ComputeIfAbsent equivalent in Java 7

Question

Is there a way to run a piece of code only if the key does not exist in ConcurrentHashMapand save the result of the code in the collection?

I cannot use Java 8 features because I am developing for Android.

Also, I would like to avoid doing long operations if I don’t need it, and I don’t want to destroy the atomic operation of the collection to do this.

Answer 1

There is no exact equivalent, but the usual approach looks something like this:

ConcurrentMap<Key,Value> map = ...

Value computeIfAbsent(Key k) {
  Value v = map.get(k);
  if (v == null) {
    Value vNew = new Value(...); // or whatever else you do to compute the value
    v = (v = map.putIfAbsent(k, vNew)) == null ? vNew : v;
  }
  return v;
}

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Answer 2

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