Cannot iterate 2 hashmaps in a hashmap

Question

I cannot iterate this structure (Java):

HashMap<HashMap<Integer, Integer>, HashMap<Integer, Integer>> last
    = new HashMap<HashMap<Integer, Integer>, HashMap<Integer, Integer>>();

These are the 2 HashMaps that I am adding into the first:

HashMap<Integer, Integer> arrival = new HashMap<Integer, Integer>();
HashMap<Integer, Integer> departure = new HashMap<Integer, Integer>();

Tried various loops, read a couple of articles but I cannot connect the dots to make it correct until now.

I want for example to be able to get/print the four integer values from the two HashMaps in the HashMap together in the loop.

Thank you!

Answer 1

First, I don't think it is a good idea to have a HashMap as a key in another HashMap. It looks like you are representing a map of key value pairs (arrival/destination?), you would be better off creating an class just for that and adding that class to a HashMap. Using your current structure will cause more confusion than benefits.

That said, it is possible to loop through this.

One way to loop through a HashMap is to loop trhough its entry set. So if h is a HashMap<Integer, Integer>, you loop as:

for(Map.Entry<Integer, Integer> entry : h.entrySet)
{
...
}

So, for a HashMap<HashMap<Integer, Integer>, HashMap<Integer, Integer>> called last

for(Map.Entry<HashMap<Integer, Integer>, HashMap<Integer, Integer>> entry : last.entrySet())
{
    HashMap<Integer, Integer> key = entry.getKey();
    for(Map.Entry<Integer, Integer> keyMapEntry : key.entrySet())
    {
        //Loop through the value HashMap. 
        //keyMapEntry.getKey() gives the key of the Hashmap that is the key in the 
        //original Hashmap, keyMapEntry.getValue() gives the value
    }

    HashMap<Integer, Integer> value = entry.getValue();
    for(Map.Entry<Integer, Integer> valueMapEntry : value.entrySet())
    {
        //Loop through the key HashMap
        //valueMapEntry.getKey() gives the key of the Hashmap that is the value in the 
        //original Hashmap, valueMapEntry.getValue() gives the value
    }
}