问题

我正在阅读有关Java中的接口的文章。其中提到我们必须实现compareTo方法才能在ArrayList容器上调用sort，例如Employee类应该实现 Comparable接口。

后面解释了为什么Employee类不能简单地提供compareTo方法而不实现Comparable接口？之所以需要接口，是因为Java编程是强类型的。在进行方法调用时，编译器需要能够检查该方法是否确实存在。

因此，当我不实现Comparable接口并使用Arrays.sort方法时，我预计会出现编译时错误，但我没有观察到编译错误，而是得到了运行时错误。请解释为什么上述场景中没有显示编译时错误

以下是代码片段

package com.vrk.inheritance;import java.time.*;
import java.util.Arrays;public class Employee
{private String name;private double salary;private LocalDate hireDay;public Employee(String name, double salary, int year, int month, int day){this.name = name;this.salary = salary;hireDay = LocalDate.of(year, month, day);}public String getName(){return name;}public double getSalary(){return salary;}public LocalDate getHireDay(){return hireDay;}public void raiseSalary(double byPercent){double raise = salary * byPercent / 100;salary += raise;}/*public int compareTo(Object otherObject) {System.out.println("Employee compareTo called");return 0;}*//*** equalTo function in employee. Created on 8th Sep 2024* @param another object to compare to this object*/public boolean equals(Object otherObject) {// quick test to check if objects are identicalif ( this == otherObject) return true;// must return false if the explicit parameter is nullif(otherObject == null) return false;// if the classes don't match, they can't be equalif (getClass() != otherObject.getClass()) return false;// now we know otherObject is a non-null Employeevar other = (Employee) otherObject;// test whether the fields have identical value// Not sure in my setup below line is not working, but online compiler it is working. // java.util.Objects.equals(this.hireDay, other.hireDay);return true;}public static void main(String[] args) {var staff = new Employee[3];// fill the staff array with Manager and Employee objectsstaff[0] = new Employee("Harry Hacker", 50000, 1989, 10, 1);staff[1] = new Employee("Tommy Tester", 40000, 1990, 3, 15);staff[1] = new Employee("Ravi Tester", 60000, 1999, 4, 16);Arrays.sort(staff);}
}

解答

如果你看一下的文档Arrays.sort，你会发现它根本没有使用泛型；它只需要一个Object[]。这就是为什么你不会收到编译错误的原因。

这是出于历史原因：该方法是在Java引入泛型之前编写的

如果我们必须能够调用在类中实现的方法，为什么我们还必须实现接口?

因为该方法必须进行一些非平凡的反射。将Object转换为Comparable要容易得多，而且正如您所指出的，这更符合Java的类型系统理念。从本质上讲，Java不实现鸭子类型。