javacomparator,javacomparator原理
Java中Comparator接口的用法
在java编程中，如果要对集合对象或数组对象进行排序，需要实现Comparator接口以达到目标，用法如下（模拟下在集合对象中对日期属性进行排序）：
一、实体类Step
package?com.ljq.entity;
/**
?*?运号单流程
?*?
?*?@author?Administrator
?*?
?*/
public?class?Step{
????/**?处理时间?*/
????private?String?acceptTime?=?"";
????/**?快件所在地点?*/
????private?String?acceptAddress?=?"";
????public?Step()?{
????????super();
????}
????public?Step(String?acceptTime,?String?acceptAddress)?{
????????super();
????????this.acceptTime?=?acceptTime;
????????this.acceptAddress?=?acceptAddress;
????}
????public?String?getAcceptTime()?{
????????return?acceptTime;
????}
????public?void?setAcceptTime(String?acceptTime)?{
????????this.acceptTime?=?acceptTime;
????}
????public?String?getAcceptAddress()?{
????????return?acceptAddress;
????}
????public?void?setAcceptAddress(String?acceptAddress)?{
????????this.acceptAddress?=?acceptAddress;
????}
}
二、实现Comparator接口
package?com.ljq.entity;
import?java.util.Comparator;
import?java.util.Date;
import?com.ljq.util.UtilTool;
/**
?*?对Step类进行排序
?*?
?*?@author?Administrator
?*
?*/
public?class?StepComparator?implements?ComparatorStep{
????/**
?????*?如果o1小于o2,返回一个负数;如果o1大于o2，返回一个正数;如果他们相等，则返回0;
?????*/
????@Override
????public?int?compare(Step?o1,?Step?o2)?{
????????Date?acceptTime1=UtilTool.strToDate(o1.getAcceptTime(),?null);
????????Date?acceptTime2=UtilTool.strToDate(o2.getAcceptTime(),?null);
????????//对日期字段进行升序，如果欲降序可采用before方法
????????if(acceptTime1.after(acceptTime2))?return?1;
????????return?-1;
????}
}
三、代码测试
package?junit;
import?java.util.Collection;
import?java.util.Collections;
import?java.util.List;
import?org.junit.Test;
public?class?StepComparatorTest?{
????@Test
????public?void?sort()?throws?Exception{
????????ListStep?steps=new?ArrayListStep;
????????//对集合对象进行排序
?????????StepComparator?comparator=new?StepComparator();
????????Collections.sort(steps,?comparator);
????????if(steps!=nullsteps.size()0){
????????????for(Step?step:steps){
???????????????System.out.println(step.getAcceptAddress());
???????????????System.out.println(step.getAcceptTime());
????????????}
????????}
????}
}
java中Comparable和Comparator两种比较器的区别
Comparable和Comparator接口都是为了对类进行比较，众所周知，诸如Integer，double等基本数据类型，java可以对他们进行比较，而对于类的比较，需要人工定义比较用到的字段比较逻辑。可以把Comparable理解为内部比较器，而Comparator是外部比较器，基本的写法如下：
class Apple implements ComparableApple{
int id;
double price;
public Apple(int id, double price) {
this.id = id;
this.price = price;
}
public int compareTo(Apple o) {
//return Double.compare(this.getPrice(),o.getPrice());
if (Math.abs(this.price-o.price)0.001)
return 0;
else
return (o.price-this.price)0?1:-1;
}
@Override
public String toString() {
return "Apple{" +
"id=" + id +
", price=" + price +
'}';
}
}
class AESComparator implements ComparatorApple{
public int compare(Apple o1, Apple o2) {
if (Math.abs(o1.price-o2.price)0.001)
return 0;
else{
return (o1.price-o2.price)0?1:-1;
}
}
}
实现了Comparable接口的类需要实现compareTo()方法，传入一个外部参数进行比对，实现了Comparator接口的方法需要实现compare()方法，对外部传入的两个类进行比较，从而让外部方法在比较时调用。
两者的区别是实现Comparator接口代码更加灵活，可以定义某个类的多个比较器，从而在排序时根据实际场景自由调用，而Comparable接口实现后便不能改动。两种接口的调用方式如下：
