CODEWITHSUNDEEP
javafunctioncollections
samba
samba

Reputation: 3111

How to implement computeIfAbsent in order to return Object value?

I have written a method that takes a name of some field and if it's an essential field, the value is calculated for the field and is put into the map for the key.

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

private void saveValues(String name) {
    if (dataStore.isEssentialField(name)) {
        valuesMap.put(name, calculateValue(name));
    }
}

Now I want to achieve the following: if there is already a value associated with the name key or the key is not an "essential field", the method must return the existing value and not calculate anything.

I suppose it may be achieved with computeIfAbsent method of a Map interface but not sure how to implement this correctly. How should the Function be implemented in this case?

private Object saveValues(String name) {
    if (dataStore.isEssentialField(name)) {
        Object newValue = valuesMap.computeIfAbsent(name, -> (Function<? super String, ?>) calculateValue(name));
    }
    return ... ? // How can the existing value be returned here?
}

Upvotes: 0

Views: 1514

Answers (1)

Neuron
Neuron

Reputation: 5841

You can use a Lambda function as follows:

private Object saveValues(String name) {
    if (dataStore.isEssentialField(name)) {
        return valuesMap.computeIfAbsent(name, this::calculateValue);
    }
    return valuesMap.get(name);
}

The more classical approach would be:

private Object saveValues(String name) {
    Object storedValue = valuesMap.get(name);
    if (dataStore.isEssentialField(name)) {
        if (storedValue != null) {
            return storedValue;
        }
        storedValue = calculateValue(name);
        valuesMap.put(name, storedValue);
    }
    return storedValue;
}

Upvotes: 4

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