The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
A List
l
may be sorted as follows.
Collections.sort(l);
If the List
consists of String
elements, it will be sorted into alphabetical order. If it consists of Date
elements, it will be sorted into chronological order. How does this happen? String
and Date
both implement the
Comparable
interface. Comparable
implementations provide a natural ordering for a class, which allows objects of that class to be sorted automatically. The
following table summarizes some of the more important Java platform classes that implement Comparable
.
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 |
Boolean |
Boolean.FALSE < Boolean.TRUE |
File |
System-dependent lexicographic on path name |
String |
Lexicographic |
Date |
Chronological |
CollationKey |
Locale-specific lexicographic |
If you try to sort a list, the elements of which do not implement Comparable
, Collections.sort(list)
will throw a
ClassCastException
. Similarly, Collections.sort(list, comparator)
will throw a ClassCastException
if you try to sort a list whose elements cannot be compared to one another using the comparator
. Elements that can be compared to one another are called mutually comparable. Although elements of different types may be mutually comparable, none of the classes listed here permit interclass comparison.
This is all you really need to know about the Comparable
interface if you just want to sort lists of comparable elements or to create sorted collections of them. The next section will be of interest to you if you want to implement your own Comparable
type.
The Comparable
interface consists of the following method.
public interface Comparable<T> { public int compareTo(T o); }
The compareTo
method compares the receiving object with the specified object and returns a negative integer, 0, or a positive integer depending on whether the receiving object is less than, equal to, or greater than the specified object. If the specified object cannot be compared to the receiving object, the method throws a ClassCastException
.
The
following class representing a person's name
implements Comparable
.
import java.util.*; public class Name implements Comparable<Name> { private final 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(Name n) { int lastCmp = lastName.compareTo(n.lastName); return (lastCmp != 0 ? lastCmp : firstName.compareTo(n.firstName)); } }
To keep the preceding 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 the following important points:
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 in Set
s or as keys in Map
s. These collections will break if you modify their elements or keys while they're in the collection.null
. This ensures that all Name
objects are well formed so that none of the other methods will ever throw a NullPointerException
.hashCode
method is redefined. This is essential for any class that redefines the equals
method. (Equal objects must have equal hash codes.)equals
method returns false
if the specified object is null
or of an inappropriate type. The compareTo
method throws a runtime exception under these circumstances. Both of these behaviors are required by the general contracts of the respective methods.toString
method has been redefined so it prints the Name
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 the toString
methods of their elements, keys, and values.Since this section is about element ordering, let's talk a bit more about Name
's compareTo
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 indeed if the natural ordering were unnatural!
Take a look at how compareTo
is implemented, because it's quite typical. First, 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 a String
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 program that builds a list of names and sorts them
.
import java.util.*; public class NameSort { public static void main(String[] args) { Name nameArray[] = { new Name("John", "Smith"), new Name("Karl", "Ng"), new Name("Jeff", "Smith"), new Name("Tom", "Rich") }; List<Name> names = Arrays.asList(nameArray); Collections.sort(names); System.out.println(names); } }
If you run this program, here's what it prints.
[Karl Ng, Tom Rich, Jeff Smith, John Smith]
There are four restrictions on the behavior of the compareTo
method, which we won't go into now because they're fairly technical and boring and are better left in the API documentation. It's really important that all classes that implement Comparable
obey these restrictions, so read the documentation for Comparable
if you're writing a class that implements it. Attempting to sort a list of objects that violate the restrictions has undefined behavior. Technically speaking, these restrictions ensure that the natural ordering is a total order on the objects of a class that implements it; this is necessary to ensure that sorting is well defined.
What if you want to sort some objects in an order other than their natural ordering? Or what if you want to sort some objects that don't implement Comparable
? To do either of these things, you'll need to provide a
Comparator
an object that encapsulates an ordering. Like the Comparable
interface, the Comparator
interface consists of a single method.
public interface Comparator<T> { int compare(T o1, T o2); }
The compare
method compares its two arguments, returning a negative integer, 0, or a positive integer depending on whether the first argument is less than, equal to, or greater than the second. If either of the arguments has an inappropriate type for the Comparator
, the compare
method throws a ClassCastException
.
