在java中使用单个Comparator接口对具有多个字段的员工对象进行排序

Question

Consider scenario below :

Suppose there is Employee Class having attributes : eID, eName, eAge, eAddress, eDeptName and can be added more in future based on requirement.

This Employee object is added in to hashmap as key and string as value.

Consider the below code in main class :

     class Test
    {
        public static void main(String[] args)
       {
          Map<Employee,String> m = new HashMap<>(); 
          //Adding object in to map
          m.put(e1,"one"); 
          m.put(e2,"two"); 
          m.put(e3,"othree");
        }
    }

Now I need to use the comparator interface to sort the map based on employee object.

My Questions is :-

1) what is the best common used way to sort the employee object : by using seperate comparator for each field or using the single comparator for all fields

2) How to use the single comparator for all fields and how to invoke it ?

Thanks in Advance

Answer 1

1) what is the best common used way to sort the employee object : by using seperate comparator for each field or using the single comparator for all fields

There is no best way, but only what you need. You can have separate comparator for each field so that you can sort your list by specific field. You will have more choices over how your list is to be sorted.

Or you can use multiple fields in the same comparator when you need them as tie-breaker(s). But you have to decide which is the primary sorting criteria, which are the secondary criteria(s).

When making a decision, ask yourself:

• about the reusability of the design. Is grouping them as 1 comparator more useful than separating them?
• what is the likelihood of using it again if you were to group them as 1 comparator?
• which one is more flexible to be used?
• which one is easier to maintain?

2) How to use the single comparator for all fields and how to invoke it ?

If you want to use all the fields, say 4 fields: name , age , salary , gender ^{(using my own fields as example)}

You choose only 1 field which you are interest to use as the sorting criteria. The other 3 fields will be used as tie-breakers.

Assuming you sort by name . When 2 similar names appear, you can use a 2nd field as the tie-breaker such as age .

If there are multiple employees with same name and age, you sort them by the 3rd criteria, maybe by salary . If all 3 fields are similar, then you sort them by the 4th criteria gender .

//example:
class EmployeeComparator implements Comparator<Employee>{
    @Override
    public int compare(Employee e1, Employee e2){
        int cmp = e1.getName().compareTo(e2.getName());
        if(cmp == 0)
            cmp = e1.getAge() - e2.getAge();
        if(cmp == 0)
            cmp = e1.getSalary() - e2.getSalary();
        if(cmp == 0)
            cmp = e1.getGender().compareTo(e2.getGender());
        return cmp;
    }   
}

Answer 2

The implementation is up to you. However, you'll have to determine how to define how to compare an employee to another. For example are they compared by ID? Name? Age?

You can use a combination of all of the fields in the compare method, a few of them that make sense or just one of them.

Since Comparator is an interface, you'll have to override the compare and equals method and define the logic in which attributes you'll compare in there.

Answer 3

Let Employee implement Comparable interface, and include all fields you want in compareTo() method. Then, if you just use TreeMap instead of HashMap, your map items will be automatically sorted.