Java HashMap: How to get a key and value by index?

Question

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

HashMap>

Basical

Answer 1

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.

Answer 2

Here is the general solution if you really only want the first key's value

Object firstKey = myHashMap.keySet().toArray()[0];
Object valueForFirstKey = myHashMap.get(firstKey);

Answer 3

A solution is already selected. However, I post this solution for those who want to use an alternative approach:

// use LinkedHashMap if you want to read values from the hashmap in the same order as you put them into it
private ArrayList<String> getMapValueAt(LinkedHashMap<String, ArrayList<String>> hashMap, int index)
{
    Map.Entry<String, ArrayList<String>> entry = (Map.Entry<String, ArrayList<String>>) hashMap.entrySet().toArray()[index];
    return entry.getValue();
}

Answer 4

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.