What does Class<? extends ParentClass> mean?

Question

I found a method like the one below.

public void simpleTest(Class <? extends ParentClass> myClass){

}

I didn't understand the expression : Class <? extends ParentClass> myClass Class <? extends ParentClass> myClass here.

Can anyone explain it?

Answer 1

Class <? extends ParentClass> myClass Class <? extends ParentClass> myClass is a method argument whose type is a Class that is parameterized to ensure that what's passed is a Class that represents some type that is a subtype of ParentClass.

ie given:

class ParentParentClass {}
class ParentClass extends ParentParentClass {}
class ChildClass extends ParentClass {}
class ChildChildClass extends ChildClass {}

public void simpleTest(Class <? extends ParentClass> myClass) {}

These are valid:

simpleTest(ParentClass.class);
simpleTest(ChildClass.class);
simpleTest(ChildChildClass.class);

These aren't valid because the argument doesn't "fit" inside the required type:

simpleTest(ParentParentClass.class);
simpleTest(String.class);
simpleTest(Date.class);
simpleTest(Object.class);

Answer 2

From the javadoc of java.lang.Class<T> :

T - the type of the class modeled by this Class object. For example, the type of String.class is Class<String> . Use Class<?> if the class being modeled is unknown.

Now replace String with your class: it's a Class of a type which is a ParentClass or a subclass of ParentClass.

Answer 3

Class myClass - means that myClass should be aa sub type of ParentClass.

class MyClass extends ParentClass {
}

simpleTest(ParentClass.class); // this is ok
simpleTest(MyClass.class); // this is ok

class SomeOtherClass {
}
simpleTest(SomeOtherClass.class); // will result in compiler error

Answer 4

It's a generic method taking a parametrized class.

It might be helpful to first consider a simpler generics example: List<? extends ParentClass> List<? extends ParentClass> means it's a List that only takes objects that fulfill the is-a relationship with ParentClass. Ie the parameter of the List implementation is-a ParentClass, but the type of the whole thing List<? extends ParentClass> List<? extends ParentClass> is List.

In your example, just substitute "Class" instead of "List". myClass is a Class, ie you can call the method with something like "MyClassName.class". Further, the parameter of the Class of my Class is-a ParentClass. This basically means that you can only pass the Class of ParentClass or its subclasses to the method.

See also: http://docs.oracle.com/javase/7/docs/api/java/lang/Class.html