I am new to Generics and trying to understand why this code compiles:
public Collection<Class<Subclass>> testFunction() {
return Collections.singleton(Subclass.class);
}
And this code doesn't:
public Collection<Class<? extends SuperClass>> testFunction() {
return Collections.singleton(Subclass.class);
}
My SubClass looks like this:
public class Subclass extends SuperClass{
}
The above compiles fine with Java8:
class SuperClass { }
class Subclass extends SuperClass{ }
class Test {
public Collection<Class<? extends SuperClass>> testFunction() {
return Collections.singleton(Subclass.class);
}
}
The point is: with Java 8, type inference was heavily reworked and improved.
So my guess here is: this doesn't compile for you because you are using Java 7; where simply spoken the compiler wasn't "good enough" to correctly resolve such kind of code.
Collections.singleton(Integer.class);should create aCollection<Class<? extends Number>>but if it doesn't (and older compilers had problems here) you'll get a type conflict.eclipsec), but it works fine for mars. You can help out the compiler by providing the explicit type:Collections.<Class<? extends SuperClass>>singleton(Subclass.class);