Much of what was said about Comparable
applies to Comparator
as well. Writing a compare
method is nearly identical to writing a compareTo
method, except that the former gets both objects passed in as arguments. The compare
method has to obey the same four technical restrictions as Comparable
's compareTo
method for the same reason a Comparator
must induce a total order on the objects it compares.
Suppose you have a class called Employee
, as follows.
public class Employee implements Comparable<Employee> { public Name name() { ... } public int number() { ... } public Date hireDate() { ... } ... }
Let's assume that the natural ordering of Employee
instances is Name
ordering (as defined in the previous example) on employee name. Unfortunately, the boss has asked for a list of employees in order of seniority. This means we have to do some work, but not much. The following program will produce the required list.
import java.util.*; public class EmpSort { static final Comparator<Employee> SENIORITY_ORDER = new Comparator<Employee>() { public int compare(Employee e1, Employee e2) { return e2.hireDate().compareTo(e1.hireDate()); } }; // Employee database static final Collection<Employee> employees = ... ; public static void main(String[] args) { List<Employee> e = new ArrayList<Employee>(employees); Collections.sort(e, SENIORITY_ORDER); System.out.println(e); } }
The Comparator
in the program is reasonably straightforward. It relies on the natural ordering of Date
applied to the values returned by the hireDate
accessor method. Note that the Comparator
passes the hire date of its second argument to its first rather than vice versa. The reason is that the employee who was hired most recently is the least senior; sorting in the order of hire date would put the list in reverse seniority order. Another technique people sometimes use to achieve this effect is to maintain the argument order but to negate the result of the comparison.
// Don't do this!! return -r1.hireDate().compareTo(r2.hireDate());
You should always use the former technique in favor of the latter because the latter is not guaranteed to work. The reason for this is that the compareTo
method can return any negative int
if its argument is less than the object on which it is invoked. There is one negative int
that remains negative when negated, strange as it may seem.
-Integer.MIN_VALUE == Integer.MIN_VALUE
The Comparator
in the preceding program works fine for sorting a List
, but it does have one deficiency: It cannot be used to order a sorted collection, such as TreeSet
, because it generates an ordering that is not compatible with equals. This means that this Comparator
equates objects that the equals
method does not. 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 the Comparator
to order a sorted collection, it's fatal. If you use this Comparator
to insert multiple employees hired on the same date into a TreeSet
, only the first one will be added to the set; the second will be seen as a duplicate element and will be ignored.
To fix this problem, simply tweak the Comparator
so that it produces an ordering that is compatible with equals
. In other words, tweak it so that the only elements seen as equal when using compare
are those that are also seen as equal when compared using equals
. The way to do this is to perform a two-part comparison (as for Name
), where the first part is the one we're interested in in this case, the hire date and the second part is an attribute that uniquely identifies the object. Here the employee number is the obvious attribute. This is the Comparator
that results.
static final Comparator<Employee> SENIORITY_ORDER = new Comparator<Employee>() { public int compare(Employee e1, Employee e2) { int dateCmp = e2.hireDate().compareTo(e1.hireDate()); if (dateCmp != 0) return dateCmp; return (e1.number() < e2.number() ? -1 : (e1.number() == e2.number() ? 0 : 1)); } };
One last note: You might be tempted to replace the final return
statement in the Comparator
with the simpler:
return e1.number() - e2.number();
Don't do it unless you're absolutely sure no one will ever have a negative employee number! This trick does not work in general because the signed integer type is not big enough to represent the difference of two arbitrary signed integers. If i
is a large positive integer and j
is a large negative integer, i - j
will overflow and will return a negative integer. The resulting comparator
violates one of the four technical restrictions we keep talking about (transitivity) and produces horrible, subtle bugs. This is not a purely theoretical concern; people get burned by it.