Lazy Map initialization

Here is one Java idiom I am happy to see go away with the tools made available in Java 8. In this scenario we have a Map with mutable values for which not all keys are known at creation time. Before Java 8, the way to add a mapping would look like this:

    private final Map< String, List< String > > map = new HashMap<>();

    public void addValue(String key, String value) {
        List valueList = map.get(key);
        if (valueList == null) {
            valueList = new ArrayList<>();
            map.put(key, valueList);
        }
        valueList.add(value);
    }

The equivalent logic with Java 8 is:

    public void addValue(String key, String value) {
        map.computeIfAbsent(key, v -> new ArrayList<>()).add(value);
    }

If the Map values are immutable, the following also works:

    private final Map< String, String > map = new HashMap<>();

    public void addValue(String key, String value) {
        map.merge(key, value, (v1, v2) -> value);
    }