Equivalent to computeifabsent in Java 7

Is there any way to run a piece of code only when there is no key in the concurrenthashmap and save the results of the code to the collection?

I can't use Java 8 features because I'm developing for Android

In addition, I want to avoid running long operations. If I don't need them, I don't want to break the atomic operations of the collection to do so

Solution

There is no exact equivalent, but the usual method is as follows:

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;
}

This is functionally equivalent to the computeifabsent call in Java 8. The only difference is that sometimes you construct a value object that will never enter the map - because another thread puts it first It will never result in the return of an incorrect object or something like that - in any case, the function always returns the correct value, but this may not be acceptable if the construction of value has a side effect *

Extra instances are usually not a performance issue because the initial get () check eliminates most calls to putifabsent Usually, this method is much faster than computeifabsent, because when the object already exists, the call will lock the object unnecessarily If some objects are seriously contested, I can measure them locally five times faster

If you really need the computing behavior integrated into the map (keep the internal lock so that a thread can create a new object), you can use guava's cachebuilder to get the loadingcache It is basically the same as CHM in Java 8, but there are a lot of additional configuration options