Fork-Join Pool not producing any result whereas the serial codes works optimally

Question

The other serial code I wrote works perfectly, but this parallel version is not giving any output, it just prints hello, I am testing it with "hey" it never even gets to that line ie it gets stuck on invoke method, I couldn't find any helpful resources online.

Please help me in understanding where I am going wrong I am new to parallel programming.

The code is a median filter program, that when given an array x=[2,80,6,3] the filtered array is y=[2,6,6,3] calculated as:

y[1] = Median[2 2 80] = 2

y[2] = Median[2 80 6] = Median[2 6 80] = 6

y[3] = Median[80 6 3] = Median[3 6 80] = 6

y[4] = Median[6 3 3] = Median[3 3 6] = 3

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
import java.io.PrintWriter;

public class ParallelFilter extends RecursiveAction {

static final int SEQUENTIAL_THRESHOLD=500; 
ArrayList<Float> inputArray;
int sizeOfFilter;
ArrayList<Float> outputlist;
ArrayList<Float> arrayFiltered;

float high;
float low;

public ParallelFilter(ArrayList<Float> inputArray,int sizeOfFilter,float           high,float low)
                 {

  this.inputArray=inputArray;
   this.outputlist=outputlist;
  this.sizeOfFilter=sizeOfFilter;
  this.arrayFiltered=arrayFiltered;
  //this.index=index;
  this.high=high;
  this.low=low;
 }




 protected void compute()
 {
  int index=1;

  if((high - low) < SEQUENTIAL_THRESHOLD )
  {
      while(inputArray.size() > sizeOfFilter){
     for(int i=0; i<sizeOfFilter;i++){

         arrayFiltered.add(i,inputArray.get(i));

     }

         Collections.sort(arrayFiltered);
         float median = arrayFiltered.get(arrayFiltered.size()/2);
         outputlist.add(index,median);
         inputArray.remove(inputArray.get(0));
         arrayFiltered.clear();
         index=index+1;

      }

      outputlist.add(inputArray.get(inputArray.size()-1));
  }


    else{

      ParallelFilter leftTask = new       ParallelFilter(inputArray,sizeOfFilter,low,(low+high)/2);
      ParallelFilter rightTask = new ParallelFilter(inputArray,sizeOfFilter,(low+high)/2,high);
      leftTask.fork();
      rightTask.compute();
      leftTask.join();
  }

  }



   public static void main(String[] args){

  try
  {
      Scanner sc;
      sc = new Scanner(new File("inp1.txt"));
      sc.useDelimiter(" ");
      ArrayList<Float> inputlist = new ArrayList<>();

      //POPULATE LIST FROM FILE
      while (sc.hasNextLine()) {

      if(sc.nextLine().length() == 1){continue;}
      String[] parts = sc.nextLine().split(" "); // split each line by " "

      inputlist.add(Float.parseFloat(parts[1]))  ; 
      // System.out.println(parts[1]);
                           }

      System.out.println("Enter filter size : ");
      Scanner in = new Scanner(System.in);
      int sizeOfFilter = in.nextInt();
      if (sizeOfFilter < 3 || sizeOfFilter / 2 == 0) {
             System.out.println("Filter size should be odd and bigger than 3");
         }

      float low = inputlist.get(0);
      float high = inputlist.get(inputlist.size()-1);

      ParallelFilter pf = new ParallelFilter(inputlist,sizeOfFilter,low,high);
      System.out.println("hello");
      ForkJoinPool forkJoinPool = new ForkJoinPool();
      forkJoinPool.invoke(pf);//suspect the problem is here...

      for (int i=0; i<pf.outputlist.size();i++){

      System.out.println(pf.outputlist.get(i));}
      System.out.println("hey");

    }
  catch(Exception e){}

 }



  }

Answer 1

If you type "hey" when run this program you'll get parsing exception which is ignored because of empty catch block.

Also in your constructor you have this.arrayFiltered = arrayFiltered; which is actually do nothing because you have no parameter with name arrayFiltered. It lead to arrayFiltered will not be initialized which lead to NPE in arrayFiltered.add(i, inputArray.get(i));

I suggest yo to put e.printStackTrace(); in catch block it will immediately show all errors. And of course you need to learn how to use debugger.

Also this part

            if (sc.nextLine().length() == 1) {
                continue;
            }
            String[] parts = sc.nextLine().split(" ");

is wrong. because you read two line here. You need to read line once, remember result then test it and pars it.

Like this

            String line = sc.nextLine();
            if (line.length() == 1) {
                continue;
            }
            String[] parts = line.split(" ");