假设你有一些对象，它们有几个字段可以比较:
public class Person {
private String firstName;
private String lastName;
private String age;
/* Constructors */
/* Methods */
}
所以在这个例子中，当你问if:
a.compareTo(b) > 0
你可能会问a的姓是不是在b的姓之前，或者a的年龄是不是比b大，等等……
在不增加不必要的混乱或开销的情况下，在这些类型的对象之间进行多重比较的最干净的方法是什么?
comparable接口只允许通过一个字段进行比较 在我看来，添加大量的比较方法(如compareByFirstName()， compareByAge()等)是混乱的。
那么最好的解决办法是什么呢?
当前回答
使用java8:
Comparator.comparing((Person p)->p.firstName)
.thenComparing(p->p.lastName)
.thenComparingInt(p->p.age);
如果你有访问方法:
Comparator.comparing(Person::getFirstName)
.thenComparing(Person::getLastName)
.thenComparingInt(Person::getAge);
如果一个类实现Comparable，那么这个比较器可以用在compareTo方法中:
@Override
public int compareTo(Person o){
return Comparator.comparing(Person::getFirstName)
.thenComparing(Person::getLastName)
.thenComparingInt(Person::getAge)
.compare(this, o);
}
其他回答
我认为如果你的比较算法是“聪明的”，那就更令人困惑了。我会选择你建议的众多比较方法。
对我来说唯一的例外就是平等。对于单元测试，重写. equals(在.net中)对我来说很有用，以便确定两个对象之间的几个字段是否相等(而不是引用是否相等)。
与比较方法不同，您可能只想在Person类中定义几种类型的“Comparator”子类。这样就可以将它们传递到标准的Collections排序方法中。
您应该实现Comparable <Person>。假设所有字段都不为空(为了简单起见)，年龄是int型，比较排名是第一，最后，年龄，compareTo方法非常简单:
public int compareTo(Person other) {
int i = firstName.compareTo(other.firstName);
if (i != 0) return i;
i = lastName.compareTo(other.lastName);
if (i != 0) return i;
return Integer.compare(age, other.age);
}
下面的博客给出了一个很好的链式比较器的例子
http://www.codejava.net/java-core/collections/sorting-a-list-by-multiple-attributes-example
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
/**
* This is a chained comparator that is used to sort a list by multiple
* attributes by chaining a sequence of comparators of individual fields
* together.
*
*/
public class EmployeeChainedComparator implements Comparator<Employee> {
private List<Comparator<Employee>> listComparators;
@SafeVarargs
public EmployeeChainedComparator(Comparator<Employee>... comparators) {
this.listComparators = Arrays.asList(comparators);
}
@Override
public int compare(Employee emp1, Employee emp2) {
for (Comparator<Employee> comparator : listComparators) {
int result = comparator.compare(emp1, emp2);
if (result != 0) {
return result;
}
}
return 0;
}
}
打电话比较器:
Collections.sort(listEmployees, new EmployeeChainedComparator(
new EmployeeJobTitleComparator(),
new EmployeeAgeComparator(),
new EmployeeSalaryComparator())
);
//Following is the example in jdk 1.8
package com;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
class User {
private String firstName;
private String lastName;
private Integer age;
public Integer getAge() {
return age;
}
public User setAge(Integer age) {
this.age = age;
return this;
}
public String getFirstName() {
return firstName;
}
public User setFirstName(String firstName) {
this.firstName = firstName;
return this;
}
public String getLastName() {
return lastName;
}
public User setLastName(String lastName) {
this.lastName = lastName;
return this;
}
}
public class MultiFieldsComparision {
public static void main(String[] args) {
List<User> users = new ArrayList<User>();
User u1 = new User().setFirstName("Pawan").setLastName("Singh").setAge(38);
User u2 = new User().setFirstName("Pawan").setLastName("Payal").setAge(37);
User u3 = new User().setFirstName("Anuj").setLastName("Kumar").setAge(60);
User u4 = new User().setFirstName("Anuj").setLastName("Kumar").setAge(43);
User u5 = new User().setFirstName("Pawan").setLastName("Chamoli").setAge(44);
User u6 = new User().setFirstName("Pawan").setLastName("Singh").setAge(5);
users.add(u1);
users.add(u2);
users.add(u3);
users.add(u4);
users.add(u5);
users.add(u6);
System.out.println("****** Before Sorting ******");
users.forEach(user -> {
System.out.println(user.getFirstName() + " , " + user.getLastName() + " , " + user.getAge());
});
System.out.println("****** Aftre Sorting ******");
users.sort(
Comparator.comparing(User::getFirstName).thenComparing(User::getLastName).thenComparing(User::getAge));
users.forEach(user -> {
System.out.println(user.getFirstName() + " , " + user.getLastName() + " , " + user.getAge());
});
}
}
