jdk之Comparator

comparator 是javase中的接口，位于java.util包下，该接口表示可以比较。

典型的运用是数组工具类和集合工作类中都有含Comparator接口的sort重载方法。

jdk源码：

public interface Comparator<T> {    /**     * 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.<p>     *     * In the foregoing description, the notation     * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical     * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,     * <tt>0</tt>, or <tt>1</tt> according to whether the value of     * <i>expression</i> is negative, zero or positive.<p>     *     * The implementor must ensure that <tt>sgn(compare(x, y)) ==     * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This     * implies that <tt>compare(x, y)</tt> must throw an exception if and only     * if <tt>compare(y, x)</tt> throws an exception.)<p>     *     * The implementor must also ensure that the relation is transitive:     * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies     * <tt>compare(x, z)>0</tt>.<p>     *     * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>     * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all     * <tt>z</tt>.<p>     *     * It is generally the case, but <i>not</i> strictly required that     * <tt>(compare(x, y)==0) == (x.equals(y))</tt>.  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."     *     * @param o1 the first object to be compared.     * @param o2 the second object to be compared.     * @return a negative integer, zero, or a positive integer as the     *         first argument is less than, equal to, or greater than the     *         second.     * @throws NullPointerException if an argument is null and this     *         comparator does not permit null arguments     * @throws ClassCastException if the arguments' types prevent them from     *         being compared by this comparator.     */    int compare(T o1, T o2);    /**     * Indicates whether some other object is "equal to" this     * comparator.  This method must obey the general contract of     * {@link Object#equals(Object)}.  Additionally, this method can return     * <tt>true</tt> <i>only</i> if the specified object is also a comparator     * and it imposes the same ordering as this comparator.  Thus,     * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,     * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference     * <tt>o1</tt> and <tt>o2</tt>.<p>     *     * Note that it is <i>always</i> safe <i>not</i> to override     * <tt>Object.equals(Object)</tt>.  However, overriding this method may,     * in some cases, improve performance by allowing programs to determine     * that two distinct comparators impose the same order.     *     * @param   obj   the reference object with which to compare.     * @return  <code>true</code> only if the specified object is also     *          a comparator and it imposes the same ordering as this     *          comparator.     * @see Object#equals(Object)     * @see Object#hashCode()     */    boolean equals(Object obj);}

可以看出Comparator声明了两个方法equals和compare两个方法

事例说明：

用户实体：

package com.sun.utils.comparator;public class User {public User(String name, Long id) {super();this.name = name;this.id = id;}private String name;private Long id;public String getName() {return name;}public void setName(String name) {this.name = name;}public Long getId() {return id;}public void setId(Long id) {this.id = id;}@Overridepublic String toString() {return "User [name=" + name + ", id=" + id + "]";}}

比较器：

如果u1比u2大则返回大于0的值

u1等于u2则返回0

u1小于u2则返回小于0的值

package com.sun.utils.comparator;import java.util.Comparator;public class TestComparator implements Comparator<User>{public int compare(User u1, User u2) {// TODO Auto-generated method stubif(u1.getId()>u2.getId()) {return 1;} else if(u1.getId()<u2.getId()) {return -1;}return 0;}}

运行类：

package com.sun.utils.comparator;import java.util.ArrayList;import java.util.Collections;import java.util.List;public class RunTest{public static void main(String[] args) { List<User> list= new ArrayList<>(); list.add(new User("zxx", 2l)); list.add(new User("www", 1l));  TestComparator comparator = new TestComparator(); Collections.sort(list, comparator); System.out.println(list);}}

打印结果：

[User [name=www, id=1], User [name=zxx, id=2]]

从比较结果可以看出sort方法会根据comparator的compare方法进行排序