Add element(s) while creation into ArrayList

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.

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.

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);