Java HashMap: How to get the first key's value?

Question

Hello, I am trying to use a HashMap to map a unique string to a string ArrayList like this:

HashMap<String, ArrayList<String>>

Basically, I want to be able to access the keys by number, not by using the key's name. And I want to be able to access said key's value, to iterate over it. I'm imagining something like this:

for(all keys in my hashmap) {
    for(int i=0; i < myhashmap.currentKey.getValue.size(); i++) {
        // do things with the hashmaps elements
    }
}

Is there an easy way to do this?

Thanks, Derek

Answer 1

HashMap's are not ordered, unless you use a LinkedHashMap or SortedMap. In this case, it sounds like you want a LinkedHashMap which will iterate in order of insertion (or in order of last access if you prefer). In this case, it would be

int index = 0;
for ( Map.Entry<String,ArrayList<String>> e : myHashMap.iterator().entrySet() ) {
    String key = e.getKey();
    ArrayList<String> val = e.getValue();
    index++;
}

There is no direct get(index) in a map because it is an unordered list of key/value pairs. LinkedHashMap is a special case that keeps the order.

Answer 2

You can do:

for(String key: hashMap.keySet()){
    for(String value: hashMap.get(key)) {
        // use the value here
    }
}

This will iterate over every key, and then every value of the list associated with each key.

Answer 3

HashMaps don't keep your key/value pairs in a specific order. They are ordered based on the hash that each key's returns from its Object.hashCode() method. You can however iterate over the set of key/value pairs using an iterator with:

for (String key : hashmap.keySet()) 
{
    for (list : hashmap.get(key))
    {
        //list.toString()
    }
}

Answer 4

You can iterate over keys by calling map.keySet(), or iterate over the entries by calling map.entrySet(). Iterating over entries will probably be faster.

for (Map.Entry<String, List<String>> entry : map.entrySet()) {
    List<String> list = entry.getValue();
    // Do things with the list
}

If you want to ensure that you iterate over the keys in the same order you inserted them then use a LinkedHashMap.

By the way, I'd recommend changing the declared type of the map to <String, List<String>>. Always best to declare types in terms of the interface rather than the implementation.