[英]Descending sort of object using comparable or comparator
So I have a Hill object that contains information about hills that is obtained from a CSV file. 因此,我有一个Hill对象,其中包含有关从CSV文件获取的hills的信息。 The file looks like this: 该文件如下所示:
import java.io.IOException;
import java.net.URL;
import java.util.*;
public class Hill implements Comparable<Hill>{
public static final String CSV_FILE_URL = "HIDDEN";
public static final String DELIMITER = ",";
public int number;
public String name,county;
public double height,lat,lon;
public Hill(int number,String name,String county,double height,double lat,double lon){
this.number = number;
this.name = name;
this.county = county;
this.height = height;
this.lat = lat;
this.lon = lon;
}
public String toString(){
return(number + ", " + name + ", " + county + ", " + height + ", " + lat + ", " + lon);
}
public static List<Hill> readHills() throws IOException{
String[] fields = new String[0];
URL url = new URL(CSV_FILE_URL);
Scanner input = new Scanner(url.openConnection().getInputStream());
List<Hill> hillList = new ArrayList<>();
input.nextLine();
while(input.hasNextLine()){
fields = input.nextLine().split(DELIMITER);
Hill hill = new Hill(Integer.parseInt(fields[0]),fields[1],fields[2],Double.parseDouble(fields[3]),Double.parseDouble(fields[4]),Double.parseDouble(fields[5]));
hillList.add(hill);
}
return hillList;
}
@Override
public int compareTo(Hill o) {
return this.name.compareTo(o.name);
}
}
Inside my exercise class I am currently able to sort the hills alphabetically using the Comparable interface. 在我的运动课中,我目前能够使用Comparable接口按字母顺序对丘陵进行排序。 I then get it to print the first 20 hills. 然后我得到它来打印前20个山丘。
Now I wish to sort the hills list by height but descending so that I can print out the 20 tallest hills. 现在,我想按高度排序,但降序排序,以便打印出20个最高的山丘。
import java.io.IOException;
import java.util.*;
public class Exercise5 {
public static void exercise5d() throws IOException {
System.out.println("### Exercise 5D ###");
List listOfHills = Hill.readHills();
Collections.sort(listOfHills);
for(int x=0;x<20;x++){
System.out.println(listOfHills.get(x));
}
System.out.println("");
//Attempt at reversing
Comparator<Hill> HillComparator = Collections.reverseOrder();
Collections.sort(listOfHills,HillComparator);
for(int x=0;x<20;x++){
System.out.println(listOfHills.get(x));
}
}
public static void main(String[] args) throws IOException {
exercise5d();
}
}
Here is an example that shows how you can use Comparable
or a Comparator
: 这是显示如何使用Comparable
或Comparator
的示例:
/*
** Use the Collections API to sort a List for you.
**
** When your class has a "natural" sort order you can implement
** the Comparable interface.
**
** You can use an alternate sort order when you implement
** a Comparator for your class.
*/
import java.util.*;
public class Person implements Comparable<Person>
{
String name;
int age;
public Person(String name, int age)
{
this.name = name;
this.age = age;
}
public String getName()
{
return name;
}
public int getAge()
{
return age;
}
public String toString()
{
return name + " : " + age;
}
/*
** Implement the natural order for this class
*/
public int compareTo(Person p)
{
return getName().compareTo(p.getName());
}
static class AgeComparator implements Comparator<Person>
{
public int compare(Person p1, Person p2)
{
return p1.getAge() - p2.getAge();
}
}
public static void main(String[] args)
{
List<Person> people = new ArrayList<Person>();
people.add( new Person("Homer", 38) );
people.add( new Person("Marge", 35) );
people.add( new Person("Bart", 15) );
people.add( new Person("Lisa", 13) );
// Sort by natural order
Collections.sort(people);
System.out.println("Sort by Natural order");
System.out.println("\t" + people);
// Sort by reverse natural order
Collections.sort(people, Collections.reverseOrder());
System.out.println("Sort by reverse natural order");
System.out.println("\t" + people);
// Use a Comparator to sort by age
Collections.sort(people, new Person.AgeComparator());
System.out.println("Sort using Age Comparator");
System.out.println("\t" + people);
// Use a Comparator to sort by descending age
Collections.sort(people, Collections.reverseOrder(new Person.AgeComparator()));
System.out.println("Sort using Reverse Age Comparator");
System.out.println("\t" + people);
// Use a Comparator with lambda expression to sort by age
// Collections.sort(people, (o1, o2) -> o1.getAge() - o2.getAge());
// Collections.sort(people, Comparator.comparingInt(p -> p.getAge()));
Collections.sort(people, Comparator.comparingInt(Person::getAge));
System.out.println("Sort using Lambda Age Comparator");
System.out.println("\t" + people);
}
}
So you just need to implement the Comparator
for your Hill
class on whatever property(s) you want. 因此,您只需要在所需的任何属性上为Hill
类实现Comparator
。
Your basic implementation of Comparable
compares the names of the hills, it has nothing to do with the height 您的Comparable
基本实现方式Comparable
了山丘的名称,与高度无关
How can I adjust Comparable so that it allows me to sort by name and then by height after? 如何调整“可比性”,以便我可以按名称排序,然后按身高排序?
Create a new Comparator which implements the logic you need to compare the heights and use that with Collections.sort 创建一个新的Comparator,它实现比较高度并将其与Collections.sort结合使用所需的逻辑
Collections.sort(listOfHills, new Comparator<Hill>() {
@Override
public int compare(Hill o1, Hill o2) {
return o2.height > o1.height ? (o2.height == o1.height ? 0 : 1) : -1;
}
});
This is one of the reasons I don't normally implement Comparator
on objects, unless their is a good business rule associated with how the object should be compared, it's easily to provide a number of custom Comparator
s (or allow other developers to devise their own) which implement "common" algorithms - but that's me 这是我通常不对对象实施Comparator
的原因之一,除非它们是与如何比较对象相关的良好业务规则,否则很容易提供许多自定义Comparator
(或允许其他开发人员设计他们的对象)。自己)来实现“通用”算法-就是我
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