Java Reference
In-Depth Information
•
compareTo()
must be transitive
:Foranynonnullreferencevalues
x
,
y
,and
z
, if
x
.compareTo(
y
) > 0
is true, and if
y
.compareTo(
z
) > 0
is
true, then
x
.compareTo(
z
) > 0
must also be true.
Also,
compareTo()
shouldthrow
NullPointerException
whenthenullref-
erenceispassedtothismethod.However,youdonotneedtocheckfornullbecausethis
methodthrows
NullPointerException
whenitattemptstoaccessanullreferen-
ce's members.
Note
Before Java 5 and its introduction of generics,
compareTo()
's argument
was of type
java.lang.Object
and had to be cast to the appropriate type before
the comparison could be made. The cast operator would throw a
java.lang.ClassCastException
instance when the argument's type was not
compatible with the cast.
You might occasionally need to store in a collection objects that are sorted in some
orderthatdiffersfromtheirnaturalordering.Inthiscase,youwouldsupplyacompar-
ator to provide that ordering.
A
comparator
is an object whose class implements the
Comparator
interface.
Thisinterface,whosegenerictypeis
Comparator<T>
,providesthefollowingpairof
methods:
•
int compare(T o1, T o2)
compares both arguments for order. This
methodreturns0when
o1
equals
o2
,anegativevaluewhen
o1
islessthan
o2
,
and a positive value when
o1
is greater than
o2
.
•
boolean equals(Object o)
returns true when
o
“equals” this
Com-
parator
in that
o
is also a
Comparator
and imposes the same ordering.
Otherwise, this method returns false.
Note
Comparator
declares
equals()
becausethisinterfaceplacesanextracon-
dition on this method's contract.
Additionally, this method can return true only if the
specified object is also a comparator and it imposes the same ordering as this com-
parator.
Youdonothavetooverride
Object
's
equals()
method,butdoingso
may
improve performance by allowing programs to determine that two distinct comparators
impose the same order
.
will discover another example later in this chapter. Also, this chapter will present ex-
amples of implementing
Comparator
.
