is the Java HashMap keySet() iteration order consistent?

Question

I understand that the Set returned from a Map's keySet() method does not guarantee any particular order.

My question is, does it guarantee the same order over multiple iterations. For example

Map<K,V> map = getMap();

for( K k : map.keySet() )
{
}

...

for( K k : map.keySet() )
{
}

In the above code, assuming that the map is not modified, will the iteration over the keySets be in the same order. Using Sun's jdk15 it does iterate in the same order, but before I depend on this behavior, I'd like to know if all JDKs will do the same.

EDIT

I see from the answers that I cannot depend on it. Too bad. I was hoping to get away with not having to build some new Collection to guarantee my ordering. My code needed to iterate through, do some logic, and then iterate through again with the same ordering. I'll just create a new ArrayList from the keySet which will guarantee order.

Answer 1

The API for Map does not guarantee any ordering whatsoever, even between multiple invocations of the method on the same object.

In practice I would be very surprised if the iteration order changed for multiple subsequent invocations (assuming the map itself did not change in between) - but you should not (and according to the API cannot) rely on this.

EDIT - if you want to rely on the iteration order being consistent, then you want a SortedMap which provides exactly these guarantees.

Answer 2

Logically, if the contract says "no particular order is guaranteed", and since "the order it came out one time" is a particular order, then the answer is no, you can't depend on it coming out the same way twice.

Answer 3

Hashmap does not guarantee that the order of the map will remain constant over time.

Answer 4

It doesn't have to be. A map's keySet function returns a Set and the set's iterator method says this in its documentation:

"Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee)."

So, unless you are using one of those classes with a guarantee, there is none.

Answer 5

I think that if it is not stated to be guaranteed in the API documentation, then you shouldn't depend on it. The behavior might even change from one release of the JDK to the next, even from the same vendor's JDK.

You could easily get the set and then just sort it yourself, right?

Answer 6

Map is an interface and it does not define in the documentation that order should be the same. That means that you can't rely on the order. But if you control Map implementation returned by the getMap(), then you can use LinkedHashMap or TreeMap and get the same order of keys/values all the time you iterate through them.

Answer 7

Map is only an interface (rather than a class), which means that the underlying class that implements it (and there are many) could behave differently, and the contract for keySet() in the API does not indicate that consistent iteration is required.

If you are looking at a specific class that implements Map (HashMap, LinkedHashMap, TreeMap, etc) then you could see how it implements the keySet() function to determine what the behaviour would be by checking out the source, you'd have to really take a close look at the algorithm to see if the property you are looking for is preserved (that is, consistent iteration order when the map has not had any insertions/removals between iterations). The source for HashMap, for example, is here (open JDK 6): http://www.docjar.com/html/api/java/util/HashMap.java.html

It could vary widely from one JDK to the next, so i definitely wouldn't rely on it.

That being said, if consistent iteration order is something you really need, you might want to try a LinkedHashMap.

Answer 8

Just for fun, I decided to write some code that you can use to guarantee a random order each time. This is useful so that you can catch cases where you are depending on the order but you should not be. If you want to depend on the order, than as others have said, you should use a SortedMap. If you just use a Map and happen to rely on the order then using the following RandomIterator will catch that. I'd only use it in testing code since it makes use of more memory then not doing it would.

You could also wrap the Map (or the Set) to have them return the RandomeIterator which would then let you use the for-each loop.

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

public class Main
{
    private Main()
    {
    }

    public static void main(final String[] args)
    {
        final Map<String, String> items;

        items = new HashMap<String, String>();
        items.put("A", "1");
        items.put("B", "2");
        items.put("C", "3");
        items.put("D", "4");
        items.put("E", "5");
        items.put("F", "6");
        items.put("G", "7");

        display(items.keySet().iterator());
        System.out.println("---");

        display(items.keySet().iterator());
        System.out.println("---");

        display(new RandomIterator<String>(items.keySet().iterator()));
        System.out.println("---");

        display(new RandomIterator<String>(items.keySet().iterator()));
        System.out.println("---");
    }

    private static <T> void display(final Iterator<T> iterator)
    {
        while(iterator.hasNext())
        {
            final T item;

            item = iterator.next();
            System.out.println(item);
        }
    }
}

class RandomIterator<T>
    implements Iterator<T>
{
    private final Iterator<T> iterator;

    public RandomIterator(final Iterator<T> i)
    {
        final List<T> items;

        items = new ArrayList<T>();

        while(i.hasNext())
        {
            final T item;

            item = i.next();
            items.add(item);
        }

        Collections.shuffle(items);
        iterator = items.iterator();
    }

    public boolean hasNext()
    {
        return (iterator.hasNext());
    }

    public T next()
    {
        return (iterator.next());
    }

    public void remove()
    {
        iterator.remove();
    }
}