Write a Java program to sort custom objects using Comparable and Comparator interfaces.

import java.util.*;

class Employee implements Comparable<Employee> {
    int id;
    String name;
    double salary;

    Employee(int id, String name, double salary) {
        this.id = id; this.name = name; this.salary = salary;
    }

    // Define natural ordering — sort by ID ascending
    @Override
    public int compareTo(Employee other) {
        return this.id - other.id;  // negative=this before other, 0=equal, positive=other before this
    }

    @Override
    public String toString() {
        return "[" + id + "] " + name + " ($" + salary + ")";
    }
}

public class ComparableDemo {
    public static void main(String[] args) {
        List<Employee> employees = new ArrayList<>();
        employees.add(new Employee(3, "Charlie", 75000));
        employees.add(new Employee(1, "Alice",   90000));
        employees.add(new Employee(2, "Bob",     80000));

        Collections.sort(employees);  // uses compareTo()
        System.out.println("Sorted by ID (natural order):");
        employees.forEach(System.out::println);
    }
}

Sorted by ID (natural order):
[1] Alice ($90000.0)
[2] Bob ($80000.0)
[3] Charlie ($75000.0)

import java.util.Comparator;

public class ComparatorDemo {
    public static void main(String[] args) {
        List<Employee> employees = new ArrayList<>(Arrays.asList(
            new Employee(3, "Charlie", 75000),
            new Employee(1, "Alice",   90000),
            new Employee(2, "Bob",     80000)
        ));

        // Sort by name alphabetically
        Comparator<Employee> byName = Comparator.comparing(e -> e.name);
        employees.sort(byName);
        System.out.println("By name:");
        employees.forEach(System.out::println);

        // Sort by salary descending
        employees.sort(Comparator.comparingDouble((Employee e) -> e.salary).reversed());
        System.out.println("\nBy salary (descending):");
        employees.forEach(System.out::println);

        // Sort by salary, then by name (chained)
        employees.sort(
            Comparator.comparingDouble((Employee e) -> e.salary)
                      .thenComparing(e -> e.name)
        );
        System.out.println("\nBy salary then name:");
        employees.forEach(System.out::println);
    }
}

By name:
[1] Alice ($90000.0)
[2] Bob ($80000.0)
[3] Charlie ($75000.0)

By salary (descending):
[1] Alice ($90000.0)
[2] Bob ($80000.0)
[3] Charlie ($75000.0)

By salary then name:
[3] Charlie ($75000.0)
[2] Bob ($80000.0)
[1] Alice ($90000.0)

// Sort strings by length
List<String> tools = Arrays.asList("Selenium", "TestNG", "Cucumber", "Maven");

tools.sort((a, b) -> a.length() - b.length());
System.out.println("By length: " + tools);
// → [Maven, TestNG, Cucumber, Selenium]

// Using Comparator.comparingInt
tools.sort(Comparator.comparingInt(String::length).reversed());
System.out.println("By length desc: " + tools);
// → [Selenium, Cucumber, TestNG, Maven]