one_array_two_threads.java

import java.util.Arrays;
import java.util.logging.Level;
import java.util.logging.Logger;

class Worker implements Runnable {
    private final float[] data;
    private final int startPos;
    
    Worker(float[] array, int start) {
        startPos = start;
        data = array;
    }

    @Override
    public void run() {
        for (int i=0; i<data.length; ++i) {
            int j = startPos + i;
            data[i] = (float) (data[i] * Math.sin(0.2 + j/5.0)* Math.cos(0.2 + j/5.0) * Math.cos(0.4 + j/2.0));
        }
    }    
}

public class Main {
    static final int SIZE = 100_000_000;

    public static void main(String[] args) {
        for (int i=0; i<100; ++i) {
            float[] arr1 = singleThread();
            float[] arr2 = doubleThread();
            System.out.println(Arrays.equals(arr1, arr2));
        }
    }

    static float[] singleThread() {
        long t0 = System.currentTimeMillis();
        float[] arr = new float[SIZE];
        for (int i=0; i<arr.length; ++i)
            arr[i] = 1.0f;
        
        for (int i=0; i<arr.length; ++i)
            arr[i] = (float) (arr[i] * Math.sin(0.2 + i/5.0)* Math.cos(0.2 + i/5.0) * Math.cos(0.4 + i/2.0));
        long t1 = System.currentTimeMillis();
        System.out.println("singleThread dt = " + (t1-t0));
        return arr;
    }
    
    static float[] doubleThread() {
        long t0 = System.currentTimeMillis();
        float[] arr = new float[SIZE];
        
        for (int i=0; i<arr.length; ++i)
            arr[i] = 1.0f;
        final int HSIZE = SIZE/2;
        float[] arr1 = new float[HSIZE];
        float[] arr2 = new float[HSIZE];
        System.arraycopy(arr, 0, arr1, 0, HSIZE);
        System.arraycopy(arr, HSIZE, arr2, 0, HSIZE);
        Thread th1 = new Thread(new Worker(arr1, 0));
        Thread th2 = new Thread(new Worker(arr2, HSIZE));
        th1.start();
        th2.start();
        try {
            th1.join();
            th2.join();
            System.arraycopy(arr1, 0, arr, 0, HSIZE);
            System.arraycopy(arr2, 0, arr, HSIZE, HSIZE);
            long t1 = System.currentTimeMillis();
            System.out.println("doubleThread dt = " + (t1-t0));                    
        } catch (InterruptedException ex) {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
        }
        return arr;
    }
}