Question: I get this error when I try to compile my program. It has to do with the way I'm constructing the map, but I really can't see something wrong:

```
error: expected identifier before numeric constant 
map <int , vector <int> >  radixM (10 , vector <int> (100));

```

Any ideas?

Answer: 
When the access pattern is lots of reads and occasional writes, you can protect an unsyncronized data structure with a `ReentrantReadWriteLock`. It allows multiple readers, but only a single writer.

Depending on your implementation, you might also benefit from using a `ConcurrentHashMap`.

You *might* be able to cheat a bit and use either an `AtomicIntegerArray` or an `AtomicReferenceArray` of `Futures/CompletionStages`.
Instead of having the same shared memory, you can have multiple threads with individual memories in a stack. Eventually, add all these up together (or occasionally) with one thread!