n System.out.println(colorEnum.toString());\n }\n}\n\nenum ColorEnum {\n RED(\"紅色\", 1), GREEN(\"綠色\", 2), BLANK(\"白色\", 3), YELLOW(\"黃色\", 4);\n // 成員變量\n private String name;\n private int index;\n\n // 構造方法\n private ColorEnum(String name, int index) {\n this.name = name;\n this.index = index;\n }\n\n //覆蓋方法\n @Override\n public String toString() {\n return this.index + \"：\" + this.name;\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上程序的執行結果爲："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"1：紅色"}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"用法五：實現接口"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"枚舉類可以用來實現接口，但不能用於繼承類，因爲枚舉默認繼承了 "},{"type":"codeinline","content":[{"type":"text","text":"java.lang.Enum"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 類，在 Java 語言中允許實現多接口，但不能繼承多個父類，實現代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class EnumTest {\n public static void main(String[] args) {\n ColorEnum colorEnum = ColorEnum.RED;\n colorEnum.print();\n System.out.println(\"顏色：\" + colorEnum.getInfo());\n }\n}\n\ninterface Behaviour {\n void print();\n\n String getInfo();\n}\n\nenum ColorEnum implements Behaviour {\n RED(\"紅色\", 1), GREEN(\"綠色\", 2), BLANK(\"白色\", 3), YELLOW(\"黃色\", 4);\n private String name;\n private int index;\n\n private ColorEnum(String name, int index) {\n this.name = name;\n this.index = index;\n }\n\n @Override\n public void print() {\n System.out.println(this.index + \"：\" + this.name);\n }\n\n @Override\n public String getInfo() {\n return this.name;\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上程序的執行結果爲："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"1：紅色"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"顏色：紅色"}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"用法六：在接口中組織枚舉類"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們可以在一個接口中創建多個枚舉類，用它可以很好的實現“多態”，也就是說我們可以將擁有相同特性，但又有細微實現差別的枚舉類聚集在一個接口中，實現代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class EnumTest {\n public static void main(String[] args) {\n // 賦值第一個枚舉類\n ColorInterface colorEnum = ColorInterface.ColorEnum.RED;\n System.out.println(colorEnum);\n // 賦值第二個枚舉類\n colorEnum = ColorInterface.NewColorEnum.NEW_RED;\n System.out.println(colorEnum);\n }\n}\n\ninterface ColorInterface {\n enum ColorEnum implements ColorInterface {\n GREEN, YELLOW, RED\n }\n enum NewColorEnum implements ColorInterface {\n NEW_GREEN, NEW_YELLOW, NEW_RED\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上程序的執行結果爲："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"RED"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"NEW_RED"}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"用法七：使用枚舉集合"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在 Java 語言中和枚舉類相關的，還有兩個枚舉集合類 "},{"type":"codeinline","content":[{"type":"text","text":"java.util.EnumSet"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 和 "},{"type":"codeinline","content":[{"type":"text","text":"java.util.EnumMap"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":"，使用它們可以實現更多的功能。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"使用 "},{"type":"codeinline","content":[{"type":"text","text":"EnumSet"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 可以保證元素不重複，並且能獲取指定範圍內的元素，示例代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"import java.util.ArrayList;\nimport java.util.EnumSet;\nimport java.util.List;\n\npublic class EnumTest {\n public static void main(String[] args) {\n List list = new ArrayList();\n list.add(ColorEnum.RED);\n list.add(ColorEnum.RED); // 重複元素\n list.add(ColorEnum.YELLOW);\n list.add(ColorEnum.GREEN);\n // 去掉重複數據\n EnumSet enumSet = EnumSet.copyOf(list);\n System.out.println(\"去重：\" + enumSet);\n\n // 獲取指定範圍的枚舉（獲取所有的失敗狀態）\n EnumSet errorCodeEnums = EnumSet.range(ErrorCodeEnum.ERROR, ErrorCodeEnum.UNKNOWN_ERROR);\n System.out.println(\"所有失敗狀態：\" + errorCodeEnums);\n }\n}\n\nenum ColorEnum {\n RED(\"紅色\", 1), GREEN(\"綠色\", 2), BLANK(\"白色\", 3), YELLOW(\"黃色\", 4);\n private String name;\n private int index;\n\n private ColorEnum(String name, int index) {\n this.name = name;\n this.index = index;\n }\n}\n\nenum ErrorCodeEnum {\n SUCCESS(1000, \"success\"),\n ERROR(2001, \"parameter error\"),\n SYS_ERROR(2002, \"system error\"),\n NAMESPACE_NOT_FOUND(2003, \"namespace not found\"),\n NODE_NOT_EXIST(3002, \"node