How to retain/get only the duplicate elements in a list

Let’s say you have a list of duplicate (and non-duplicate) items, and you want a new collection with only the duplicate items in there. The easiest way is to extend the HashSet.

public class DuplicatesOnlySet<E> extends HashSet<E>
{
    private final Set<E> uniques = new HashSet<E>();

    public  DuplicatesOnlySet(Collection<? extends E> c) 
    {
        super.addAll(c);
    }

    @Override
    public boolean add(E e) 
    {                
        if(!this.uniques.add(e))
             return super.add(e);

    return false;
    }
}

Call it like

List<String> duplicates = new ArrayList<String>(new DuplicatesOnlySet<String>(original)) ;

where original is the Collection with the duplicate items.

Compliments to Markos for his implementation.

How to sort a list of objects

Ever wondered how to sort a list full of custom objects? The answer is to use a Comparator (or Comparable).

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

public class Sorter
{
    private static Comparator<Product> COMPARATOR = new Comparator<Product>()
    {
	// This is where the sorting happens.
        public int compare(Product o1, Product o2)
        {
            return o1.getPrice() - o2.getPrice();
        }
    };
    
    public static void main(String[] args)
    {
	Product product1 = new Product(1, 20);
	Product product2 = new Product(1, 1);
	Product product3 = new Product(3, 30);
	Product product4 = new Product(6, 6);
	Product product5 = new Product(4, 45);
	Product product6 = new Product(6, 3);
	
	List<Product> products = new ArrayList<Product>(6);
	products.add(product1);
	products.add(product2);
	products.add(product3);
	products.add(product4);
	products.add(product5);
	products.add(product6);
	
	for (Product product : products)
	    System.out.println("Before sorting on price: " + product.getPrice());
	
	Collections.sort(products, COMPARATOR);
	
	for (Product product : products)
	    System.out.println("After sorting on price: " + product.getPrice());
    }
}

class Product
{
    int code, price;

    public Product(int code, int price)
    {
	this.code = code;
	this.price = price;
    }

    public int getCode()
    {
        return code;
    }

    public int getPrice()
    {
        return price;
    }
}

What if you want to sort based on two (or more) properties in the class. Lets say that we want to sort first on code and then on price (a product might have many prices, depending on who is buying it). Then we need to change the compare method to the following

// This is where the sorting happens.
	public int compare(Product o1, Product o2)
	{
	    int codeDifference = o1.getCode() - o2.getCode();
	    // If the code is the same then sort on the price.
	    if (codeDifference == 0)
		return o1.getPrice() - o2.getPrice();
	    else
		return codeDifference;
	}

It will first sort on the product code and, if it is the same between two products, it will further sort on the price of each product.