Java 中的 Comparable 与 Comparator\
java programming java8object oriented programming
Comparable
当我们想要使用对象的一个属性来比较对象时，可以使用 Comparable 接口。它被认为是对象的自然排序。考虑一个员工列表，我们希望按姓名排序作为默认排序顺序。Comparable 接口具有目标类必须实现的 compareTo() 方法。
class Employee implements Comparable {
String name;
public int compareTo(Employee o) {
return name.compareTo(o.name);
}
}
现在，可以使用 Collections.sort() 方法按名称对上述对象的集合进行排序。
Collections.sort(employees);
Comparator
Comparator 接口提供了一个 compare() 方法来比较同一类的两个对象。在这里，它提供了灵活性，因为我们可以为要比较的对象的每个属性创建 n 个比较器。请考虑以下示例:
class IdComparator implements Comparator<Employee> {
public int compare(Employee o1, Employee o2) {
if (o1.getId() < o2.getId()) {
return -1;
}else if (o1.getId() > o2.getId()) {
return 1;
} else {
return 0;
}
}
}
class AgeComparator implements Comparator<Employee> {
public int compare(Employee o1, Employee o2) {
if (o1.getAge() < o2.getAge()) {
return -1;
}else if (o1.getAge() > o2.getAge()) {
return 1;
} else {
return 0;
}
}
}
示例
在此示例中，我们展示了如何使用可比较接口和比较器接口分别根据员工姓名、ID 和年龄对员工列表进行排序。
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Tester {
public static void main(String[] args) {
List<Employee> employees = new ArrayList<>();
employees.add(new Employee(1, 20, "A"));
employees.add(new Employee(4, 26, "D"));
employees.add(new Employee(2, 23, "C"));
employees.add(new Employee(3, 25, "B"));
employees.add(new Employee(5, 24, "E"));
//无序列表
System.out.println("无序列表");
System.out.println(employees);
//按员工姓名的自然顺序排序，
//由 compareTo 方法提供
Collections.sort(employees);
//有序列表
System.out.println("按姓名排序列表");
System.out.println(employees);
//按员工 ID 排序，
//由 IdComparator 提供
Collections.sort(employees, new IdComparator());
//有序列表
System.out.println("Sorted List on Id");
System.out.println(employees);
//按员工年龄排序，
//由 AgeComparator 提供
Collections.sort(employees, new AgeComparator());
//有序列表
System.out.println("Sorted List on Age");
System.out.println(employees);
}
}
class Employee implements Comparable<Employee> {
private int id;
private int age;
private String name;
public Employee(int id, int age, String name) {
this.id = id;
this.age = age;
this.name = name;
}
public int compareTo(Employee o) {
return name.compareTo(o.name);
}
public String toString() {
return "
[Id: " + id + ", age: " + age + ", name: " + name +" ]";
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
class IdComparator implements Comparator<Employee> {
public int compare(Employee o1, Employee o2) {
if (o1.getId() < o2.getId()) {
return -1;
}else if (o1.getId() > o2.getId()) {
return 1;
} else {
return 0;
}
}
}
class AgeComparator implements Comparator<Employee> {
public int compare(Employee o1, Employee o2) {
if (o1.getAge() < o2.getAge()) {
return -1;
}else if (o1.getAge() > o2.getAge()) {
return 1;
} else {
return 0;
}
}
}
输出
Unsorted List [ [Id: 1, age: 20, name: A ], [Id: 4, age: 26, name: D ], [Id: 2, age: 23, name: C ], [Id: 3, age: 25, name: B ], [Id: 5, age: 24, name: E ]] Sorted List on Name [ [Id: 1, age: 20, name: A ], [Id: 3, age: 25, name: B ], [Id: 2, age: 23, name: C ], [Id: 4, age: 26, name: D ], [Id: 5, age: 24, name: E ]] Sorted List on Id [ [Id: 1, age: 20, name: A ], [Id: 2, age: 23, name: C ], [Id: 3, age: 25, name: B ], [Id: 4, age: 26, name: D ], [Id: 5, age: 24, name: E ]] Sorted List on Age [ [Id: 1, age: 20, name: A ], [Id: 2, age: 23, name: C ], [Id: 5, age: 24, name: E ], [Id: 3, age: 25, name: B ], [Id: 4, age: 26, name: D ]]
