Effective Java

// List as a typesafe alternative to a generic varargs parameter
static  List flatten(
List> lists
) {
List result = new ArrayList<>();
for (List list : lists)
result.addAll(list);
return result;
}

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CHAPTER 5
GENERICS
150
This method can then be used in conjunction with the static factory method
List.of
to allow for a variable number of arguments. Note that this approach
relies on the fact that the 
List.of
declaration is annotated with 
@SafeVarargs
:
audience = flatten(List.of(friends, romans, countrymen));
The advantage of this approach is that the compiler can 
prove
that the method
is typesafe. You don’t have to vouch for its safety with a 
SafeVarargs
annotation,
and you don’t have worry that you might have erred in determining that it was
safe. The main disadvantage is that the client code is a bit more verbose and may
be a bit slower.
This trick can also be used in situations where it is impossible to write a safe
varargs method, as is the case with the 
toArray
method on page 147. Its 
List
ana-
logue 
is
the 
List.of
method, so we don’t even have to write it; the Java libraries
authors have done the work for us. The 
pickTwo
method then becomes this:
static  
List
pickTwo(T a, T b, T c) {
switch(rnd.nextInt(3)) {
case 0: return 
List.of(a, b)
;
case 1: return 
List.of(a, c)
;
case 2: return 
List.of(b, c)
;
}
throw new AssertionError();
}
and the main method becomes this:
public static void main(String[] args) {
List
attributes = pickTwo("Good", "Fast", "Cheap");
}
The resulting code is typesafe because it uses only generics, and not arrays.
In summary, varargs and generics do not interact well because the varargs
facility is a leaky abstraction built atop arrays, and arrays have different type rules
from generics. Though generic varargs parameters are not typesafe, they are legal.
If you choose to write a method with a generic (or parameterized) varargs parame-
ter, first ensure that the method is typesafe, and then annotate it with 
@Safe-
Varargs
so it is not unpleasant to use.

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ITEM 33: CONSIDER TYPESAFE HETEROGENEOUS CONTAINERS
151