HashSet, TreeSet
================
import java.util.*;

public class Main {
        public static void main(String[] args) {

                Set<String> s1 = new HashSet<>(); // HashSet implements Set

                s1.add("Apple");
                s1.add("Bear");
                System.out.println(s1); // Output#1: [Apple, Bear]

                s1.add("Cat");
                s1.add("Dog");
                s1.add("Apple");
                s1.add("Bear");
                System.out.println(s1); // Output#2: [Apple, Cat, Bear, Dog]
        }
}


Comparator
===========
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class Product {
	private int part_number; 	//sometimes we want to compare part_number
	private String product_name;	//sometimes we want compare product name
	public Product(String pName, int pNum) {part_number=pNum; product_name=pName;}
	public int getPNum() {return part_number;}
	public String getPName() {return product_name;}
	public String toString() {return String.format("%s(%d)",product_name,part_number);}
}

public class Main{

	public static void main(String [] args) 
	{	
		ArrayList<Product> arr = new ArrayList<>();
		arr.add(new Product("Widget",4562));
		arr.add(new Product("Modem",9912));
		arr.add(new Product("Toaster",1234));
		
		Collections.sort(arr, new Comparator<Product>() {
			public int compare(Product a, Product b) {
				return Integer.compare(a.getPNum(),b.getPNum());
			}
		});
		
		System.out.println(arr); //Output: [Toaster(1234), Widget(4562), Modem(9912)]
		
		Collections.sort(arr, new Comparator<Product>() {
			public int compare(Product a, Product b) {
				return a.getPName().compareTo(b.getPName());
			}
		});

		System.out.println(arr); //Output: ___________
	}
}