As a method parameter in Java, what does the question mark allow you to do in Class<?>

Question

With the code segment below what does the question mark allow you to do, since it is in angle brackets by a class type? Does it allow for any class type to be passed in as a parameter?

Also, what does method 1 return? Does it return an object that can be cast to an ObjectService type?

method 1

public Object getInstance(Class<?> clazz) {
    if (org.service.ObjectService.class.equals(clazz))
        return getObjectService();
    return null;
}

method 2

public ObjectService getObjectService() {
    ObjectService service;

    service = (ObjectService) context.getBean("ObjectService");

    return service;
}

Answer 1

It tells the compiler to accept any type of Class as a parameter. Basically, this way you choose to turn off compiler's type check. Alternatively, you can omit <?> altogether with a compiler warning or do as Luiggi suggested and specify Class<Object> , which is not always the same though.

If your generic class is defined with restrictions, <Object> would not be an acceptable parameter.

For instance, if you have the following class somewhere:

public class TheClass<T extends Component> {
}

You can't do TheClass<Object> . You can do TheClass<Component> or TheClass<?> , which are essentially the same for TheClass case.

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UPDATE TO ANSWER UPDATED QUESTION

getInstance() in your case will defer to getObjectService(), which can only return ObjectService. You're guaranteed to receive one of the following:

• ObjectService instance.
• null.
• ClassCastException from method getObjectService(), where casting is performed.