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AList
l
may be sorted as follows:If theCollections.sort(l);list
consists ofString
elements, it will be sorted into lexicographic (alphabetical) order. If it consists ofDate
elements, it will be sorted into chronological order. How does Java know how to do this? It's magic! Well, no. ActuallyString
andDate
both implement theComparable
interface. TheComparable
interfaces provides a natural ordering for a class, which allows objects of that class to be sorted automatically. The following table summarizes the JDK classes that implementComparable
:
Class Natural Ordering Byte
signed numerical Character
unsigned numerical Long
signed numerical Integer
signed numerical Short
signed numerical Double
signed numerical Float
signed numerical BigInteger
signed numerical BigDecimal
signed numerical File
system-dependent lexicographic on pathname. String
lexicographic Date
chronological CollationKey
locale-specific lexicographic If you try to sort a list whose elements do not implement
Comparable
,Collections.sort(list)
will throw aClassCastException
. Similarly, if you try to sort a list whose elements cannot be compared to one another,Collections.sort
will throw aClassCastException
. Elements that can be compared to one another are called mutually comparable. While it is possible to have elements of different types be mutually comparable, none of the JDK types listed above permit inter-class comparison.This is all you really need to know about the
Comparable
interface if you just want to sort lists of comparable elements, or create sorted collections of them. The next section will be of interest to you if you want to implement your ownComparable
type.
Comparable
TypesTheComparable
interface consists of a single method:Thepublic interface Comparable { public int compareTo(Object o); }compareTo
method compares the receiving object with the specified object, and returns a negative integer, zero, or a positive integer as the receiving object is less than, equal to, or greater than the specifiedObject
. If the specified object cannot be compared to the receiving object, the method throws aClassCastException
.Here's
a class representing a person's name
that implements
Comparable
:To keep the example short, the class is somewhat limited: It doesn't support middle names, it demands both a first and a last name, and it is not internationalized in any way. Nonetheless, it illustrates several important points:import java.util.*; public class Name implements Comparable { private String firstName, lastName; public Name(String firstName, String lastName) { if (firstName==null || lastName==null) throw new NullPointerException(); this.firstName = firstName; this.lastName = lastName; } public String firstName() {return firstName;} public String lastName() {return lastName;} public boolean equals(Object o) { if (!(o instanceof Name)) return false; Name n = (Name)o; return n.firstName.equals(firstName) && n.lastName.equals(lastName); } public int hashCode() { return 31*firstName.hashCode() + lastName.hashCode(); } public String toString() {return firstName + " " + lastName;} public int compareTo(Object o) { Name n = (Name)o; int lastCmp = lastName.compareTo(n.lastName); return (lastCmp!=0 ? lastCmp : firstName.compareTo(n.firstName)); } }Since this section is about element ordering, let's talk a bit more about
Name
objects are immutable. All other things being equal, immutable types are the way to go, especially for objects that will be used as elements inSet
s, or keys inMap
s. These collections will break if you modify their elements or keys while they're in the collection.- The constructor checks its arguments for
null
. This ensures that allName
objects are well-formed, so that none of the other methods will ever throw aNullPointerException
.- The
hashCode
method is redefined. This is essential for any class that redefines theequals
method. It is required by the general contract for Object.equals. (Equal objects must have equal hash codes.)- The
equals
method returnsfalse
if the specified object isnull
, or of an inappropriate type. ThecompareTo
method throws a runtime exception under these circumstances. Both of these behaviors are required by the general contracts of the respective methods.- The
toString
method has been redefined to print theName
in human-readable form. This is always a good idea, especially for objects that are going to get put into collections. The various collection types'toString
methods depend on thetoString
methods of their elements, keys and values.Name
'scompareTo
method. It implements the standard name-ordering algorithm, where last names take precedence over first names. This is exactly what you want in a natural ordering. It would be very confusing if the natural ordering were unnatural!Take a look at how
compareTo
is implemented, because it's quite typical. First, you cast theObject
argument to the appropriate type. This throws the appropriate exception (ClassCastException
) if the arguments type is inappropriate. Then you compare the most significant part of the object (in this case, the last name). Often, you can just use the natural ordering of the part's type. In this case, the part is aString
, and the natural (lexicographic) ordering is exactly what's called for. If the comparison results in anything other than zero (which represents equality), you're done: you just return the result. If the most significant parts are equal, you go on to compare the next-most-significant parts. In this case, there are only two parts (first name and last name). If there were more parts, you'd proceed in the obvious fashion, comparing parts until you found two that weren't equal (or you were comparing the least-significant parts), at which point you'd return the result of the comparison.Just to show that it all works, here's
a little program that builds a list of
Name
objects and sorts them:
If you run this program, here's what it prints:import java.util.*; class NameSort { public static void main(String args[]) { Name n[] = { new Name("John", "Lennon"), new Name("Karl", "Marx"), new Name("Groucho", "Marx"), new Name("Oscar", "Grouch") }; List l = Arrays.asList(n); Collections.sort(l); System.out.println(l); } }There are four restrictions on the behavior of the[Oscar Grouch, John Lennon, Groucho Marx, Karl Marx]compareTo
method, which we won't go over now because they're fairly technical and boring and are better left in the API documentation. It's really important that all classes that implementComparable
obey these restrictions, so read the documentation forComparable
if you're writing a class that implements it. Attempting to sort a list of objects that violate these restrictions has undefined behavior. Technically speaking, these restrictions ensure that the natural ordering is a partial order on the objects of a class that implements it; this is necessary to ensure that sorting is well-defined.
