The reason is that Test.class is of the type Class<Test>. You cannot assign a reference of type Class<Test> to a variable of type Class<T> as they are not the same thing. This, however, works:
Class<? extends Test> testType = type == null ? Test.class : type;
The wildcard allows both Class<T> and Class<Test> references to be assigned to testType.
There is a ton of information about Java generics behavior at Angelika Langer Java Generics FAQ. I'll provide an example based on some of the information there that uses the
Number class heirarchy Java's core API.
Consider the following method:
public <T extends Number> void testNumber(final Class<T> type)
This is to allow for the following statements to be successfully compile:
But the following won't compile:
Now consider these statements:
Class<Number> numberClass = Number.class;
Class<Integer> integerClass = numberClass;
The second line fails to compile and produces this error
Type mismatch: cannot convert from Class<Number> to Class<Integer>. But
Number, so why does it fail? Look at these next two statements to see why:
Number anumber = new Long(0);
Integer another = anumber;
It is pretty easy to see why the 2nd line doesn't compile here. You can't assign an instance of
Number to a variable of type
Integer because there is no way to guarantee that the
Number instance is of a compatible type. In this example the
Number is actually a
Long, which certainly can't be assigned to an
Integer. In fact, the error is also a type mismatch:
Type mismatch: cannot convert from Number to Integer.
The rule is that an instance cannot be assigned to a variable that is a subclass of the type of the instance as there is no guarantee that is is compatible.
Generics behave in a similar manner. In the generic method signature,
T is just a placeholder to indicate what the method allows to the compiler. When the compiler encounters
testNumber(Integer.class) it essentially replaces
Wildcards add additional flexibility, as the following will compile:
Class<? extends Number> wildcard = numberClass;
Class<? extends Number> indicates any type that is a
Number or a subclass of
Number this is perfectly legal and potentially useful in many circumstances.