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javajava-8hashmap

Put in the map if key exists and value is not null, else throw exception

void addToMapIfKeyExists(String k, String v) {
    if (!map.containsKey(k)) {
        throw new NoSuchElementException(k + " does not exist in the map");
    }

    // else ignore
    if (v != null) {
        map.put(k, v);
    }
}

Can I write this in a better way in Java 8? Can I merge it into one expression in some way?

Solution

  • You can, in a single statement:

    public void overwrite(String k, String v) {
    map.compute(k, (key, value) -> {
        if (value == null) throw new NoSuchElementException(key + " does not exist in the map");
        return v == null ? value : v;
    });
}

    The above does:

    [1] if key is not in the map, or is mapped to null, it throws NoSuchElementException.

    [2] Otherwise, will do nothing if you're trying to set null.

    [3] otherwise, will set the key to the new value

    Which is apparently what you want, but that's a bizarre combination of requirements.