OK, so now you know about natural ordering. But what if you want to sort some objects in some order other than their natural order? Or what if you want to sort some objects that don't implementComparable
? To do either of these things, you'll need to provide aComparator
. AComparator
is simply an object that encapsulates an ordering. Like theComparable
interface, theComparator
interface consists of a single method:Thepublic interface Comparator { int compare(Object o1, Object o2); }compare
method compares its two arguments, returning a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second. If either of the arguments has an inappropriate type for theComparator
, thecompare
method throws aClassCastException
.Much of what was said about
Comparable
in the previous section applies toComparator
as well. Writing acompare
method is nearly identical to writing acompareTo
method, except that the former gets both objects passed in as arguments. Thecompare
method has to obey the same four "technical restrictions" asComparable
'scompareTo
method, for the same reason: aComparator
must induce a partial order on the objects it compares.Suppose you have a class called
EmployeeRecord
:Let's assume that the natural ordering ofpublic class EmployeeRecord implements Comparable { public Name name(); public int employeeNumber(); public Date hireDate(); ... }EmployeeRecord
objects isName
-ordering (as defined in the previous example) on employee name. Unfortunately the boss has asked us for a list of employees in order of seniority. This means we actually have to do some work, but not much. Here's a program that will produce the required list:Theimport java.util.*; class EmpSort { static final Comparator SENIORITY_ORDER = new Comparator() { public int compare(Object o1, Object o2) { EmployeeRecord r1 = (EmployeeRecord) o1; EmployeeRecord r2 = (EmployeeRecord) o2; return r2.hireDate().compareTo(r1.hireDate()); } }; static final Collection employees = ... ; // Employee Database public static void main(String args[]) { List emp = new ArrayList(employees); Collections.sort(emp, SENIORITY_ORDER); System.out.println(emp); } }Comparator
in the above program is reasonably straightforward. It casts its arguments toEmployeeRecord
, and relies on the natural ordering ofDate
applied to thehireDate()
accessor method. Note that theComparator
passes the hire-date of its second argument to its first, rather than vice-versa. This is because the employee who was hired most recently is least senior: sorting in order of hire-date would put the list in reverse seniority-order. Another way to achieve the same effect would be to maintain the argument order but negate the result of the comparison:The two techniques are equally preferable. Use whichever looks best to you.return -r1.hireDate().compareTo(r2.hireDate());The
Comparator
in the above program works fine for sorting aList
, but it does have one deficiency: it cannot be used to order a sorted collection (such as TreeSet) because it generates a strictly partial ordering. What this means is that this comparator equates unequal objects. In particular, any two employees who were hired on the same date will compare as equal. When you're sorting a
List
, this doesn't matter, but when you're using theComparator
to order a sorted collection, it's fatal. If you insert multiple employees who were hired on the same date into aTreeSet
with thisComparator
, only the first one will be added to the set. The second will be seen as a duplicate element, and ignored.To fix this problem, all you have to do is tweak the
Comparator
so that it produces a total ordering. In other words, tweak it so that the only elements that are seen as equal when usingcompare
are those that are also seen as equal when compared usingequals
. The way to do this is to do a two-part comparison (like we did forName
) where the first part is the one that we're actually interested in (in this case, the hire-date), and the second part is attribute that uniquely identifies the object. In this case, the employee number is the obvious attribute to use as the second part. Here's theComparator
that results:One last note. You might be tempted to replace the finalstatic final Comparator SENIORITY_ORDER = new Comparator() { public int compare(Object o1, Object o2) { EmployeeRecord r1 = (EmployeeRecord) o1; EmployeeRecord r2 = (EmployeeRecord) o2; int dateCmp = r2.hireDate().compareTo(r1.hireDate()); if (dateCmp != 0) return dateCmp; return (r1.employeeNumber() < r2.employeeNumber() ? -1 : (r1.employeeNumber() == r2.employeeNumber() ? 0 : 1)); } };return
statement in theComparator
with the simpler:Don't do it unless you're absolutely sure that no one will ever have a negative employee number! This trick does not work in general, as the signed integer type is not big enough to represent the difference of two arbitrary signed integers. Ifreturn r1.employeeNumber() - r2.employeeNumber();i
is a large positive integer andj
is a large negative integer,i-j
will overflow and return a negative integer. The resultingComparator
violates one of the four technical restrictions that we keep talking about (transitivity), and produces horrible, subtle bugs. This is not a purely theoretical concern; people get burned by it.
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