CODEWITHSUNDEEP
javagenericsraw-typesunbounded-wildcardparameterized-class
Levin Kwong
Levin Kwong

Reputation: 393

Java Generic For Parameterized Class : Unbounded Wildcards vs Raw Type

I have a parameterized interface RestHandler. 

public interface RestHandler<T> {
   blah blah blah...
}

And I need to create a class from config using Class.forName. Now I come up with three version , which ALL compiles successfully.

Version 1:

@SuppressWarnings("unchecked")
public static <T> RestHandler<T> create(final String handlerImplFqcn) throws ClassNotFoundException, 
                                                                    IllegalAccessException, 
                                                                    InstantiationException {
    Class<?> handlerClass = Class.forName(handlerImplFqcn);
    return (RestHandler<T>) handlerClass.newInstance();
}

Version 2:

public static RestHandler<?> create(final String handlerImplFqcn) throws ClassNotFoundException, 
                                                                    IllegalAccessException, 
                                                                    InstantiationException {
    @SuppressWarnings("rawtypes")
    Class handlerClass = Class.forName(handlerImplFqcn);
    return (RestHandler<?>) handlerClass.newInstance();
}

Version 3:

public static RestHandler<?> create(final String handlerImplFqcn) throws ClassNotFoundException, 
                                                                    IllegalAccessException, 
                                                                    InstantiationException {
    Class<?> handlerClass = Class.forName(handlerImplFqcn);
    return (RestHandler<?>) handlerClass.newInstance();
}

My question is , why they ALL work and which one would be best practice ?

Upvotes: 3

Views: 141

Answers (2)

Dave
Dave

Reputation: 591

Version 1, re-written to this:

public static <T extends RestHandler<?>> RestHandler<T> create(final String handlerImplFqcn) throws ClassNotFoundException, IllegalAccessException, InstantiationException, ClassCastException {
    Class<T> handlerClass = (Class<T>) Class.forName(handlerImplFqcn);
    return (RestHandler<T>) handlerClass.newInstance();
}

Upvotes: 2

OldCurmudgeon
OldCurmudgeon

Reputation: 65889

Use version 1.

Version 2 uses raw types - avoid these if at all possible.

Version 3 uses <?> which is also often an anti pattern as it rarely achieves what you mean to achieve.

Upvotes: 0

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