Java Collection How to - Parallelize search in a Java set

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Question

We would like to know how to parallelize search in a Java set.

Answer

Code revised from http://stackoverflow.com/questions/14552297/parallelize-search-in-a-java-set

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/*ww  w.  ja v a  2s . c o  m*/
public class Main {
  public static void main(String[] args) throws InterruptedException,
      ExecutionException {
    List<String> searchList = new ArrayList<String>(7);
    searchList.add("hello");
    searchList.add("world");
    searchList.add("CSS");
    searchList.add("debian");
    searchList.add("linux");
    searchList.add("HTML");
    searchList.add("stack");

    Set<String> targetSet = new HashSet<String>(searchList);
    Set<String> matchSet = findMatches(searchList, targetSet);
    for (String match : matchSet) {
      System.out.println("match:  " + match);
    }
  }
  public static Set<String> findMatches(List<String> searchList,
      Set<String> targetSet) throws InterruptedException, ExecutionException {
    Set<String> locatedMatchSet = new HashSet<String>();

    int threadCount = Runtime.getRuntime().availableProcessors();
    List<List<String>> partitionList = getChunkList(searchList, threadCount);

    if (partitionList.size() == 1) {
      // if we only have one "chunk" then don't bother with a thread-pool
      locatedMatchSet = new ListSearcher(searchList, targetSet).call();
    } else {
      ExecutorService executor = Executors.newFixedThreadPool(threadCount);
      CompletionService<Set<String>> completionService = new ExecutorCompletionService<Set<String>>(
          executor);
      for (List<String> chunkList : partitionList)
        completionService.submit(new ListSearcher(chunkList, targetSet));

      for (int x = 0; x < partitionList.size(); x++) {
        Set<String> threadMatchSet = completionService.take().get();
        locatedMatchSet.addAll(threadMatchSet);
      }
      executor.shutdown();
    }
    return locatedMatchSet;
  } 
  private static <T> List<List<T>> getChunkList(List<T> unpartitionedList,
      int splitCount) {
    int totalProblemSize = unpartitionedList.size();
    int chunkSize = (int) Math.ceil((double) totalProblemSize / splitCount);

    List<List<T>> chunkList = new ArrayList<List<T>>(splitCount);

    int offset = 0;
    int limit = 0;
    for (int x = 0; x < splitCount; x++) {
      limit = offset + chunkSize;
      if (limit > totalProblemSize)
        limit = totalProblemSize;
      List<T> subList = unpartitionedList.subList(offset, limit);
      chunkList.add(subList);
      offset = limit;
    }
    return chunkList;
  }
}
class ListSearcher implements Callable<Set<String>> {

  private final List<String> searchList;
  private final Set<String> targetSet;
  private final Set<String> matchSet = new HashSet<String>();

  public ListSearcher(List<String> searchList, Set<String> targetSet) {
    this.searchList = searchList;
    this.targetSet = targetSet;
  }

  @Override
  public Set<String> call() {
    for (String searchValue : searchList) {
      if (targetSet.contains(searchValue))
        matchSet.add(searchValue);
    }

    return matchSet;
  }

}

The code above generates the following result.

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