Effective Java

---

CHAPTER 7
LAMBDAS AND STREAMS
218
The 
Collection
interface is a subtype of 
Iterable
and has a 
stream
method,
so it provides for both iteration and stream access. Therefore, 
Collection
 or an
appropriate subtype is generally the best return type for a public, sequence-
returning method.
Arrays also provide for easy iteration and stream access with
the 
Arrays.asList
and 
Stream.of
methods. If the sequence you’re returning is
small enough to fit easily in memory, you’re probably best off returning one of the
standard collection implementations, such as 
ArrayList
or 
HashSet
. But
 do not
store a large sequence in memory just to return it as a collection.
If the sequence you’re returning is large but can be represented concisely, con-
sider implementing a special-purpose collection. For example, suppose you want
to return the 
power set
of a given set, which consists of all of its subsets. The
power set of {
a
, 
b
, 
c
} is {{}, {
a
}, {
b
}, {
c
}, {
a
, 
b
}, {
a,
c
}, {
b
, 
c
}, {
a
, 
b
, 
c
}}. If a
set has 
n
elements, its power set has 2
n
. Therefore, you shouldn’t even consider
storing the power set in a standard collection implementation. It is, however, easy
to implement a custom collection for the job with the help of 
AbstractList
.
The trick is to use the index of each element in the power set as a bit vector,
where the 
n
th bit in the index indicates the presence or absence of the 
n
th element
from the source set. In essence, there is a natural mapping between the binary
numbers from 0 to 2
n
−
1 and the power set of an 
n
-element set. Here’s the code: