There are three class,class IEntity is an abstract class,which is only used for generic.
Class one:
public abstract class IEntity {
}
class two:
public class Config {
private Class<? extends IEntity> clazz;
public Class<? extends IEntity> getClazz(){
return this.clazz;
}
}
class three:
public class EntityTest<T extends IEntity> {
private Class<T> clazz;
public void init(Config config){
//Here is the wrong report
// Failed to compile
this.clazz=config.getClazz();
}
}
It seems like you have not understood the difference between <? extends IEntity>
<? extends IEntity>
and <T extends IEntity>
.
Let's introduce two subclasses of IEntity
, and a constructor for Config
for a clearer explanation:
class Entity1 extends IEntity {}
class Entity2 extends IEntity {}
// in Config class
public Config(Class<? extends IEntity> clazz) {
this.clazz = clazz;
}
At the line this.clazz=config.getClazz();
, you are trying to assign a Class<? extends IEntity>
Class<? extends IEntity>
to a Class<T>
, where T
is a subclass of IEntity
or IEntity
itself. The problem is, we don't know exactly what type of class getClazz
returns. It could be Class<Entity1>
, or Class<Entity2>
. On the other hand, we do know what type we need - Class<T>
. How can we make sure that whatever getClazz
returns, is the same type of class as Class<T>
? T
could be Entity2
but getClazz
could return Class<Entity2>
, couldn't it?
Here's a concrete example with code:
Config c = new Config(Entity1.class);
// c.config now contains Entity1.class
// the line below will attempt to assign an Entity1.class to a variable of type Class<Entity2>
new EntityTest<Entity2>().init(c);
You should now see why there is an error in init
.
One way to make sure that getClazz
returns the same type of class as Class<T>
is to make Config
generic too:
class Config<T extends IEntity> {
private Class<T> clazz;
public Class<T> getClazz(){
return this.clazz;
}
}
class EntityTest<T extends IEntity> {
private Class<T> clazz;
public void init(Config<T> config){
this.clazz=config.getClazz();
}
}
<? extend IEntity>
<? extend IEntity>
indicates that the class must be IEntity or it's subclass. The "T" in your code is just one subclass of IEntity(there can be many subclasses of <? extend IEntity>
), you could not be sure that <? extend IEntity>
<? extend IEntity>
is same as "T" or is subclass of "T". so the type cast is illegale.
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