Question

import java.util.*;

public class Employee
{
    private int empId;
    private String name;
    private int age;
    public Employee(String name, int id, int age )
    {
        this.name = name;
        this.empId = id;
        this.age = age;
    }
    public int getId()
    {
        return empId;
    }
    public int getAge()
    {
        return age;
    }
    public String getName()
    {
        return name;
    }

}

class SortById extends Employee implements Comparable<SortById>
{
    public SortById(String name, int id, int age)
    {
        super(name, id, age);
    }
    public int compareTo(SortById other)
    {
        if (this.getId() > other.getId()) return 1;
        if (this.getId() < other.getId()) return -1;
        return 0;

    }
}

class SortByName extends Employee implements Comparable<SortByName>
{
    public SortByName(String name, int id, int age)
    {
        super(name, id, age);
    }

    public int compareTo(SortByName other)
    {
        if (this.getName().compareTo(other.getName()) > 0) return 1;
        if (this.getName().compareTo(other.getName()) < 0) return -1;
        return 0;

    }
}

class Test
{
    public static void main(String args[])
    {
        Employee[] array = new SortById[3];
        array[0] = new SortById("Gautam", 1222, 20);
        array[1] = new SortById("Shivam", 1221, 20);
        array[2] = new SortById("Ankit", 1223, 21);
        System.out.println(array[0] instanceof SortByName);

        Arrays.sort(array);
        for (int i = 0; i < array.length; i++)
            System.out.println("ID: " + array[i].getId() + " Name: " + array[i].getName() + " Age: " + array[i].getAge());
        Employee[] array2 = new SortByName[3];
        array2[0] = new SortByName("Gautam", 1222, 20);
        array2[1] = new SortByName("Shivam", 1221, 20);
        array2[2] = new SortByName("Ankit", 1223, 21);
        Arrays.sort(array2);
        for (int i = 0; i < array2.length; i++)
            System.out.println("ID: " + array2[i].getId() + " Name: " + array2[i].getName() + " Age: " + array2[i].getAge());

    }
}

This program runs fine, I just wanted to ask since I’m using the parameterized version of Comparable, should the reference I pass into the compareTo be of SortById type or SortByName type?

The code runs fine even if the reference is of type Employee, although pointing to its subclass (SortByName or SortById).

How is an implicit cast happening? I have read is not possible i.e., it’s not possible to cast a superclass type to subclass implicitly.

Answer 1

You never call compareTo which explains why you don’t need the cast. The actual call is within Arrays.sort which takes care of it (by actually using the raw Comparable)

Also, in such a case the compiler will generate 2 compareTo methods: the one you explicitly define with the SortByXX parameter, and another one with an Object parameter which delegates to the first one.

As @martijno says it, you will run into problems if you add a simple Employee in your array, which will lead to ClassCastException (from Employee to either Comparable or SortByXX). Same will occur if you mix SortByName and SortById.