Generic method as functional programming using java.util.Function

Question

I am not sure can we write the generic method as a function programming using Java 8 lamda expression as below

Generic method

public <T> MongoCollection<T> getCollection(String collectionName, Class<T> typeParameterClass) {
    return mongoClient
            .getDatabase(mongodbConfiguration.getDatabase())
            .getCollection(collectionName, typeParameterClass);
}

I want to convert this generic method to generic functional lamda expression as below

public BiFunction<String, Class<T>, T> getCollection = (collectionName, typeParameterClass) -> {
    return mongoClient
            .getDatabase(mongodbConfiguration.getDatabase())
            .getCollection(collectionName, typeParameterClass);
};

From the above code I have an error on type T and typeParameterClass

Update 1

While implementing the method suggested by @Clashsoft the second argumenent is Class<Object> instead of Class<T>

repository.getCollectionFactory().apply("", Category.class);

Your signature should at minimum be BiFunction<String, Class<T>, MongoCollection<T>> (though I don't think that'll work either), as the return type is MongoCollection<T>, and not T. — Mark Rotteveel
– Mark Rotteveel, Commented Feb 19, 2021 at 9:45

Clashsoft · Accepted Answer · 2021-02-19 10:33:11Z

2

That's not possible with a field. You can however create another method / getter with a type parameter:

public <T> BiFunction<String, Class<T>, MongoCollection<T>> getCollectionFactory() {
    return  (collectionName, typeParameterClass) -> {
        return mongoClient
                .getDatabase(mongodbConfiguration.getDatabase())
                .getCollection(collectionName, typeParameterClass);
    };
};

Keep in mind you have to specify the type when calling the method:

repository.<Category>.getCollectionFactory().apply("test", Category.class)

Or use a variable with an explicit type:

BiFunction<String, Class<Category>, MongoCollection<Category>> factory = repository.getCollectionFactory();
factory.apply("test", Category.class)

Of course, you can already do this with the regular old getCollection method:

BiFunction<String, Class<Category>, MongoCollection<Category>> factory = repository::getCollection;

So the usefulness of the factory method is questionable.

edited Feb 19, 2021 at 10:33

answered Feb 19, 2021 at 9:27

Clashsoft

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7 Comments

khelwood Over a year ago

I think that should be BiFunction<String, Class<T>, MongoCollection<T>> since the function returns a MongoCollection<T>.

Clashsoft Over a year ago

@khelwood thanks, I copied that part from the question. Fixed!

San Jaisy Over a year ago

@Clashsoft While implementing this method repository.getCollectionFactory().apply("test" ,Category.class)); the second argument is Class<Object> but it should be Class<T>

Clashsoft Over a year ago

@SanJaisy yes that's a problem you'll have a lot with this approach. You need to specify the type argument in the getCollectionFactory call: repository.<Category>.getCollectionFactory().apply("test", Category.class). That's why I'd try to stick with the regular method.

Yassin Hajaj Over a year ago

It's possible with a field if the class is generic no?

|

Nikolas · Accepted Answer · 2021-02-19 10:48:10Z

1

The BiFunction has wrong the third type (the result type) that should be MongoCollection<T>:

public BiFunction<String, Class<T>, MongoCollection<T>> getCollection = (name, type) -> 
    mongoClient
        .getDatabase(mongodbConfiguration.getDatabase())
        .getCollection(name, type);

Also note the T must be the class generic parameter as long as getCollection is no longer a method but a field.

public class ClassHavingGetCollectionField<T> {
   ...
}

... otherwise you need to define a method with a generic type returning such functional interface implementation:

public <T> BiFunction<String, Class<T>, MongoCollection<T>> getCollectionBiFunction() {
    return (name, type) ->
         mongoClient
             .getDatabase(mongodbConfiguration.getDatabase())
             .getCollection(name, type);
}

edited Feb 19, 2021 at 10:48

answered Feb 19, 2021 at 10:39

Nikolas

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1 Comment

San Jaisy Over a year ago

This doesn't work, what is the type for T ?

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