Array sort not working, no instance(s) of variable(s) T exist so that Employee conforms to Comparable

Question

I created this program. I'm trying to get it to sort my list, but it will not sort. I have included the main program as well as my Employee class.

t spits out this when I try to sort it:

sort(java.util.list) in Collections cannot be applied to (java.util.ArrayList )

Here is the error code I am given : no instance(s) of variable(s) T exist so that Employee conforms to Comparable<? super T> Comparable<? super T>

import java.util.*;
import java.lang.*;

public class Employee {
    String name;

    public Employee(String name) {
        this.name = name;
    }

    // Used to print student details in main()

    public String toString() {
        return this.name;
    }
}


public class Schedule {
    public static void main (String args[] ) {

        ArrayList<Employee> listOfEmployees = new ArrayList<Employee>();

        listOfEmployees.add(new Employee("Lauren"));
        listOfEmployees.add(new Employee("Greg"));
        listOfEmployees.add(new Employee("Brenda"));
        listOfEmployees.add(new Employee("Derek"));
        listOfEmployees.add(new Employee("Brittany"));

        System.out.println("Unsorted");


        for (int i=0; i<listOfEmployees.size(); i++) {

            System.out.println(listOfEmployees.get(i));
        }

        Collections.sort(listOfEmployees);

        System.out.println("Sorted Alphabetically");

        for (int i=0; i<listOfEmployees.size(); i++) {
            System.out.println(listOfEmployees.get(i));
        }
    }

}

Answer 1

You need to implement Comparable interface and override its compareTo method like following:

public class Employee implements Comparable<Employee> {
    String name;

    public Employee(String name) {
        this.name = name;
    }

    public String toString() {
        return this.name;
    }

    @Override
    public int compareTo(Employee employee) {
        return this.name.compareTo(employee.name);
    }
}

And if you don't want to implement Comparable interface then you have to create a custom Comparator like follwing:

Comparator<Employee> employeeNameComarator = new Comparator<Employee>() {
    @Override
    public int compare(Employee employee1, Employee employee2) {
        return employee1.name.compareTo(employee2.name);
    }
};

And the pass the comparator object while sorting like:

Collections.sort(listOfEmployees, employeeNameComarator);

Answer 2

您还可以使用Comparator 。

listOfEmployees.sort(Comparator.comparing(Employee::getName));

Answer 3

You need to implement the Comparable interface in order to use Collections.sort(). Please go through below oracle docs for more information:

https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#sort(java.util.List)