Effective Java

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CHAPTER 8
METHODS
248
same 
immutable
empty collection repeatedly, as immutable objects may be shared
freely (Item 17). Here is the code to do it, using the 
Collections.emptyList
method. If you were returning a set, you’d use 
Collections.emptySet
; if you
were returning a map, you’d use 
Collections.emptyMap
. But remember, this is
an optimization, and it’s seldom called for. If you think you need it, measure
performance before and after, to ensure that it’s actually helping:
// Optimization - avoids allocating empty collections
public List getCheeses() {
return cheesesInStock.isEmpty() ? Collections.emptyList()
: new ArrayList<>(cheesesInStock);
}
The situation for arrays is identical to that for collections. Never return null
instead of a zero-length array. Normally, you should simply return an array of the
correct length, which may be zero. Note that we’re passing a zero-length array
into the 
toArray
method to indicate the desired return type, which is 
Cheese[]
:
//The right way to return a possibly empty array
public Cheese[] getCheeses() {
return cheesesInStock.toArray(new Cheese[0]);
}
If you believe that allocating zero-length arrays is harming performance, you
can return the same zero-length array repeatedly because all zero-length arrays are
immutable:
// Optimization - avoids allocating empty arrays
private static final Cheese[] EMPTY_CHEESE_ARRAY = new Cheese[0];
public Cheese[] getCheeses() {
return cheesesInStock.toArray(EMPTY_CHEESE_ARRAY);
}
In the optimized version, we pass 
the same
empty array into every 
toArray
call,
and this array will be returned from 
getCheeses
whenever 
cheesesInStock
is
empty. Do 
not
preallocate the array passed to 
toArray
in hopes of improving
performance. Studies have shown that it is counterproductive [Shipilëv16]: