How to use Java comparator properly?

You need to implement it so that it orders by preferred elements. That is, you need to compare by name, then if that comparison is equal, compare by age, etc. An example is listed below:

public class EmployeeComparator implements Comparator<Employee> {

  @Override
  public int compare(Employee e1, Employee e2) {
    int nameDiff = e1.getName().compareTo(e2.getName());

    if(nameDiff != 0) {
      return nameDiff;
    }

    int ageDiff = e1.getAge() - e2.getAge();

    if(ageDiff != 0) {
      return ageDiff;
    }

    int idDiff = e1.getEmpId() - e2.getEmpId();

    return idDiff;
  }
}

Update

Came across this a moment ago: How to compare objects by multiple fields One of the answers linked to ComparatorChain which will invoke multiple comparators in sequence until a non-zero result is received from a comparator or all comparators are invoked. This should probably be your preferred solution.

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Perhaps this (untested) implementation of Comparator#compare() will do the trick.

int compare(Employee e, Employee f)
{
    int val = e.name.compareTo(f.name);

    if(val == 0)
    {
        val = e.age - f.age;

        if(val == 0)
        {
            val = e.empId - f.empId;
        }
    }

    return val;
}