class AESComparator implements ComparatorApple{
public int compare(Apple o1, Apple o2) {
if (Math.abs(o1.price-o2.price)0.001)
return 0;
else{
return (o1.price-o2.price)0?1:-1;
}
}
}
class DESComparator implements ComparatorApple{
public int compare(Apple o1, Apple o2) {
if (Math.abs(o1.price-o2.price)0.001)
return 0;
else {
return (o1.price-o2.price)0?-1:1;
}
}
}
public static void main(String[] args) {
Apple apple1 = new Apple(1,4.8);
Apple apple2 = new Apple(2,5.9);
Apple apple3 = new Apple(3,8.5);
ListApple list = new ArrayListApple();
list.add(apple1);
list.add(apple3);
list.add(apple2);
System.out.println("Comparable==========");
System.out.printf("this list of apples: %s\n",list);
Collections.sort(list);
System.out.printf("this list of apples: %s\n",list);
System.out.println("Comparator==========");
System.out.printf("this list of apples: %s\n",list);
Collections.sort(list,new DESComparator());
System.out.printf("this list of apples: %s\n",list);
Collections.sort(list,new AESComparator());
System.out.printf("this list of apples: %s\n",list);
}
}
上述代码存在的问题，不能在比较器中进行double类型的减法操作，因为对于值比较大的double，减法操作容易导致值的溢出，java7对每一种包装类型的比较新增了compare()方法，改造后的代码如下：
class Apple implements ComparableApple{
int id;
double price;
public Apple(int id, double price) {
this.id = id;
this.price = price;
}
public int compareTo(Apple o) {
return Double.compare(this.price,o.price);
}
@Override
public String toString() {
return "Apple{" +
"id=" + id +
", price=" + price +
'}';
}
}
class AESComparator implements ComparatorApple{
public int compare(Apple o1, Apple o2) {
return Double.compare(o1.price,o2.price);
}
}
class DESComparator implements ComparatorApple{
public int compare(Apple o1, Apple o2) {
return Double.compare(o2.price,o1.price);
}
}
查看Double.compare的源码如下
public static int compare(double d1, double d2) {
if (d1 d2)
return -1; // Neither val is NaN, thisVal is smaller
if (d1 d2)
return 1; // Neither val is NaN, thisVal is larger
// Cannot use doubleToRawLongBits because of possibility of NaNs.
long thisBits = Double.doubleToLongBits(d1);
long anotherBits = Double.doubleToLongBits(d2);
return (thisBits == anotherBits ? 0 : // Values are equal
(thisBits anotherBits ? -1 : // (-0.0, 0.0) or (!NaN, NaN)
1)); // (0.0, -0.0) or (NaN, !NaN)
}
请教：JAVA中，comparator接口如何对double型数据进行排序。
返回值是int
-1，0或1。1表示第一个数比第二个数大，0表示相等，-1表示第一个数比第二个数小。这个int跟你要比较的数值类型没关系。举例：
import
java.util.Comparator;
public
class
Test1
implements
Comparator
{
public
int
compare(Double
o1,
Double
o2)
{
return
o1.compareTo(o2);
}
public
static
void
main(String[]
args)
{
Test1
t
=
new
Test1();
double
i=10,j=9;
System.out.println(t.compare(j,
i));
}
}
Java编写程序实现Comparator接口，实现字符串按 降序排序怎么弄？
Comparator升序降序实现 int compare(Object o1, Object o2) 返回一个基本类型的整型 1) 如果要按照升序排序, 则o1 小于o2,返回-1(负数),相等返回0,01大于02
java中Comparator接口，为什么o1.length()-o2.length()就是升序，o2.length() - o1.lengh()就是降序呢?
并不是因为o1.length() - o2.length() 就是升序，o2.length() - o1.lengh()就是降序，而是因为
你实现了用Comparator这个接口的时候，就有compare这个方法，JDK会在比较的时候自动调用这个方法，当该方法返回正数时，以第一个参数大于第二个，反之亦然。
这里只是用了2个String参数的长度来决定哪个比较大，你也可以自己来实现比较的逻辑。比如传入的2个参数的第一个字母比较之类的。javase的问题多看书。看帖子和问人得来的收获太小了。
建议啃完《thinking in java》