铸造不';t使用类在java中工作<;T>;及<;T扩展A>;
我对Java编译器有一个奇怪的问题。代码如下:铸造不';t使用类在java中工作<;T>;及<;T扩展A>;,java,generics,Java,Generics,我对Java编译器有一个奇怪的问题。代码如下: private <T extends IdentifiedBusinessTransversalEntity> T getOrCreateTransversalEntity(Class<T> classT, String id) { ...} private <T extends IdentifiedBusinessDSEntity> T getOrCreateDSEntity(Class&l
private <T extends IdentifiedBusinessTransversalEntity> T
getOrCreateTransversalEntity(Class<T> classT, String id) {
...}
private <T extends IdentifiedBusinessDSEntity> T
getOrCreateDSEntity(Class<T> classT, String id) {
...}
public abstract class IdentifiedBusinessDSEntity extends
BusinessDSEntity implements IdentifiedEntity {
...}
public abstract class IdentifiedBusinessTransversalEntity extends
BusinessTransversalEntity implements IdentifiedEntity {
...}
public <T extends IdentifiedEntity> T getOrCreate(Class<T> classT, String id)
{
if (IdentifiedBusinessDSEntity.class.isAssignableFrom(classT))
{
return getOrCreateDSEntity(classT.asSubclass(IdentifiedBusinessDSEntity.class),id);
} else if (IdentifiedBusinessTransversalEntity.class.isAssignableFrom(classT))
{ //must cast explicitly to T here but works well just above and is exactly the same. Strange
return (T) getOrCreateTransversalEntity(classT.asSubclass(IdentifiedBusinessTransversalEntity.class),id);
}
return null;
}
private T
GetOrCreateTraversalEntity(类classT,字符串id){
...}
私家侦探
getOrCreateDSEntity(类classT,字符串id){
...}
公共抽象类IdentifiedBusinessDSEntity扩展
BusinessDSEntity实现标识性{
...}
公共抽象类标识的BusinessTransversalEntity扩展
BusinessTransversalEntity实现了IdentifiedEntity{
...}
公共T getOrCreate(类classT,字符串id)
{
if(IdentifiedBusinessDSEntity.class.isAssignableFrom(classT))
{
返回getOrCreateDSEntity(classT.asSubclass(IdentifiedBusinessDSEntity.class),id);
}else if(IdentifiedBusinessTransversalEntity.class.isAssignableFrom(classT))
{//这里必须显式地强制转换为T,但在上面工作得很好,并且完全相同。奇怪
return(T)getOrCreateTransversalEntity(classT.asSubclass(IdentifiedBusinessTransversalEntity.class),id);
}
返回null;
}
我不明白为什么在getOrCreate
函数中,编译器允许第一次返回而不使用(t)
,但不允许第二次返回。错误是:
类型不匹配:无法从IdentifiedBusinessTransversalEntity
转换为T
应该可以:getOrCreateTRansversalEntity
返回IdentifiedBusinessTransversalEntity
的子类,该子类实现了IdentifiedEntity
。
最奇怪的是,第一回还行,这是完全对称的。有什么区别吗?我试试看。这个解释是基于观察而不是明确的知识,所以如果我被纠正了,不要感到惊讶。我认为您混淆的是
T
返回类型声明的含义。在我看来,这个返回声明正在创建一个全新的类型,而不是描述一个已经声明的类型。换句话说,您可以将Class1作为Class1扩展ClassA
,但是当您返回T扩展ClassA
时,您不是描述Class1,而是一个全新的类(或类型)。您不能期望这个新类与Class1相同,而是说您返回了一个ClassX,其中ClassX扩展了ClassA
:Class1和ClassX不是相同的类
看起来您正在尝试使用缓存实现工厂设计模式。这段代码可以做到这一点,应该与上面的代码非常相似。假设工厂应该生成接口实现,则无需将返回类型声明为扩展接口的类,只需返回接口即可
public class Factory {
interface I {}
List<I> iCache = new ArrayList<I>();
abstract class ClassA {}
abstract class ClassB {}
class Class1 extends ClassA implements I {}
