Question

I created a class Foo that has the method toArray() that returns an Array<Int>.

Now, I have a HashMap mapping Strings to HashMaps, which map Objects to Foo. That is:

HashMap<String,HashMap<Object,Foo>>

And I want to create a new object of type:

HashMap<String,HashMap<Object,Array<Int>>>

That is obtained by calling the function toArray() for every element Foo in the original HashMAp.

To do so I normally would do something like:

    public static HashMap<String,HashMap<Object,Array<Int>>> changeMap(Map mpOld) {
        Object key2;
        String key1;
        Iterator it2;
        HashMap<String,HashMap<Object,Array<Int>>> mpNew= 
            new HashMap<String,HashMap<Object,Array<Int>>>()
        Iterator it1 = mpOld.keySet().iterator();
        while (it1.hasNext()) {
            key1=it1.next();
            it2= mpOld.get(key1).keySet().iterator();
            mpNew.put(key1,new HashMap<Object,Array<Int>>())
            while (it2.hasNext()) {
                key2=it2.next();
                mpNew.get(key1).put(key2,mpOld.get(key1).get(key2).toArray());
                //TODO clear entry mpOld.get(key1).get(key2)
            }
            //TODO clear entry mpOld.get(key1)
        }
        return mpNew;
    }

A similar code works just fine, but the Size of the HashMap is too big to hold two of them in memory. As you can see I added two points where I want to clear some entries. The problem is, if I do, I get either a concurrency error, or the iterator loop just terminates.

I wonder if there is a better way to iterate through the Maps and copy the information.

Also, I’m working in a Scala project but here I have to use Java types for some compatibility issues. Although Java.util.HashMap is not an iterator, maybe Scala has some hidden functinality to deal with this?

Thanks,

Answer 1

Iterators offer remove(..) methods that safely removes the previously accessed item. Iterate over the Key/Value entries of the map, converting them and adding them to the new map, and removing the old ones as you go.

/**
 * Transfers and converts all entries from <code>map1</code> to 
 * <code>map2</code>.  Specifically, the {@link Foo} objects of the 
 * inner maps will be converted to integer arrays via {@link Foo#toArray}.
 * 
 * @param map1 Map to be emptied.
 * @param map2 Receptacle for the converted entries.
 */
private static void transfer(Map<String, Map<Object, Foo>> map1
        , Map<String, Map<Object, int[]>> map2) {

    final Iterator<Entry<String, Map<Object, Foo>>> mapIt
        = map1.entrySet().iterator();
    while (mapIt.hasNext()) {
        final Entry<String, Map<Object, Foo>> mapEntry = mapIt.next();
        mapIt.remove();
        final Map<Object, int[]> submap = new HashMap<Object,int[]>();
        map2.put(mapEntry.getKey(), submap);
        final Iterator<Entry<Object,Foo>> fooIt 
            = mapEntry.getValue().entrySet().iterator();
        while (fooIt.hasNext()) {
            final Entry<Object,Foo> fooEntry = fooIt.next();
            fooIt.remove();
            submap.put(fooEntry.getKey(), fooEntry.getValue().toArray());
        }
    }
}