multithreading - Java ExecutorService to solve Recursive Fibonacci Series -

i need find out number based on index in fibonacci series recursively using threads , tried following code, program never ends. please let me know if missing something.

code:

  import java.math.biginteger;   import java.util.concurrent.*;    public class multithreadedfib {      private executorservice executorservice;      public multithreadedfib(final int numberofthreads) {       executorservice = executors.newfixedthreadpool(numberofthreads);     }      public biginteger getfibnumberatindex(final int index)        throws interruptedexception, executionexception {        future<biginteger> indexminusone = executorservice.submit(         new callable<biginteger>() {           public biginteger call()            throws interruptedexception, executionexception {             return getnumber(index - 1);           }       });        future<biginteger> indexminustwo = executorservice.submit(         new callable<biginteger>() {           public biginteger call()            throws interruptedexception, executionexception {             return getnumber(index - 2);           }       });        return indexminusone.get().add(indexminustwo.get());     }      public biginteger getnumber(final int index)      throws interruptedexception, executionexception {       if (index == 0 || index == 1)         return biginteger.valueof(index);        return getfibnumberatindex(index - 1).add(getfibnumberatindex(index - 2));     }   } 

fixed it (thanks fiver)

instead of calling getnumber(int) call method, calling dynamic programming algorithm computes number @ index.

the code is:

public class dynamicfib implements ifib {  private map<integer, biginteger> memoize = new hashmap<integer, biginteger>();  public dynamicfib() {   memoize.put(0, biginteger.zero);   memoize.put(1, biginteger.one); }  public biginteger getfibnumberatindex(final int index) {    if (!memoize.containskey(index))     memoize.put(index, getfibnumberatindex(index - 1).add(getfibnumberatindex(index - 2)));    return memoize.get(index);   } } 

this recursion overflow stack fast. because computing lower fibonacci numbers on , on again - exponentially many number of times.

one effective way avoid use memoized recursion (a dynamic programming approach)

basically use static array hold computed fibonacci numbers , whenever need one, take array, if it's computed. if not, compute , store in array. way each number computed once.

(you can use other data structure instead of array, of course, i.e. hashtable)