class Class2 extends ClassB implements I {}
I getOrCreateTypeA() {
for( I cls: iCache ) {
if( cls instanceof ClassA ) return cls;
}
Class1 cls = new Class1();
iCache.add(cls);
return cls;
}
I getOrCreateTypeB() {
for( I cls: iCache ) {
if( cls instanceof ClassB ) return cls;
}
Class2 cls = new Class2();
iCache.add(cls);
return cls;
}
I getOrCreate(Class<?> cls) {
if ( ClassA.class.isAssignableFrom(cls)) {
return getOrCreateTypeA();
} else if ( ClassB.class.isAssignableFrom(cls)) {
return getOrCreateTypeB();
}
return null;
}
void run() {
I classI1 = getOrCreate(Class1.class);
System.out.println(classI1);
I classI2 = getOrCreate(Class2.class);
System.out.println(classI2);
I classI3 = getOrCreate(Class1.class);
System.out.println(classI3);
System.out.println(iCache);
}
public static void main(String... args) {
new Factory().run();
}
}
公共类工厂{
接口I{}
List iCache=new ArrayList();
抽象类ClassA{}
抽象类ClassB{}
类Class1扩展ClassA实现I{}
类Class2扩展了类B实现I{}
I getOrCreateTypeA(){
对于(I cls:iCache){
如果(ClassA的cls实例)返回cls;
}
Class1 cls=新的Class1();
添加(cls);
返回cls;
}
I getOrCreateTypeB(){
对于(I cls:iCache){
如果(B类的cls实例)返回cls;
}
Class2 cls=新的Class2();
添加(cls);
返回cls;
}
I获取或创建(类cls){
if(ClassA.class.isAssignableFrom(cls)){
返回getOrCreateTypeA();
}else if(ClassB.class.isAssignableFrom(cls)){
返回getOrCreateTypeB();
}
返回null;
}
无效运行(){
I classI1=getOrCreate(Class1.class);
系统输出打印项次(1级);
I Class2=获取或创建(Class2.class);
系统输出打印LN(Class2);
I classI3=getOrCreate(Class1.class);
系统输出打印项次(3级);
系统输出打印项次(iCache);
}
公共静态void main(字符串…参数){
新工厂().run();
}
}
该代码不应编译
我相信您要么犯了一个错误,使您相信它是有效的,要么您的编译器中有bug。getOrCreate
中的两个返回语句都为我提供了Eclipse4.5.1和JavaC1.8.0中的编译错误
解释
编辑:我更改了这个解释,我想我一开始误解了这个问题
在解释中,我将getAndCreate
类型参数的名称更改为C
,以避免与其他类型参数混淆
问题是在asSubclass
方法中,类是C
的子类的信息丢失;剩下的唯一信息是该类是例如IdentifiedBusinessDSEntity
的子类
asSubclass
具有以下类型:
<U> Class<? extends U> asSubclass(Class<U> clazz)
这将是类型安全的,并且使用该返回类型,您的代码将在编译时不使用强制转换。但是,Java中不允许使用多个类型参数作为边界
代码
以下是我用来调查问题的代码:
class Test {
interface IdentifiedEntity {}
class BusinessDSEntity {}
class BusinessTransversalEntity {}
private <T extends IdentifiedBusinessTransversalEntity> T getOrCreateTransversalEntity(Class<T> classT, String id) {
return null;
}
private <T extends IdentifiedBusinessDSEntity> T getOrCreateDSEntity(Class<T> classT, String id) {
return null;
}
public abstract class IdentifiedBusinessDSEntity
extends BusinessDSEntity implements IdentifiedEntity {}
public abstract class IdentifiedBusinessTransversalEntity
extends BusinessTransversalEntity implements IdentifiedEntity {}
public <C extends IdentifiedEntity> C getOrCreate(Class<C> classT, String id) {
if (IdentifiedBusinessDSEntity.class.isAssignableFrom(classT)) {
// Error here. Note that the type of s does not contain C.
