为什么valueOf有两个参数?
在Java documentation for valueOf
public static <T extends Enum<T>> T valueOf(Class<T> enumType,String name)
Parameters:
enumType – the Class object of the enum type from which to return a constant
name – the name of the constant to return
但我在网上看到的大多数例子都说:
enum WorkDays { MONDAY,TUESDAY,WednESDAY,THURSDAY,FRIDAY; }
测试:
WorkDays day1 = WorkDays.valueOf("MONDAY"); System.out.println(day1); // >>> MONDAY
解决方法
你可以检查字节码,看看编译枚举时会发生什么:
public enum TestEnum {A,B}
而valueOf的字节码:
// access flags 0x9 public static valueOf(Ljava/lang/String;)LTestEnum; L0 LINENUMBER 1 L0 LDC LTestEnum;.class ALOAD 0 INVOKESTATIC java/lang/Enum.valueOf (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; CHECKCAST TestEnum ARETURN L1 LOCALVARIABLE name Ljava/lang/String; L0 L1 0 MAXSTACK = 2 MAXLOCALS = 1
我不是字节码的专家,但你可以看到这一行:
INVOKESTATIC java/lang/Enum.valueOf (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
实际上调用java.lang.Enum.valueOf. Java等价物看起来像这样:
public static TestEnum myValueOf(String name) { return Enum.valueOf(TestEnum.class,name); }
字节码证实了这一点:
// access flags 0x9 public static myValueOf(Ljava/lang/String;)LTestEnum; L0 LINENUMBER 6 L0 LDC LTestEnum;.class ALOAD 0 INVOKESTATIC java/lang/Enum.valueOf (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; CHECKCAST TestEnum ARETURN L1 LOCALVARIABLE name Ljava/lang/String; L0 L1 0 MAXSTACK = 2 MAXLOCALS = 1
比较这两个片段,您可以看到差异是……是的,名称(和行号):