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comparator interface 可以被用作集合外部實現的排序。
它是在java.util package。
它有兩個methods:
分別是compare(Object obj1, Object obj2)
和equals(Object element)

我們可以利用Comparator去為物件的屬性重新排序(例如 ID號碼、名字、電話等等)。
而且完成Comparator的資料可以通過Collections.sort 或者Arrays.sort實現排序。

compare()

public interface Comparator<T> {
    int compare(T o1, T o2);
}

• 一個負數，即是第一個參數比第二個小。
• 零，即是第一個參數等於第二個。
• 一個正數，即是第一個參數比第二個大。

例子:

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class ComparatorExample {
    public static void main(String[] args) {
        List<Employee> employees = new ArrayList<>();

        employees.add(new Employee(1010, "Atom", 100000.00, LocalDate.of(2020, 1, 1)));
        employees.add(new Employee(1004, "Ben", 95000.50, LocalDate.of(2020, 1, 11)));
        employees.add(new Employee(1015, "Coco", 134000.00, LocalDate.of(2020, 1, 28)));
        employees.add(new Employee(1009, "Doggy", 100000.00, LocalDate.of(2020, 1, 18)));

        System.out.println("Employees : " + employees);

        // 按員工的名字排序
        Comparator<Employee> employeeNameComparator = new Comparator<Employee>() {
            @Override
            public int compare(Employee e1, Employee e2) {
                return e1.getName().compareTo(e2.getName());
            }
        };

        Collections.sort(employees, employeeNameComparator);
        System.out.println("\nEmployees (Sorted by Name) : " + employees);

        //按員工的工資排序
        Comparator<Employee> employeeSalaryComparator = new Comparator<Employee>() {
            @Override
            public int compare(Employee e1, Employee e2) {
                if(e1.getSalary() < e2.getSalary()) {
                    return -1;
                } else if (e1.getSalary() > e2.getSalary()) {
                    return 1;
                } else {
                    return 0;
                }
            }
        };
        
        Collections.sort(employees, employeeSalaryComparator);
        System.out.println("\nEmployees (Sorted by Salary) : " + employees);

        //按員工加入的日期排序
        Comparator<Employee> employeeJoiningDateComparator = new Comparator<Employee>() {
            @Override
            public int compare(Employee e1, Employee e2) {
                return e1.getJoiningDate().compareTo(e2.getJoiningDate());
            }
        };

        Collections.sort(employees, employeeJoiningDateComparator);
        System.out.println("\nEmployees (Sorted by JoiningDate) : " + employees);
    }
}

參考文章/網站/書本:

• Interface Comparator