多级比较器

Question

Please can you show me the simplest way to implement multi-level comparator: I have an employee class and I want to compare them by salary, this is done by implementing a comparator :

public class SalaryComparator implements Comparator<Employee> {

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

Now if the salary is the same I want to compare them by Lastname or ID

How can I do it ?

Thank you

Answer 1

You can use the Comparator.thenComparing method. The Comparator.comparing method also simplifies this so you don't have to write your own classes implementing Comparator :

Comparator<Employee> cmp = new SalaryComparator()
    .thenComparing(Comparator.comparing(Employee::getLastName))
    .thenComparing(Comparator.comparing(Employee::getID));

You can even eliminate your SalaryComparator class entirely:

Comparator<Employee> cmp = Comparator.comparing(Employee::getSalary)
    .thenComparing(Comparator.comparing(Employee::getLastName))
    .thenComparing(Comparator.comparing(Employee::getID));

If your methods return primitive types (eg if getSalary returns double ), use eg the comparingDouble method instead of comparing . There are also comparingInt and comparingLong .