问题
In Java SE 7 (and most probably in previous versions) the Enum class is declared like this:
public abstract class Enum<E extends Enum<E>>
extends Object
implements Comparable<E>, Serializable
The Enum class has a static method with this signature:
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.
The question is where does valueOf(String) come from ?
Is it a feature of the language, i.e. a feature built in the compiler ?
回答1:
This method is implicitly defined by the compiler.
From the documentation:
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.
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:
/**
* 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2:
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:
public enum myEnum { red, blue, green }
That alone, is a language feature or else you would need to do this:
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).
That is what would seem possible given that the new Enum is as much a language feature as it is a class to extend.
来源：https://stackoverflow.com/questions/11914792/where-does-the-enum-valueofstring-method-come-from