Why does toString function of a HashMap prints itself with a different order?

Question

I have this very simple piece of code, and I was just trying to play a bit with different kind of objects inside a Map.

//There's a bit of spanish, sorry about that
//just think 'persona1' as an object with
//a string and an int
Map mapa = new HashMap();
mapa.put('c', 12850);
mapa.put(38.6, 386540);
mapa.put("Andrés", 238761);
mapa.put(14, "Valor de 14");
mapa.put("p1", persona1);
mapa.put("Andrea", 34500);

System.out.println(mapa.toString());

And then I expect from console something like:

{c=12850, 38.6=386540, Andrés=238761, 14=Valor de 14, p1={nombre: Andres Perea, edad: 10}, Andrea=34500}

But susprisingly for me I got same data in different order:

{38.6=386540, Andrés=238761, c=12850, p1={nombre: Andres Perea, edad: 10}, Andrea=34500, 14=Valor de 14}

It doesn't matter if I try other kind of objects, even just Strings or numeric types, it always does the same, it makes a different without-apparently-any-sense order.

Can someone give me a hint why this happens? Or may be something too obvious I'm missing?

I'm using Java 1.7 and Eclipse Juno.

Answer 1

HashMap not guaranteed the order of element. If you want to keep order use LinkedHashMap.

See following case

    Map<Integer,String> unOrderedMap=new HashMap<>();
    unOrderedMap.put(1,"a");
    unOrderedMap.put(3,"a");
    unOrderedMap.put(2,"a");
    System.out.println("HashMap output: "+unOrderedMap.toString());

    Map<Integer,String> orderedMap=new LinkedHashMap<>();
    orderedMap.put(1,"a");
    orderedMap.put(3,"a");
    orderedMap.put(2,"a");
    System.out.println("LinkedHashMap output: "+orderedMap.toString());

Output:

   HashMap output: {1=a, 2=a, 3=a}
   LinkedHashMap output: {1=a, 3=a, 2=a}