Redesigning method to use Java 8 Map.computeIfAbsent() with thrown exception

Question

I am trying to rework some of my methods to make them more concise using Java 8, whose new features I am trying to slowly absorb.

It is a method with the purpose of adding a value to a Map<Key, Set<Value>>. There are three posibilities:

1. The key doesn't exist: it is added and a new set containing the value is associated to it.
2. The key exists: the value is added to the existing set. Note that a set will never be null, because I have some preconditions to deal with that.
3. The key exists and the value is already contained in the set: an IllegalArgumentException is thrown.

The code implementing this behaviour is the following, and it does not use Java 8 features:

public void addValue(Key key, Value value) {
    // irrelevant preconditions...

    Set<Value> valuesForKey = myMap.get(key);
    if (valuesForKey != null && valuesForKey.contains(value))
        throw new IllegalArgumentException("Association exists already");

    if (valuesForKey == null)
        myMap.put(key, new HashSet<Value>(Arrays.asList(value)));
    else
        valuesForKey.add(value);
}

I would like to shorten this code using Java 8 methods like computeIfAbsent.

I could sum up the last if-else block, but I can't get past the redundancy of the value set to which key maps to having already being retrieved when performing the precondition checks.

public void addValue(Key key, Value value) {
    // irrelevant preconditions...

    Set<Value> valuesForKey = myMap.get(key);
    if (valuesForKey != null && valuesForKey.contains(value))
        throw new IllegalArgumentException("Association exists already");

    myMap.computeIfAbsent(key, v -> new HashSet<Value>()).add(value);
}

Is there anyway I could merge all that in one instruction?

Answer 1

You can take advantage of the fact that the add(element) method will return true if the set didn't contain the specified element. If this call returns false, it means the element wasn't added because it was already present. Therefore, you can use:

public void addValue(Key key, Value value) {
    boolean added = myMap.computeIfAbsent(key, k -> new HashSet<>()).add(value);
    if (!added) {
        throw new IllegalArgumentException("Association exists already");
    }
}