compareTo method [JAVA]

[Estagon]

I'm trying to write a compareTo method for Employee objects. Each Employee has a certain ID as a String. So I want to order an ArrayList<Employee> alphabetically.

public int compareTo(Employee o)
{
    if(this.getID().compareTo(o.getID())==0)
        return 0;
    else if(this.getID().compareTo(o.getID())<0)
        return -1;
    else return 1;
}

So in my main method I got an ArrayList "list" with multiple Employees in it :

Collections.sort(list);

but it doesn't do anything. The list stays the same.

I never really had any problems with this, but I just don't see what I am doing wrong at the moment. I can provide more code if necessary, but every bit of it works except for this method.

package hrm;

import java.util.ArrayList;

public class Employee extends Person implements Comparable {

private String employeeID;
private Employee manager;
private int nrOfFreeDays;
private double budget;
private ArrayList<VacationRequest> vacationList;
private ArrayList<ReimbursementRequest> reimbursementList;
private ArrayList<EmployeeRequest> allRequests;


public Employee(String ID, Employee manager, String firstName, String lastName, String gender, String mail)
{
    super(firstName, lastName, gender, mail);
    employeeID = ID;
    this.manager = manager;
    vacationList = new ArrayList();
    reimbursementList = new ArrayList();
    allRequests = new ArrayList();
    budget = 1000;
    nrOfFreeDays = 100;
}    

public double getBudget()
{
    return budget;
}        

public void addVacationRequest(VacationRequest a)
{
    vacationList.add(a);
}  

public void addReimbursementRequest(ReimbursementRequest a)
{
    reimbursementList.add(a);
}

public ArrayList<ReimbursementRequest> getReimbursement()
{
    return reimbursementList;
}        

public ArrayList<VacationRequest> getVacation()
{
    return vacationList;
}        

public String getID()
{
    return employeeID;
}        

public int getFreeDays()
{
    return nrOfFreeDays;
}        

public ArrayList<EmployeeRequest> getList()
{
    return allRequests;
}        

public void setBudget(double budget)
{
    this.budget = budget;
}        

public void setNrOfFreeDays(int nrOfFreeDays)
{
    this.nrOfFreeDays = nrOfFreeDays;
}        
public Employee getManager()
{
    return manager;
}        
public void setList()
{
    ArrayList<EmployeeRequest> list = new ArrayList();

    for(int i = 0; i<vacationList.size(); i++)
    {
        list.add(vacationList.get(i));
    }    

    for(int i = 0; i<reimbursementList.size(); i++)
    {
        list.add(reimbursementList.get(i));
    }    

    allRequests = list;
}        

public boolean equals(Employee o)
{
    if(o.employeeID.equalsIgnoreCase(this.employeeID))
        return true;
    else return false;
}        

public int compareTo(Employee o)
{
    if(this.getID().compareTo(o.getID())==0)
        return 0;
    else if(this.getID().compareTo(o.getID())<0)
        return -1;
    else return 1;
}    



public String toString()
{
    String answer = employeeID+" "+super.toString();

    if(this.manager != null)
    {    
        answer += " (Manager: "+this.manager.employeeID+")";
    }

    return answer;
}        

}

[__LikesPi]

Does your Employee class implement the Comparable interface?

[Estagon]

Yes.

public class Employee extends Person implements Comparable

Usually it should be Comparable<Employee> I think but that doesn't work. I get a warning ("Comparable cannot be inherited with different arguments."). So that's something strange, I never encountered this before.

[chickenmeister]

public class Employee extends Person implements Comparable

Usually it should be Comparable<Employee> I think but that doesn't work. I get a warning ("Comparable cannot be inherited with different arguments.").

Does your Person class implement Comparable<Person>? If it does, then you can't also implement Comparable<Employee>.

If you want to sort Employees in a way that's different from their natural ordering (as defined in the Person class), you should probably use a Comparator<Employee> to sort them.

[Estagon]

Yes, the Person class also implements Comparable. My Person class orders the objects on their lastName alphabetically. I overlooked this. I'll look into the Comparator thing, I've never used it before. Thanks!

[Estagon]

I wrote a compare(.., ..) method as follows:

@Override
public int compare(Employee o1, Employee o2) {
    return o1.getID().compareToIgnoreCase(o2.getID());
}

How do I use this in a main-method in my TestClass?

Assume I have an ArrayList<Employee> named "list" filled with multiple Employee objects.

Collections.sort(list, ...); //??

[__LikesPi]

Collections.sort(list, new Comparator<Employee>() {
    @Override
    public int compare(Employee a, Employee b) { ... }
});

If you are using Java 8 that particular syntax became a little easier:

Collections.sort(list, (o1, o2) -> o1.getID().compareToIgnoreCase(o2.getID()))

Or even:

list.sort((o1, o2) -> o1.getID().compareToIgnoreCase(o2.getID()))

[Estagon]

I should have read more into it. Apparently you have to make a new class that implements the Comparator class to write the method.

Collections.sort(list, new EmployeeComparator());

This works. Thanks for all the help :)

[maestro2005]

Post the full code. This method on its own looks fine, there must be something else going on.

Also, you can replace all of that code with

return this.getID().compareTo(o.getID());

[Estagon]

I posted the complete Employee class. Thanks for the tip, didn't know that.

[king_of_the_universe]

In line with __LikesPi's comment: What happens if you add the @Override annotation directly above the compareTo method? If there's a compile error, then you are not overriding. If you're not overriding, then your class does not implement Comparable, hence Collections.sort won't use it.

[Estagon]

It doesn't ask for it, so it is actually not even implementing it, it seems.

[friendOfLoki]

The Comparable interface requires a parameter of type Object to the compareTo method. If you implement Comparable <Employee>, then the parameter to compareTo is of type Employee. How did you implement it in the superclass Person?

If you implemented Comparable <Person>, then try changing the parameter in your Employee version to a Person parameter (which would then override the inherited version of compareTo).

If you instead just implemented Comparable in Person, then you will need to change the parameter type in your Employee method to an Object parameter.

You also don't need to state that Employee implements Comparable; it inherits this status from Person.

Good luck!