http://download.oracle.com/javase/6/docs/api/java/util/Comparator.html
public interface Comparator<T>
A comparison function, which imposes a total ordering on some collection of objects. Comparators can be passed to a sort method (such asCollections.sort
or
Arrays.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such assorted
sets or
sorted maps), or to provide an ordering for collections of objects that don't have anatural ordering.
The ordering imposed by a comparator c on a set of elementsS is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value ase1.equals(e2) for every e1 and e2 in S.
Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit comparatorc is used with elements (or keys) drawn from a set S. If the ordering imposed byc on S is inconsistent with equals, the sorted set (or sorted map) will behave "strangely." In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms ofequals.
For example, suppose one adds two elements
a and b such that (a.equals(b) && c.compare(a, b) != 0) to an emptyTreeSet with comparator
c. The second adda andb are not equivalent from the tree set's perspective, even though this is contrary to the specification of theSet.add
method. operation will return true (and the size of the tree set will increase) because
Note: It is generally a good idea for comparators to also implementjava.io.Serializable, as they may be used as ordering methods in serializable data structures (likeTreeSet,
TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implementSerializable.
| Method Summary | |
|---|---|
int |
compare(T o1,T o2)Compares its two arguments for order. |
boolean |
equals(Object obj)Indicates whether some other object is "equal to" this comparator. |
For the mathematically inclined, the relation that defines theimposed ordering that a given comparator c imposes on a given set of objectsS is:
{(x, y) such that c.compare(x, y) <= 0}.
The quotient for this total order is:
{(x, y) such that c.compare(x, y) == 0}.
It follows immediately from the contract for compare that the quotient is anequivalence relation on
S, and that the imposed ordering is a total order on S. When we say that the ordering imposed by
c onS is consistent with equals, we mean that the quotient for the ordering is the equivalence relation defined by the objects'equals(Object)
method(s):
{(x, y) such that x.equals(y)}.
This interface is a member of the Java Collections Framework.
- Since:
- 1.2
- See Also:
Comparable,Serializable
compare
int compare(T o1, T o2)
- Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
In the foregoing description, the notation sgn(expression) designates the mathematicalsignum function, which is defined to return one of -1, 0, or1 according to whether the value of expression is negative, zero or positive.
The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x)) for allx and y. (This implies that compare(x, y) must throw an exception if and only ifcompare(y, x) throws an exception.)
The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) impliescompare(x, z)>0.
Finally, the implementor must ensure that compare(x, y)==0 implies thatsgn(compare(x, z))==sgn(compare(y, z)) for all z.
It is generally the case, but not strictly required that (compare(x, y)==0) == (x.equals(y)). Generally speaking, any comparator that violates this condition should clearly indicate this fact. The recommended language is "Note: this comparator imposes orderings that are inconsistent with equals."
-
- Parameters:
o1- the first object to be compared.o2- the second object to be compared.- Returns:
- a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
- Throws:
ClassCastException- if the arguments' types prevent them from being compared by this comparator.
equals
boolean equals(Object obj)
- Indicates whether some other object is "equal to" this comparator. This method must obey the general contract of
Object.equals(Object). Additionally, this method can return true only if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus,comp1.equals(comp2)implies that sgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2)) for every object referenceo1 and o2.Note that it is always safe not to override Object.equals(Object). However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct comparators impose the same order.
-
- Parameters:
obj- the reference object with which to compare.- Returns:
trueonly if the specified object is also a comparator and it imposes the same ordering as this comparator.- See Also:
Object.equals(Object),Object.hashCode()；
package test;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
public class ComparatorTest {
class Dog {
private String name;
private int weight;
private int height;
public Dog(String name, int weight, int height) {
this.setName(name);
this.weight = weight;
this.height = height;
}
public int getHeight() {
return height;
}
public void setHeight(int height) {
this.height = height;
}
public int getWeight() {
return weight;
}
public void setWeight(int weight) {
this.weight = weight;
}
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
class ByWeightComparator implements Comparator<Object> {
public int compare(Object firstDog, Object secondDog) {
int firstWeight = ((Dog) firstDog).getWeight();
int secondWeight = ((Dog) secondDog).getWeight();
return signum(firstWeight - secondWeight);
}
public final int signum(int diff) {
if (diff > 0)
return 1;
if (diff < 0)
return -1;
else
return 0;
}
}
class ByHeightComparator implements Comparator<Object> {
public int compare(Object firstDog, Object secondDog) {
int firstHeight = ((Dog) firstDog).getHeight();
int secondHeight = ((Dog) secondDog).getHeight();
return signum(firstHeight - secondHeight);
}
public final int signum(int diff) {
if (diff > 0)
return 1;
if (diff < 0)
return -1;
else
return 0;
}
}
/**
* @param args
*/
public static void main(String[] args) {
ComparatorTest test = new ComparatorTest();
Dog o1 = test.new Dog("dog1", 1, 5);
Dog o2 = test.new Dog("dog2", 2, 4);
Dog o3 = test.new Dog("dog3", 3, 3);
Dog o4 = test.new Dog("dog4", 4, 2);
Dog o5 = test.new Dog("dog5", 5, 1);
Dog[] dogs = new Dog[] { o1, o4, o3, o5, o2 };
System.out.println("未排序前");
for (int i = 0; i < dogs.length; i++) {
Dog dog = dogs[i];
System.out.println(dog.getName());
}
Arrays.sort(dogs, test.new ByHeightComparator());
//Collections.sort(dogs,comp);
System.out.println("使用高度排序之後：");
for (int i = 0; i < dogs.length; i++) {
Dog dog = dogs[i];
System.out.println(dog.getName());
}
Arrays.sort(dogs, test.new ByWeightComparator());
System.out.println("使用重量排序之後：");
for (int i = 0; i < dogs.length; i++) {
Dog dog = dogs[i];
System.out.println(dog.getName());
}
}
}