java 类对象排序 java对象怎么排序

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网线小游侠 2023-06-30 15:31:42

文章标签 java 类对象排序 Java-对象排序 System java List 文章分类 Java 后端开发

在业务逻辑中，我们经常需要对list进行排序，就像下面这样：

Collections.sort(l);

如果l中的元素是String类型，你会发现sort方法将使用字母顺序排序。如果l中的元素是Date类型，sort方法将使用日历顺序排序。这是因为String和Date都实现了Comparable接口，也就是说，如果你想对某种对象进行排序，那么它必须实现Comparable接口。在Java语言中，实现该接口的类罗列如下：

Classes Implementing 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

如果某个类是别人写的，它确实没有实现该接口，那就对排序问题无能为力了么？不是的，sort还有另一种形式：

Collections.sort(list, comparator)

只有这两种方法。如果以上两种方法你都没有做，那么sort方法将抛出异常。

Comparable接口

Comparable接口形式如下：

public interface Comparable<T> {
    public int compareTo(T o);
}
是的，它只有一个方法。你必须在该方法中定义对象是如何比较的。下面是一个demo：
SortDemo.java

package Colloections;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class SortDemo {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        new SortDemo().sort();
    }   private void sort(){
        Person p1 = new Person("bob", 5);
        Person p2 = new Person("albert", 8);
        Person p3 = new Person("bob", 13);
        
        List<Person> list = new ArrayList<Person>();
        list.add(p1);
        list.add(p2);
        list.add(p3);
        System.out.printf("排序前：%n");
        for (Person person : list) {
            System.out.printf(person.toString());
        }
        
        Collections.sort(list);
        System.out.printf("排序后：%n");
        for (Person person : list) {
            System.out.printf(person.toString());
        }
    }
}


class Person implements Comparable<Person>{

    public String name;
    public int age;
    
    public Person(String n, int a){
        name = n;
        age = a;
    }
    
    public  String toString() {
        return String.format("Name is %s, Age is %d%n", name, age);
    }
    
    @Override
    public int compareTo(Person o) {
        // TODO Auto-generated method stub
        //排序优先级为：姓名/年龄
        int nameComp = this.name.compareTo(o.name);
        return (nameComp != 0 ? nameComp : (this.age - o.age));
    }
}

程序输出如下：

排序前：
Name is bob, Age is 5
Name is albert, Age is 8
Name is bob, Age is 13
排序后：
Name is albert, Age is 8
Name is bob, Age is 5
Name is bob, Age is 13

Comparator

Comparator接口提供一个独立的排序功能，这有两个用处：1.你不想使用某个类自带的compareTo逻辑进行排序；2.某个类并没有继承Comparable接口。可见，Comparator接口使得排序更加灵活。它的形式如下所示：

public interface Comparator<T> {
    int compare(T o1, T o2);
}

是的，一个方法就够了。当o1比o2小于，等于，大于时，compare方法将返回一个负数，零或者正数。使用demo如下：

SortDemo.java

package Colloections;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class SortDemo {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        //new SortDemo().sort();
        new SortDemo().sortByComparatpr();
    }
    
    private void sortByComparatpr(){
        Person p1 = new Person("bob", 5);
        Person p2 = new Person("albert", 8);
        Person p3 = new Person("bob", 13);
        
        List<Person> list = new ArrayList<Person>();
        list.add(p1);
        list.add(p2);
        list.add(p3);
        System.out.printf("排序前：%n");
        for (Person person : list) {
            System.out.printf(person.toString());
        }
        
        PersonComparator comparator = new PersonComparator();
        Collections.sort(list, comparator);
        System.out.printf("排序后：%n");
        for (Person person : list) {
            System.out.printf(person.toString());
        }
    }
    
    private void sort(){
        Person p1 = new Person("bob", 5);
        Person p2 = new Person("albert", 8);
        Person p3 = new Person("bob", 13);
        
        List<Person> list = new ArrayList<Person>();
        list.add(p1);
        list.add(p2);
        list.add(p3);
        System.out.printf("排序前：%n");
        for (Person person : list) {
            System.out.printf(person.toString());
        }
        
        Collections.sort(list);
        System.out.printf("排序后：%n");
        for (Person person : list) {
            System.out.printf(person.toString());
        }
    }
}


class Person implements Comparable<Person>{

    public String name;
    public int age;
    
    public Person(String n, int a){
        name = n;
        age = a;
    }
    
    public  String toString() {
        return String.format("Name is %s, Age is %d%n", name, age);
    }
    
    @Override
    public int compareTo(Person o) {
        // TODO Auto-generated method stub
        //排序优先级为：姓名/年龄
        int nameComp = this.name.compareTo(o.name);
        return (nameComp != 0 ? nameComp : (this.age - o.age));
    }
}



class PersonComparator implements Comparator<Person>{

    @Override
    public int compare(Person o1, Person o2) {
        // TODO Auto-generated method stub
        
        return o2.compareTo(o1);
    }
}

程序输出如下：

排序前：
Name is bob, Age is 5
Name is albert, Age is 8
Name is bob, Age is 13
排序后：
Name is bob, Age is 13
Name is bob, Age is 5
Name is albert, Age is 8

注意，这里的输出是降序排列的，因为在compare方法中使用o2与o1进行了比较。如果需要升序排列，则如下修改即可：