CODEWITHSUNDEEP
javaarraylistcollections
SimpleGuy
SimpleGuy

Reputation: 2894

Add element(s) while creation into ArrayList

I have a HashMap

myMap = new HashMap<String, ArrayList<String> >();

I am writing a code where I want to add items in ArrayList for the key if it exists in the map, 

myMap.get(key).add(element);

or else create a new ArrayList, add an item to the list, and add this list to the map.

ArrayList<String> tmpArrList = new ArrayList<>();
tmpArrList.add(element);
myMap.put(key, tmpArrayList);

Writing three lines for adding single element, do not look good to me, how can I write it in single line using Java 5.. ?

Upvotes: 1

Views: 2018

Answers (3)

Nicolas Filotto
Nicolas Filotto

Reputation: 44985

As you need a solution for Java 5, the best you could do is simplify this:

ArrayList<String> tmpArrList = new ArrayList<>();
tmpArrList.add(element);
myMap.put(key, tmpArrayList);

With this:

myMap.put(key, new ArrayList<String>(Collections.singletonList(element)));

Or the code allowing to add an entry to your map could be:

if (!myMap.containsKey(key)) {
    myMap.put(key, new ArrayList<String>());
}
myMap.get(key).add(element);

You could also consider using a Multimap from Google Guava but note that it is only stating from Java 6 for the most recent versions hopefully there are existing backports for Java 5 of the versions 13.0, 14.0, 16.0 and 17.0 available from here, more details here.

Here is an example of how to use it:

ListMultimap<String, String> myMap = ArrayListMultimap.create();
// This will map the provided key with the provided value whetever the
// total amount of values already mapped with this key  
myMap.put(key, element);

Upvotes: 1

Andreas
Andreas

Reputation: 159114

How can I write it in single line.. ? (from question)
I have to write something which supports Java 1.5 and above (from comment)

You can't. The shortest way to write it is:

ArrayList<String> list = myMap.get(key);
if (list == null)
    myMap.put(key, list = new ArrayList<String>());
list.add(element);

This is optimized to only perform one map lookup, unlike an implementation using containsKey() to detect if a new ArrayList is needed.

Except:

  1. Just write it all on a single line*:

    ArrayList<String> list = myMap.get(key); if (list == null) myMap.put(key, list = new ArrayList<String>()); list.add(element);

    * Yes, that's a joke, though technically a valid answer to the question.

  2. Create a helper method, e.g. in a shared utility class:

    public static <K, V> void addToMapList(Map<K, ArrayList<V>> mapList, K key, V value) {
    ArrayList<V> list = mapList.get(key);
    if (list == null)
        mapList.put(key, list = new ArrayList<V>());
    list.add(value);
}

    Then using it is a single line:

    MyUtil.addToMapList(myMap, key, element);

  3. Use Apache Commons Collections ™* class MultiValueMap:

    MultiMap<String, String> myMap = new MultiValueMap<String, String>();

myMap.put(key, element);

    * For Java 5, use version 4.0.

  4. Use Google Guava* class ArrayListMultimap:

    ListMultimap<String, String> myMap = ArrayListMultimap.create();

myMap.put(key, element);

    * For Java 5, use version 11.0.

  5. Write your own MultiMap.

Upvotes: 1

Eran
Eran

Reputation: 393856

If you are using Java 8, computeIfAbsent is what you need :

myMap.computeIfAbsent(key, ArrayList::new).add(element);

This will generate a new ArrayList and put it in the Map if the requested key is not already present in the Map.

Upvotes: 2

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