In what order is a Set stored? Random?

Question

When I store something inside a Set, the order in which it stores is random?

And how could I order a set Set?

Answer 1

The Set is just an interface. The order depends on the implementation.

The HashSet has no guarantee of any order. In practice it is random.

The LinkedHashSet maintains the order in which the elements are inserted.

The TreeSet sorts the elements according to the natural ordering (crescent, alphabetical, etc). It is possible to pass a Comparator in the constructor to specify how it should order if the natural ordering does not exist or is not the desired one.

---

To instantiate the TreeSet with a Comparator, we can use a separate class, an anonymous class or a reference to a method (java 8):

Classy:

public void instanciaElemento() {
    TreeSet<SuaClasse> = new TreeSet<>(new MeuComparator());
}

public static class MeuComparator implements Comparator<SuaClasse> {
    @Override
    public int compare(SuaClasse a, SuaClasse b) {
        // Implementação...
        // Retorna 1 se a é depois de b, -1 se a é antes de b ou 0 se os dois são iguais.
    }
}

Anonymous class:

public void instanciaElemento() {
    TreeSet<SuaClasse> = new TreeSet<>(new Comparator<> {
        @Override
        public int compare(SuaClasse a, SuaClasse b) {
            // Implementação...
            // Retorna 1 se a é depois de b, -1 se a é antes de b ou 0 se os dois são iguais.
        }
    });
}

With reference to method:

public void instanciaElemento() {
    TreeSet<SuaClasse> = new TreeSet<>(EstaClasse::metodoComparador);
}

private static int metodoComparador(SuaClasse a, SuaClasse b) {
    // Implementação...
    // Retorna 1 se a é depois de b, -1 se a é antes de b ou 0 se os dois são iguais.
}

Answer 2

Although TreeSet sort the elements, the decision to use this implementation should not be made just in case you need to sort the list at a time.

By instantiating a TreeSet, HashSet or LinkedHashSet you should consider the advantages of each, especially with regard to the cost to add and the cost to recover an element. Some implementations work best to add various elements, others to recover elements and all this depends on the variety of objects placed inside.

If you simply have a set and want to generate an ordered list, just use the ArrayList who receives a Collection whichever.

Thus:

Set<String> conjunto = new HashSet<>();
conjunto.add("Jose");
conjunto.add("Maria");
conjunto.add("Joao");
conjunto.add("Maria");
System.out.println(conjunto);

//conjunto -> lista
List<String> lista = new ArrayList<>(conjunto);
Collections.sort(lista);
System.out.println(lista);

Example in Ideone

Answer 3

To complete the @Victor response

The set by itself does not hold anything, since it is a Interface:

Article on set here

Imagery:

In what order is a Set stored? Random?

Yes

Example:

import java.util.HashSet;
public class HashSetExample {
   public static void main(String args[]) {
      // HashSet declaration
      HashSet<String> hset = 
               new HashSet<String>();

      // Adding elements to the HashSet
      hset.add("Apple");
      hset.add("Mango");
      hset.add("Grapes");
      hset.add("Orange");
      hset.add("Fig");
      //Addition of duplicate elements
      hset.add("Apple");
      hset.add("Mango");
      //Addition of null values
      hset.add(null);
      hset.add(null);

      //Displaying HashSet elements
      System.out.println(hset);
    }
}

Output:

[null, Mango, Grapes, Apple, Orange, Fig]

link to this example

If in case you intend to make one Set of some object created by you, you must implement Comparable:

For example this for an account class:

public class Conta implements Comparable<Conta> {

    private int numero;
    private String titular;
    // outros metodos e atributos

    @Override
    public int compareTo(Conta outraConta) {
        if (this.numero < outraConta.numero) {
            return -1;
        }
        if (this.numero > outraConta.numero) {
            return 1;
        }
        return 0;
    }
} 

You got a good article here