not exist\"),\n NODE_ALREADY_EXIST(3003, \"node already exist\"),\n UNKNOWN_ERROR(9999, \"unknown error\");\n\n private int code;\n private String msg;\n\n ErrorCodeEnum(int code, String msg) {\n this.code = code;\n this.msg = msg;\n }\n\n public int code() {\n return code;\n }\n\n public String msg() {\n return msg;\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上程序的執行結果爲："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"去重：[RED, GREEN, YELLOW]"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"所有失敗狀態：[ERROR, SYS_ERROR, NAMESPACE_NOT_FOUND, NODE_NOT_EXIST, NODE_ALREADY_EXIST, UNKNOWN_ERROR]"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"EnumMap"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 與 "},{"type":"codeinline","content":[{"type":"text","text":"HashMap"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 類似，不過它是一個專門爲枚舉設計的 "},{"type":"codeinline","content":[{"type":"text","text":"Map"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 集合，相比 "},{"type":"codeinline","content":[{"type":"text","text":"HashMap"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 來說它的性能更高，因爲它內部放棄使用鏈表和紅黑樹的結構，採用數組作爲數據存儲的結構。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"EnumMap"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 基本使用示例如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"import java.util.EnumMap;\n\npublic class EnumTest {\n public static void main(String[] args) {\n EnumMap enumMap = new EnumMap<>(ColorEnum.class);\n enumMap.put(ColorEnum.RED, \"紅色\");\n enumMap.put(ColorEnum.GREEN, \"綠色\");\n enumMap.put(ColorEnum.BLANK, \"白色\");\n enumMap.put(ColorEnum.YELLOW, \"黃色\");\n System.out.println(ColorEnum.RED + \":\" + enumMap.get(ColorEnum.RED));\n }\n}\n\nenum ColorEnum {\n RED, GREEN, BLANK, YELLOW;\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上程序的執行結果爲："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"RED:紅色"}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"使用注意事項"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"阿里《Java開發手冊》對枚舉的相關規定如下，我們在使用時需要稍微注意一下。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"【強制】所有的枚舉類型字段必須要有註釋，說明每個數據項的用途。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"【參考】枚舉類名帶上 Enum 後綴，枚舉成員名稱需要全大寫，單詞間用下劃線隔開。說明：枚舉其實就是特殊的常量類，且構造方法被默認強制是私有。正例：枚舉名字爲 ProcessStatusEnum 的成員名稱：SUCCESS / UNKNOWN_REASON。"}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"假如不使用枚舉"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在枚舉沒有誕生之前，也就是 JDK 1.5 版本之前，我們通常會使用 "},{"type":"codeinline","content":[{"type":"text","text":"int"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 常量來表示枚舉，實現代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public static final int COLOR_RED = 1;\npublic static final int COLOR_BLUE = 2;\npublic static final int COLOR_GREEN = 3;\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是使用 "},{"type":"codeinline","content":[{"type":"text","text":"int"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 類型可能存在兩個問題："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第一， "},{"type":"codeinline","content":[{"type":"text","text":"int"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 類型本身並不具備安全性，假如某個程序員在定義 "},{"type":"codeinline","content":[{"type":"text","text":"int"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 時少些了一個 "},{"type":"codeinline","content":[{"type":"text","text":"final"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 關鍵字，那麼就會存在被其他人修改的風險，而反觀枚舉類，它“天然”就是一個常量類，不存在被修改的風險（原因詳見下半部分）；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第二，使用 "},{"type":"codeinline","content":[{"type":"text","text":"int"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 類型的語義不夠明確，比如我們在控制檯打印時如果只輸出 1...2...3 這樣的數字，我們肯定不知道它代表的是什麼含義。"}]}]}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/bf/bfdcfe9ea61e1d267cd324894fade22c.png","alt":null,"title":"","style":[{"key":"width","value":"25%"},{"key":"bordertype","value":"boxShadow"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"那有人就說了，那就使用常量字符唄，這總不會還不知道語義吧？