java Enum.valueOf(String) 方法从何而来？
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Where does the Enum.valueOf(String) method come from?
提问by Razvan
In Java SE 7 (and most probably in previous versions) the Enum class is declared like this:
在 Java SE 7（并且很可能在以前的版本中）中，Enum 类声明如下：
public abstract class Enum<E extends Enum<E>>
extends Object
implements Comparable<E>, Serializable
The Enum class has a static method with this signature:
Enum 类有一个带有此签名的静态方法：
T static<T extends Enum<T>> valueOf(Class<T> enumType, String name)
But there is no static method : valueOf(String)defined in the Enum class nor upwards in the hierarchy Enum belongs to.
但是没有静态方法：valueOf(String)在 Enum 类中定义，也没有在 Enum 所属的层次结构中向上定义。
The question is where does valueOf(String)come from ?
Is it a feature of the language, i.e. a feature built in the compiler ?
问题是valueOf(String)从哪里来？它是语言的特性，即编译器中内置的特性吗？
采纳答案by dasblinkenlight
This method is implicitly defined by the compiler.
这个方法是由编译器隐式定义的。
From the documentation:
从文档：
Note that for a particular enum type T, the implicitly declared public static T valueOf(String) method on that enum may be used instead of this method to map from a name to the corresponding enum constant. All the constants of an enum type can be obtained by calling the implicit public static T[] values() method of that type.
请注意，对于特定的枚举类型 T，可以使用该枚举上隐式声明的 public static T valueOf(String) 方法代替此方法将名称映射到相应的枚举常量。枚举类型的所有常量都可以通过调用该类型的隐式 public static T[] values() 方法获得。
From the Java Language Specification, section 8.9.2:
In addition, if E is the name of an enum type, then that type has the following implicitly declared static methods:
此外，如果 E 是枚举类型的名称，则该类型具有以下隐式声明的静态方法：
/**
* Returns an array containing the constants of this enum
* type, in the order they're declared. This method may be
* used to iterate over the constants as follows:
*
* for(E c : E.values())
* System.out.println(c);
*
* @return an array containing the constants of this enum
* type, in the order they're declared
*/
public static E[] values();
/**
* Returns the enum constant of this type with the specified
* name.
* The string must match exactly an identifier used to declare
* an enum constant in this type. (Extraneous whitespace
* characters are not permitted.)
*
* @return the enum constant with the specified name
* @throws IllegalArgumentException if this enum type has no
* constant with the specified name
*/
public static E valueOf(String name);
回答by The Coordinator
I think it must be a feature of the language. For one, to create an enum by making one and it does not need to extend Enum:
我认为这一定是语言的一个特征。一方面，通过创建一个枚举来创建一个枚举，并且不需要扩展枚举：
public enum myEnum { red, blue, green }
That alone, is a language feature or else you would need to do this:
仅此一项就是语言功能，否则您需要这样做：
public class MyEnum extends Enum { ..... }
Secondly, the method, Enum.valueOf(Class<T> enumType, String name), must be produced by the compiler when you you use myEnum.valueOf(String name).
其次，Enum.valueOf(Class<T> enumType, String name)当您使用myEnum.valueOf(String name).
That is what would seem possible given that the new Enumis as much a language feature as it is a class to extend.
鉴于 newEnum既是一个语言特性又是一个要扩展的类，这似乎是可能的。
