Collections.sort不对我的对象列表进行排序

Question

I'm trying to sort a list of objects by its id. Here is my code:

List<Employee> employee = getEmployeeList();
Collections.sort(employee, new Comparator<Employee>(){
    @Override
    public int compare(Employee employee1, Employee employee2) {
        return employee1.getEmployeeId().compareTo(employee2.getEmployeeId());
    }
});

But nothing happens after the sort, it still shows the original employee list. Am I doing something wrong? I've searched everywhere but they all just do this and it works for them. Here is a hard coded set of Employee Ids:

public class Employee{
    private String employeeId = "";
    private String employeeName = "";
    private String contactNbr = "";

    //getters and setters
}

List<Employee> empList = new ArrayList<Employee>();
Employee emp1 = new Employee();
emp1.setEmployeeId("A1B1");
empList.add(emp1);

Employee emp2 = new Employee();
emp2.setEmployeeId("A2B1");
empList.add(emp2);

Employee emp3 = new Employee();
emp3.setEmployeeId("A3B1");
empList.add(emp3);

Collections.sort(empList, new Comparator<Employee>(){
    @Override
    public int compare(Employee employee1, Employee employee2) {
        return employee1.getEmployeeId().compareTo(employee2.getEmployeeId());
    }
});

Answer 1

Check the results of the employeeId compareTo method.

If it always returns 0, the sort will think all the elements are equal. Collections does a stable sort, so if all the elements are equal it will leave the order unchanged.

Answer 2

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;

public class MainTest {

    public static void main(String[] arg)
    {
        List<Employee> empList = new ArrayList<Employee>();
        Employee emp1 = new Employee();
        emp1.setEmployeeId("A2B1");
        empList.add(emp1);

        Employee emp2 = new Employee();
        emp2.setEmployeeId("A1B1");
        empList.add(emp2);

        Employee emp3 = new Employee();
        emp3.setEmployeeId("A3B1");
        empList.add(emp3);

        Collections.sort(empList, new Comparator<Employee>(){

            public int compare(Employee employee1, Employee employee2) {
                return employee1.getEmployeeId().compareTo(employee2.getEmployeeId());
            }
        });


        Iterator i = empList.iterator();
        while(i.hasNext()){
            System.out.println(((Employee)i.next()).getEmployeeId());
        }


    }
}
class Employee{
    private String employeeId = "";
    private String employeeName = "";
    private String contactNbr = "";
    public String getContactNbr() {
        return contactNbr;
    }
    public void setContactNbr(String contactNbr) {
        this.contactNbr = contactNbr;
    }
    public String getEmployeeId() {
        return employeeId;
    }
    public void setEmployeeId(String employeeId) {
        this.employeeId = employeeId;
    }
    public String getEmployeeName() {
        return employeeName;
    }
    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    //getters and setters
}

This is printing correctly output: A1B1 A2B1 A3B1

Answer 3

You are adding the list in the sorted order that's why you are under the impression that your list is not getting the updated. As far as your source is concerned there is no issues you are doing it right. The issue is your input. You are giving the following input in the sorted order as follows.

emp1.setEmployeeId("A1B1");
empList.add(emp1);

emp1.setEmployeeId("A2B1");
empList.add(emp2);

emp1.setEmployeeId("A3B1");
empList.add(emp3);

Your output will be as below

Before sorting: [A1B1, A2B1, A3B1]
After sorting:  [A1B1, A2B1, A3B1]

Try modifying the source as below and check

emp1.setEmployeeId("A3B1");
empList.add(emp3);

emp1.setEmployeeId("A2B1");
empList.add(emp2);

emp1.setEmployeeId("A1B1");
empList.add(emp1);

You will get the result as below

Before sorting: [A3B1, A2B1, A1B1]
After sorting:  [A1B1, A2B1, A3B1]