Question

In Java generics you can use “&” to specify multiple interfaces as type bounds for a type parameter. This allows us for example to manipulate objects of different types with common interfaces to be manipulated uniformly even if there is not an parent interface for those common ones. My question is, how can this be used? For What purposes? I can imagine using this feature for collections, but how is it really than creating a new interface? This cannot be used dynamically, nor for type parameters. Is it just a syntactic sugar, or is there real use case for this feature?

Answer 1

Here is an example taken from my Java Generics book:

public static <S extends Readable & Closeable, 
              T extends Appendable & Closeable> 
       void copy(S source , T target, int size){

    //code to copy from source to target 

}

The method above takes any source that implements both Readable and Closeable and any target that implements both Appendable and Closeable and copies from source to target. You might wonder why we can’t simplify it to:

public static void copy(Reader source, Writer target, int size)

This will indeed admit most of the
same classes but not all of them. For
instance, PrintStream implements
both Appendable and Closeable, but
is not a subclass of Writer.
Furthermore, you can’t rule out the
possibility that some programmer using
your code might have his or her own
custom class that, say, implements
Readable and Closeable but is not
a subclass of Reader.