Java中泛型类型实现的返回类型应该是什么

Question

Refering to the following code snippet, I have a interface Splitter which takes generic type parameters T and V . There is one implementation which is CompanySplitterImpl . There could be many such implementations.

public interface Splitter<T, V> {
    V[] split(T arg);
}

public class CompanySplitterImpl
implements Splitter<Company, Department> {

    @Override
    public Department[] split(Company comp) {

        return comp.getDepartment();
    }
}

I am trying to write a factory method which returns different implementation based on a key parameter value passed in to the factory method.

// Factory method to return different Implementation of Splitter
// (Is Splitter return type correct?)
public static Splitter getSplitter(String key) {

    return new CompanySplitterImpl(); // Is this correct?
}

My question is what is the correct manner of returning instance of different implementation of Splitter<K, V> ?

// Client to call factory method...
// Not sure what will be type argument for Splitter type
Splitter<?> split = getSplitter("dummyKey");

At the client side, What will be the type argument for Splitter type?

Answer 1

Splitter is a raw type. You should not use raw types. Since the key (a String ) carries no type information, it is not possible to infer the type parameters from the argument passed to getSplitter . Therefore, the only way to avoid raw types is to make the return type Splitter<?, ?> .

This is ugly and forces the caller of the method to cast:

Splitter<Company, Department> split = (Splitter<Company, Department>) getSplitter("dummyKey");

A better way of doing this is to use a key that carries type information, and the usual way to do this is to use Class<T> objects.

public static <T, V> Splitter<T, V> getSplitter(Class<T> key1, Class<V> key2) {

    if (key1 == Company.class && key2 == Department.class)
        return (Splitter<T, V>) new CompanySplitterImpl();

    // more cases
}

Then the caller could do:

Splitter<Company, Department> split = getSplitter(Company.class, Department.class);