Effective Java

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ITEM 83: USE LAZY INITIALIZATION JUDICIOUSLY
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This code may appear a bit convoluted. In particular, the need for the local
variable (
result
) may be unclear. What this variable does is to ensure that 
field
is read only once in the common case where it’s already initialized. While not
strictly necessary, this may improve performance and is more elegant by the stan-
dards applied to low-level concurrent programming. On my machine, the method
above is about 1.4 times as fast as the obvious version without a local variable.
While you can apply the double-check idiom to static fields as well, there is
no reason to do so: the lazy initialization holder class idiom is a better choice.
Two variants of the double-check idiom bear noting. Occasionally, you may
need to lazily initialize an instance field that can tolerate repeated initialization. If
you find yourself in this situation, you can use a variant of the double-check idiom
that dispenses with the second check. It is, not surprisingly, known as the 
single-
check idiom
. Here is how it looks. Note that 
field
is still declared 
volatile
: