A multithreaded java program where 4 threads are used to execute parallel search on large number of integers read from a text file and reduce the overall search time.

ParallelSearch.java

import java.util.*;
import java.io.IOException;
import java.nio.file.*;
import java.text.DecimalFormat;
import java.text.NumberFormat;

public class ParallelSearch implements Runnable {

    private int startIndex, endIndex;
    private static int nElements, key;
    public static List<Integer> array = new ArrayList<>();
    private boolean flag; // default value is false
    private String threadName;
    public static Scanner sc = new Scanner(System.in);

    public ParallelSearch(int startIndex, int endIndex, String threadName) {
        this.startIndex = startIndex;
        this.endIndex = endIndex;
        this.threadName = threadName;
        Thread thread = new Thread(this);
        thread.start();
    }

    @Override
    public void run() {
        for (int i = startIndex; i <= endIndex; i++) {
            if (key == array.get(i)) {
                // System.out.println("\nSearch is successful by " + threadName + " and " + key + " found at position " + i);
                System.out.print("\nSearch is successful by " + threadName);
                flag = true;
                return;
            }
        }
        if (flag == false) {
            System.out.print("\nSearch is unsuccessful by " + threadName);
        }
    }

    public static void main(String[] args) throws IOException {

        long start = System.currentTimeMillis();

        Path filePath = Paths.get("file.txt");
        Scanner scanner = new Scanner(filePath);
        // List<Integer> array = new ArrayList<>();
        while (scanner.hasNext()) {
            if (scanner.hasNextInt()) {
                array.add(scanner.nextInt());
            } else {
                scanner.next();
            }
        }
        nElements = array.size();

        System.out.print("\nEnter the value to be searched : ");
        key = sc.nextInt();

        // creating anonymous object
        new ParallelSearch(0, (nElements / 4) - 1, "Thread 1");
        new ParallelSearch(nElements / 4, (nElements / 2) - 1, "Thread 2");
        new ParallelSearch(nElements / 2, ((3 * nElements) / 4 - 1), "Thread 3");
        new ParallelSearch((3 * nElements) / 4, nElements - 1, "Thread 4");

        // worst case scenario - last thread tak baat chali jayegi

        scanner.close();

        long end = System.currentTimeMillis();
        NumberFormat formatter = new DecimalFormat("#0.00000");
        System.out.println("\n\nExecution time is " + formatter.format((end - start) / 1000d) + " seconds");
    }
}