java Comparable and Comparator

1.Comparable简介

此接口对实现它的每一个类的对象强加一个总排序。这种排序被称为类的天然排序，类的compareTo方法被称为其天然比较方法。能够经过

Collections.sort（和Arrays.sort）自动对实现此接口的对象的列表（和数组）进行排序。实现此接口的对象可用做有序映射中的键或有序

集中的元素，而无需指定比较器。

注：若一个类实现了该接口，说明该类自己是支持排序的。在java中倡导全部实现Comparable接口的类都应该保持与

equals()一致的排序顺序，所以还须要重写equals方法和hashcode方法。

This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's natural ordering, and the class's compareTo method is referred to as its natural comparison method. Lists (and arrays) of objects that implement this interface can be sorted automatically by Collections.sort (and Arrays.sort). Objects that implement this interface can be used as keys in a sorted map or as elements in a sorted set, without the need to specify a comparator.

2.Comparable定义

实现Comparable接口仅需实现compareTo方法。

经过x.compareTo(y)来比较x与y的大小：1）返回负数，说明x小于y；2）返回0，说明x与y相等；3）返回正数，说明x大于y。

1 package java.lang;
2 import java.util.*;
3 
4 public interface Comparable<T> {
5     public int compareTo(T o);
6 }

3.Comparator简介

该接口内部是一个比较函数，它对某些对象集合施加总排序。能够将比较器传递给排序方法（例如Collections.sort或Arrays.sort），以便

精确控制排序顺序。比较器还可用于控制某些数据结构的顺序（例若有序集或有序映射），或者为不具备天然顺序的对象集合提供排序。

注：若一个类没有实现Comparable接口，则该类自身没法排序；此时能够使用Comparator帮助这个类进行排序。

A comparison function, which imposes a total ordering on some collection of objects. Comparators can be passed to a sort method (such as Collections.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 as sorted sets or sorted maps), or to provide an ordering for collections of objects that don't have a natural ordering.

4. Comparator定义

1 package java.util;
2 
3 public interface Comparator<T> {
4 
5     int compare(T o1, T o2);
6 
7     boolean equals(Object obj);
8 }

 5. 示例

5.1 Customer.java

该类实现了Comparable接口，即该类的成员对象自身是支持排序的。

 1 import java.util.Objects;
 2 
 3 public class Customer implements Comparable<Customer>{
 4 
 5     private int customerId;
 6     private String customerName;
 7 
 8     public Customer(Integer customerId, String customerName) {
 9         this.customerId = customerId;
10         this.customerName = customerName;
11     }
12 
13     public int getCustomerId() {
14         return customerId;
15     }
16     public String getCustomerName() {
17         return customerName;
18     }
19 
20     @Override
21     public String toString() {
22         return "Customer：[Id=" + customerId + ", Name=" + customerName + "]";
23     }
24 
25      /*
26      * 重写compareTo方法
27      * 按Id或者name排序
28      * 能够对总体添加负号决定升降序
29      * */
30     @Override
31     public int compareTo(Customer o) {
32 //        return this.customerId - o.customerId;
33         return this.customerName.compareTo(o.customerName);
34     }
35 
36     /*
37     * 重写equals和hashcode方法
38     * 这里id和name相同则为同一对象
39     * */
40     @Override
41     public boolean equals(Object o) {
42         if (this == o) return true;
43         if (!(o instanceof Customer)) return false;
44         Customer customer = (Customer) o;
45         return customerId == customer.customerId &&
46                 Objects.equals(customerName, customer.customerName);
47     }
48 
49     @Override
50     public int hashCode() {
51         return Objects.hash(customerId, customerName);
52     }
53 
54 }

View Code

5.2 CustomerComparator.java

该类实现了Comparator接口，帮助Customer对象按Id排序。

 1 import java.util.Comparator;
 2 
 3 public class CustomerComparator implements Comparator<Customer> {
 4 
 5     @Override
 6     public int compare(Customer c1, Customer c2) {
 7         // 按Id排序
 8         return c1.getCustomerId() - c2.getCustomerId();
 9      }
10 }

5.3 SortFunc.java

使用两种接口的排序方法对list进行排序。

 1 import java.util.*;
 2 
 3 public class SortFunc {
 4     public static void main(String[] args){
 5         List<Customer> list = new ArrayList<>();
 6         list.add(new Customer(1, "A"));
 7         list.add(new Customer(2, "C"));
 8         list.add(new Customer(3, "D"));
 9         list.add(new Customer(4, "B"));
10 
11         // 原始排序
12         System.out.println("原始的排序："+list);
13 
14         // 使用compare接口按name排序
15         Collections.sort(list);
16         System.out.println("使用compare接口按name排序："+list);
17 
18         // 使用comparator接口按id排序
19 //        Collections.sort(list, new CustomerComparator());     // 两个方式
20         list.sort(new CustomerComparator());
21         System.out.println("使用comparator接口按id排序："+list);
22 
23         // 判断c1和c2是否引用同一个对象；判断c1和c2是否等价
24         Customer c1 = new Customer(6,"c1");
25         Customer c2 = new Customer(6,"c1");
26         System.out.println(c1==c2);
27         System.out.println(c1.equals(c2));
28     }
29 }

显示结果以下所示，附带==与equals的说明：

对于引用类型，== 判断两个变量是否引用同一个对象，而 equals() 判断引用的对象是否等价，即数据是否一致。

1 原始的排序：[Customer：[Id=1, Name=A], Customer：[Id=2, Name=C], Customer：[Id=3, Name=D], Customer：[Id=4, Name=B]]
2 使用compare接口按name排序：[Customer：[Id=1, Name=A], Customer：[Id=4, Name=B], Customer：[Id=2, Name=C], Customer：[Id=3, Name=D]]
3 使用comparator接口按id排序：[Customer：[Id=1, Name=A], Customer：[Id=2, Name=C], Customer：[Id=3, Name=D], Customer：[Id=4, Name=B]]
4 false
5 true

!!!