實現示例代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public static final String COLOR_RED = \"RED\";\npublic static final String COLOR_BLUE = \"BLUE\";\npublic static final String COLOR_GREEN = \"GREEN\";\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"但是這樣同樣存在一個問題，有些初級程序員會不按套路出牌，他們可能會直接使用字符串的值進行比較，而不是直接使用枚舉的字段，實現示例代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class EnumTest {\n public static final String COLOR_RED = \"RED\";\n public static final String COLOR_BLUE = \"BLUE\";\n public static final String COLOR_GREEN = \"GREEN\";\n public static void main(String[] args) {\n String color = \"BLUE\";\n if (\"BLUE\".equals(color)) {\n System.out.println(\"藍色\");\n }\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"這樣當我們修改了枚舉中的值，那程序就涼涼了。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"枚舉使用場景"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"枚舉的常見使用場景是單例，它的完整實現代碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class Singleton {\n // 枚舉類型是線程安全的，並且只會裝載一次\n private enum SingletonEnum {\n INSTANCE;\n // 聲明單例對象\n private final Singleton instance;\n // 實例化\n SingletonEnum() {\n instance = new Singleton();\n }\n private Singleton getInstance() {\n return instance;\n }\n }\n // 獲取實例（單例對象）\n public static Singleton getInstance() {\n return SingletonEnum.INSTANCE.getInstance();\n }\n private Singleton() {\n }\n // 類方法\n public void sayHi() {\n System.out.println(\"Hi,Java.\");\n }\n}\nclass SingletonTest {\n public static void main(String[] args) {\n Singleton singleton = Singleton.getInstance();\n singleton.sayHi();\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲枚舉只會在類加載時裝載一次，所以它是線程安全的，這也是《Effective Java》作者極力推薦使用枚舉來實現單例的主要原因。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"知識擴展"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"枚舉爲什麼是線程安全的？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這一點要從枚舉最終生成的字節碼說起，首先我們先來定義一個簡單的枚舉類："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public enum ColorEnumTest {\n RED, GREEN, BLANK, YELLOW;\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"然後我們再將上面的那段代碼編譯爲字節碼，具體內容如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public final class ColorEnumTest extends java.lang.Enum {\n public static final ColorEnumTest RED;\n public static final ColorEnumTest GREEN;\n public static final ColorEnumTest BLANK;\n public static final ColorEnumTest YELLOW;\n public static ColorEnumTest[] values();\n public static ColorEnumTest valueOf(java.lang.String);\n static {};\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"從上述結果可以看出枚舉類最終會被編譯爲被 "},{"type":"codeinline","content":[{"type":"text","text":"final"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 修飾的普通類，它的所有屬性也都會被 "},{"type":"codeinline","content":[{"type":"text","text":"static"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 和 "},{"type":"codeinline","content":[{"type":"text","text":"final"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 關鍵字修飾，所以枚舉類在項目啓動時就會被 JVM 加載並初始化，而這個執行過程是線程安全的，所以枚舉類也是線程安全的類。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#616161","name":"user"}},{"type":"bgcolor","attrs":{"color":"#FBF9FD","name":"user"}}],"text":"小貼士：代碼反編譯的過程是先用 javac 命令將 java 代碼編譯字節碼（.class），再使用 javap 命令查看編譯的字節碼。"}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"枚舉比較小技巧"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們在枚舉比較時使用 == 就夠了，因爲枚舉類是在程序加載時就創建了（它並不是 "},{"type":"codeinline","content":[{"type":"text","text":"new"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 出來的），並且枚舉類不允許在外部直接使用 "},{"type":"codeinline","content":[{"type":"text","text":"new"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 關鍵字來創建枚舉實例，所以我們在使用枚舉類時本質上只有一個對象，因此在枚舉比較時使用 == 就夠了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"並且我們在查看枚舉的 "},{"type":"codeinline","content":[{"type":"text","text":"equlas()"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 源碼會發現，它的內部其實還是直接調用了 == 方法，源碼如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public final boolean equals(Object other) {\n return this==other;\n}\n"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}],"text":"總結"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本文我們介紹了枚舉類的 7 種使用方法：常量、switch、枚舉中添加方法、覆蓋枚舉方法、實現接口、在接口中組織枚舉類和使用枚舉集合等，然後講了如果不使用枚舉類使用 "},{"type":"codeinline","content":[{"type":"text","text":"int"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 類型和 "},{"type":"codeinline","content":[{"type":"text","text":"String"}],"marks":[{"type":"color","attrs":{"color":"#35b378","name":"user"}}]},{"type":"text","text":" 類型存在的一些弊端：語義不夠清晰、容易被修改、存在被誤用的風險，所以我們在適合的環境下應該儘量使用枚舉類。並且我們還講了枚舉類的使用場景——單例，以及枚舉類爲什麼是安全的，最後我們講了枚舉比較的小技巧，希望本文對你有幫助。"}]}]}