Class<? extends IdentifiedBusinessDSEntity> s = classT.asSubclass(IdentifiedBusinessDSEntity.class);
return getOrCreateDSEntity(s, id);
} else if (IdentifiedBusinessTransversalEntity.class.isAssignableFrom(classT)) {
// Also error here
return getOrCreateTransversalEntity(classT.asSubclass(IdentifiedBusinessTransversalEntity.class), id);
}
return null;
}
}
类测试{
接口标识性{}
类BusinessDSEntity{}
类业务横向属性{}
私有T getOrCreateTransversalEntity(类classT,字符串id){
返回null;
}
私有T getOrCreateDSEntity(类classT,字符串id){
返回null;
}
公共抽象类标识的BusinessDSEntity
扩展BusinessDSEntity实现IdentifiedEntity{}
公共抽象类标识的BusinessTransversalEntity
扩展BusinessTransversalEntity实现IdentifiedEntity{}
公共C getOrCreate(类classT,字符串id){
if(IdentifiedBusinessDSEntity.class.isAssignableFrom(classT)){
//此处出错。请注意,s的类型不包含C。
class Test {
interface IdentifiedEntity {}
class BusinessDSEntity {}
class BusinessTransversalEntity {}
private <T extends IdentifiedBusinessTransversalEntity> T getOrCreateTransversalEntity(Class<T> classT, String id) {
return null;
}
private <T extends IdentifiedBusinessDSEntity> T getOrCreateDSEntity(Class<T> classT, String id) {
return null;
}
public abstract class IdentifiedBusinessDSEntity
extends BusinessDSEntity implements IdentifiedEntity {}
public abstract class IdentifiedBusinessTransversalEntity
extends BusinessTransversalEntity implements IdentifiedEntity {}
public <C extends IdentifiedEntity> C getOrCreate(Class<C> classT, String id) {
if (IdentifiedBusinessDSEntity.class.isAssignableFrom(classT)) {
// Error here. Note that the type of s does not contain C.
Class<? extends IdentifiedBusinessDSEntity> s = classT.asSubclass(IdentifiedBusinessDSEntity.class);
return getOrCreateDSEntity(s, id);
} else if (IdentifiedBusinessTransversalEntity.class.isAssignableFrom(classT)) {
// Also error here
return getOrCreateTransversalEntity(classT.asSubclass(IdentifiedBusinessTransversalEntity.class), id);
}
return null;
}
}
class Test {
interface IdentifiedEntity {}
class BusinessDSEntity {}
class BusinessTransversalEntity {}
private <R extends IdentifiedEntity, T extends IdentifiedBusinessDSEntity>
R getOrCreateDSEntity(Class<T> classT, Class<R> classR, String id)
{
// Verify that classT really is subclass of classR.
classT.asSubclass(classR);
return null;
}
private <R extends IdentifiedEntity, T extends IdentifiedBusinessTransversalEntity>
R getOrCreateTransversalEntity(Class<T> classT, Class<R> classR, String id)
{
// Verify that classT really is subclass of classR.
classT.asSubclass(classR);
return null;
}
public abstract class IdentifiedBusinessDSEntity
extends BusinessDSEntity implements IdentifiedEntity {}
public abstract class IdentifiedBusinessTransversalEntity
extends BusinessTransversalEntity implements IdentifiedEntity {}
public <C extends IdentifiedEntity> C getOrCreate(Class<C> classT, String id) {
if (IdentifiedBusinessDSEntity.class.isAssignableFrom(classT)) {
return getOrCreateDSEntity(classT.asSubclass(IdentifiedBusinessDSEntity.class), classT, id);
} else if (IdentifiedBusinessTransversalEntity.class.isAssignableFrom(classT)) {
return getOrCreateTransversalEntity(classT.asSubclass(IdentifiedBusinessTransversalEntity.class), classT, id);
}
return null;
}
}