泛型和类< extends Enum<?>>,EnumSet.allOf(class)vs class.getEnumConstants() [英] Generics and Class<? extends Enum<?>>, EnumSet.allOf(class) vs class.getEnumConstants()
问题描述
@EnumValue() enumClass = MyTestEnum.class)
private String field;
public enum MyTestEnum {
VAL1,VAL2;
}
仅当字段值为VAL1或VAL2 。
public class EnumNameValidator implements ConstraintValidator< EnumValue,String> {
private Set< String> AVAILABLE_ENUM_NAMES;
@Override
public void initialize(EnumValue enumValue){
Class< ;? extends Enum<?>> enumSelected = enumValue.enumClass();
设置< ;? extends Enum<?>> enumInstances = Sets.newHashSet(enumSelected.getEnumConstants());
AVAILABLE_ENUM_NAMES = FluentIterable
.from(enumInstances)
.transform(PrimitiveGuavaFunctions.ENUM_TO_NAME)
.toImmutableSet();
}
@Override
public boolean isValid(String value,ConstraintValidatorContext context){
if(value == null){
return true;
} else {
return AVAILABLE_ENUM_NAMES.contains(value);
}
}
}
不明白是为什么我的第一次尝试失败。使用以下代码代替 enumSelected.getEnumConstants()
:
设置< ;? extends Enum<?>> enumInstances = EnumSet.allOf(enumSelected);
Intellij 12不突出显示任何错误,但编译器说:
java:类java.util.EnumSet中的方法allOf< E>不能应用于给定类型;
必需:java.lang.Class< E>
found:java.lang.Class< capture#1 of? extends java.lang.Enum<?>>
reason:推断类型不符合声明的bound(s)
inferred:capture#1 of? extends java.lang.Enum<?>
bound(s):java.lang.Enum< capture#1 of? extends java.lang.Enum<?>>
我不明白问题,也具有工作正常的代码:
private static< T extends Enum< T& & EnumAlternativeName> T safeGetByAlternativeName(Class< T> enumClass,String alternativeName){
for(T t:EnumSet.allOf(enumClass)){
if(t.getAlternativeName()。equals(alternativeName)){
return t;
}
}
return null;
}
code>? extends Enum <?> 两个?
可以不同, allOf
code> T extends Enum< T> 其中 T
都是相同的。
例如,请考虑以下代码:
static enum MyEnum {}
static class EnumValue< T extends枚举> {
Class< T> enumClass;
EnumValue(Class< T> enumClass){
this.enumClass = enumClass;
}
Class< T> enumClass(){return enumClass; }
}
这些行将编译:
EnumValue<?> enumValue = new EnumValue(MyEnum.class); // raw constructor
Set< ;? extends Enum<?>> enumInstances = EnumSet.allOf(enumValue.enumClass());
因为我们知道两个 T
enumValue.enumClass()
是相同的,但不会:
EnumValue enumValue = new EnumValue(MyEnum.class);
Class< ;? extends Enum<?>> enumSelected = enumValue.enumClass();
设置< ;? extends Enum<?>> enumInstances = EnumSet.allOf(enumSelected);
,因为您通过使用 Class <将Enum<?>>
扩展为中间步骤。
I have the following BeanValidation code that works fine, and permits to validate that a bean annotated with:
@EnumValue(enumClass = MyTestEnum.class)
private String field;
public enum MyTestEnum {
VAL1, VAL2;
}
Will be validated only if the field value is "VAL1" or "VAL2".
public class EnumNameValidator implements ConstraintValidator<EnumValue, String> {
private Set<String> AVAILABLE_ENUM_NAMES;
@Override
public void initialize(EnumValue enumValue) {
Class<? extends Enum<?>> enumSelected = enumValue.enumClass();
Set<? extends Enum<?>> enumInstances = Sets.newHashSet(enumSelected.getEnumConstants());
AVAILABLE_ENUM_NAMES = FluentIterable
.from(enumInstances)
.transform(PrimitiveGuavaFunctions.ENUM_TO_NAME)
.toImmutableSet();
}
@Override
public boolean isValid(String value, ConstraintValidatorContext context) {
if ( value == null ) {
return true;
} else {
return AVAILABLE_ENUM_NAMES.contains(value);
}
}
}
What I don't understand is why my first attempt failed. Using instead of the enumSelected.getEnumConstants()
above the following code:
Set<? extends Enum<?>> enumInstances = EnumSet.allOf(enumSelected);
Intellij 12 doesn't highlight any error, but the compiler says:
java: method allOf in class java.util.EnumSet<E> cannot be applied to given types;
required: java.lang.Class<E>
found: java.lang.Class<capture#1 of ? extends java.lang.Enum<?>>
reason: inferred type does not conform to declared bound(s)
inferred: capture#1 of ? extends java.lang.Enum<?>
bound(s): java.lang.Enum<capture#1 of ? extends java.lang.Enum<?>>
I don't understand the problem, and I also have that code which works fine:
private static <T extends Enum<T> & EnumAlternativeName> T safeGetByAlternativeName(Class<T> enumClass, String alternativeName) {
for ( T t : EnumSet.allOf(enumClass) ) {
if ( t.getAlternativeName().equals(alternativeName) ) {
return t;
}
}
return null;
}
My guess is that in ? extends Enum<?>
the two ?
could be different whereas allOf
expects a T extends Enum<T>
where both T
are the same.
For example, consider the following code:
static enum MyEnum {}
static class EnumValue<T extends Enum<T>> {
Class<T> enumClass;
EnumValue(Class<T> enumClass) {
this.enumClass = enumClass;
}
Class<T> enumClass() { return enumClass; }
}
These lines will compile:
EnumValue<?> enumValue = new EnumValue(MyEnum.class); // raw constructor
Set<? extends Enum<?>> enumInstances = EnumSet.allOf(enumValue.enumClass());
because we know that the two T
in enumValue.enumClass()
are the same but this won't:
EnumValue enumValue = new EnumValue(MyEnum.class);
Class<? extends Enum<?>> enumSelected = enumValue.enumClass();
Set<? extends Enum<?>> enumInstances = EnumSet.allOf(enumSelected);
because you have lost information by using a Class<? extends Enum<?>>
as an intermediate step.
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