Effective Java


// BROKEN difference-based comparator - violates transitivity!



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Effective Java

// BROKEN difference-based comparator - violates transitivity!
static Comparator hashCodeOrder = new Comparator<>() {
public int compare(Object o1, Object o2) {
return o1.hashCode() - o2.hashCode();
}
};
Do not use this technique. It is fraught with danger from integer overflow and
IEEE 754 floating point arithmetic artifacts [JLS 15.20.1, 15.21.1]. Furthermore,
the resulting methods are unlikely to be significantly faster than those written
using the techniques described in this item. Use either a static 
compare
method:
// Comparator based on static compare method
static Comparator hashCodeOrder = new Comparator<>() {
public int compare(Object o1, Object o2) {
return Integer.compare(o1.hashCode(), o2.hashCode());
}
};
or a comparator construction method:
// Comparator based on Comparator construction method
static Comparator hashCodeOrder =
Comparator.comparingInt(o -> o.hashCode());

CHAPTER 3
METHODS COMMON TO ALL OBJECTS
72
In summary, whenever you implement a value class that has a sensible order-
ing, you should have the class implement the 
Comparable
interface so that its
instances can be easily sorted, searched, and used in comparison-based collec-
tions. When comparing field values in the implementations of the 
compareTo
methods, avoid the use of the 
<
and 
>
operators. Instead, use the static 
compare
methods in the boxed primitive classes or the comparator construction methods in
the 
Comparator
interface.

73
C H A P T E R
4
Classes and Interfaces
C
LASSES
and interfaces lie at the heart of the Java programming language.
They are its basic units of abstraction. The language provides many powerful
elements that you can use to design classes and interfaces. This chapter contains
guidelines to help you make the best use of these elements so that your classes and
interfaces are usable, robust, and flexible.

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