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

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?

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);

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.

HashMaps are not ordered, unless you use a LinkedHashMap or SortedMap. In this case, you may want a LinkedHashMap. This 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.

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.

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();
}

Kotlin HashMap Answer

You can get key by index. Then get value by key.

val item = HashMap<String, String>() // Dummy HashMap.

val keyByIndex = item.keys.elementAt(0) // Get key by index. I selected "0".

val valueOfElement = item.getValue(keyByIndex) // Get value.

for (Object key : data.keySet()) {
    String lKey = (String) key;
    List<String> list = data.get(key);
}

I came across the same problem, read a couple of answers from different related questions and came up with my own class.

public class IndexableMap<K, V> extends HashMap<K, V> {

    private LinkedList<K> keyList = new LinkedList<>();

    @Override
    public V put(K key, V value) {
        if (!keyList.contains(key))
            keyList.add(key);
        return super.put(key, value);
    }

    @Override
    public void putAll(Map<? extends K, ? extends V> m) {
        for (Entry<? extends K, ? extends V> entry : m.entrySet()) {
            put(entry.getKey(), entry.getValue());
        }
    }

    @Override
    public void clear() {
        keyList.clear();
        super.clear();
    }

    public List<K> getKeys() {
        return keyList;
    }

    public int getKeyIndex(K key) {
        return keyList.indexOf(key);
    }

    public K getKeyAt(int index) {
        if (keyList.size() > index)
            return keyList.get(index);
        return null;
    }

    public V getValueAt(int index) {
        K key = getKeyAt(index);
        if (key != null)
            return get(key);
        return null;
    }
}

Example (types are differing from OPs question just for clarity):

Map<String, Double> myMap = new IndexableMap<>();

List<String> keys = myMap.getKeys();
int keyIndex = myMap.getKeyIndex("keyString");
String key = myMap.getKeyAt(2);
Double value myMap.getValueAt(2);

Keep in mind that it does not override any of the complex methods, so you will need to do this on your own if you want to reliably access one of these.

Edit: I made a change to the putAll() method, because the old one had a rare chance to cause HashMap and LinkedList being in different states.

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()
    }
}

If you don't care about the actual key, a concise way to iterate over all the Map's values would be to use its values() method

Map<String, List<String>> myMap;

for ( List<String> stringList : myMap.values() ) {
    for ( String myString : stringList ) {
        // process the string here
    }
}

The values() method is part of the Map interface and returns a Collection view of the values in the map.

You can use Kotlin extension function

fun LinkedHashMap<String, String>.getKeyByPosition(position: Int) =
        this.keys.toTypedArray()[position]


fun LinkedHashMap<String, String>.getValueByPosition(position: Int) =
        this.values.toTypedArray()[position]

You'll need to create multiple HashMaps like this for example

Map<String, String> fruitDetails = new HashMap();
fruitDetails.put("Mango", "Mango is a delicious fruit!");
fruitDetails.put("Guava" "Guava is a delicious fruit!");
fruitDetails.put("Pineapple", "Pineapple is a delicious fruit!");
Map<String, String> fruitDetails2 = new HashMap();
fruitDetails2.put("Orange", "Orange is a delicious fruit!");
fruitDetails2.put("Banana" "Banana is a delicious fruit!");
fruitDetails2.put("Apple", "Apple is a delicious fruit!");
// STEP 2: Create a numeric key based HashMap containing fruitDetails so we can access them by index
Map<Integer, Map<String, String>> hashMap = new HashMap();
hashMap.put(0, fruitDetails);
hashMap.put(1, fruitDetails2);
// Now we can successfully access the fruitDetails by index like this
String fruit1 = hashMap.get(0).get("Guava");
String fruit2 = hashMap.get(1).get("Apple");
System.out.println(fruit1); // outputs: Guava is a delicious fruit!
System.out.println(fruit2); // outputs: Apple is a delicious fruit!