Java Reference
In-Depth Information
"+argMap.size());
}
}
HashMapDemo
createsahashmapof
String
keysand
Integer
values.Eachkey
is one of the command-line arguments passed to this application, and its value is the
number of occurrences of that argument on the command line.
Forexample,
java HashMapDemo how much wood could a woodchuck
chuck if a woodchuck could chuck wood
generates the following output:
{wood=2, could=2, how=1, if=1, chuck=2, a=2, woodchuck=2,
much=1}
Number of distinct arguments = 8
canuse
String
objectsaskeysinahashmap.Whenyoucreateaclasswhoseinstances
are to be used as keys, you must ensure that you override both methods.
referencefield's
hashCode()
methodandreturnitsvalue,providedthattheclassde-
clares a single reference field (and no primitive type fields).
More commonly, classes declare multiple fields, and a better implementation of the
hashCode()
method is required. The implementation should try to generate hash
codes that minimize collisions.
There is no rule on how to best implement
hashCode()
, and various
algorithms
(recipes for accomplishing tasks) have been created. My favorite algorithm appears
in
Effective Java Second Edition
, by Joshua Bloch (Addison-Wesley, 2008; ISBN:
0321356683).
Thefollowingalgorithm,whichassumestheexistenceofanarbitraryclassthatisre-
ferred to as
X
, closely follows Bloch's algorithm, but is not identical:
1. Initialize
int
variable
hashCode
(the name is arbitrary) to an arbitrary
nonzerointegervalue,suchas19.Thisvariableisinitializedtoanonzerovalue
toensurethatittakesintoaccountanyinitialfieldswhosehashcodesarezeros.
Ifyouinitialize
hashCode
to0,thefinalhashcodewillbeunaffectedbysuch
fields and you run the risk of increased collisions.
2. Foreachfield
f
thatisalsousedin
X
's
equals()
method,calculate
f
'shash
code and assign it to
int
variable
hc
as follows:
a. If
f
is of Boolean type, calculate
hc = f?1:0
.
