A DESCRIPTION OF THE REQUEST :
One thing I've found to be extremely useful since the introduction of generics into my own APIs is the use of a common factory interface. For example:

public interface Factory<T>
{
    T create();
}

Since the introduction of generics, the API designer can use this interface to accept factories for the repetative or custom creation of typed objects. For example, the API of  a multi-map might be designed using a factory to create the collections that will be used to store multiple values under one key:

public interface MultiMap<K, V, C extends Collection<V>>
extends Map<K, C>
{
    /**
     * Sets the factory that will be used for creating the collections that will
     * store multiple values under each key.
     */
    void setCollectionFactory(Factory<? extends C> factory);

    /**
     * Puts the specified value into the collection that is mapped to the
     * specified key and uses the defined factory to create a new collection
     * if the mapping does not exist.
     */
    void putValue(K, V);
}

The use of the factory allows the developer to define the collection implementation that best fits his needs while the multi-map need not care what type of collection it is.

JUSTIFICATION :
Factories are frequently used to hide implementation details and simplify code. I believe the frequency of use justifies the need to define a common interface.
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