Chapter 30 Exercise 17, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

30.17 (Parallel matrix multiplication) Programming Exercise 7.6 describes how to perform matrix multiplication. Suppose you have multiple processors, so you
can speed up the matrix multiplication. Implement the following method in
parallel.
public static double[][] parallelMultiplyMatrix(double[][] a, double[][] b)

Write a test program that measures the execution time for multiplying two
2,000 * 2,000 matrices using the parallel method and sequential method,
respectively.

import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;


public class Exercise17 {

 public static void main(String[] args) {
  int size = 2000;
  double[][] a = new double[size][size];
  double[][] b = new double[size][size];
  for (int i = 0; i < a.length; i++) {
   for (int j = 0; j < b[i].length; j++) {
    a[i][j] = Math.random();
    b[i][j] = Math.random();
   }
  }
  long time = System.currentTimeMillis();
  parallelMultiplyMatrix(a, b);
  System.out.println((System.currentTimeMillis() - time) + " msec - parallelMultiplyMatrix()");
  time = System.currentTimeMillis();
  multiplyMatrix(a, b);
  System.out.println((System.currentTimeMillis() - time) + " msec - addMatrix()");
 }

 public static double[][] parallelMultiplyMatrix(double[][] a, double[][] b) {
  double[][] c = new double[a.length][a.length];

  RecursiveAction mainTask = new MultiplyMatrix(a, b, c, 0, a.length, 0, a.length);
  ForkJoinPool pool = new ForkJoinPool();
  pool.invoke(mainTask);
  return c;
 }
 
 private static class MultiplyMatrix extends RecursiveAction {
  private static final long serialVersionUID = 1L;
  private final static int THRESHOLD = 100;
  private double[][] a;
  private double[][] b;
  private double[][] c;
  private int x1;
  private int x2;
  private int y1;  
  private int y2;

  public MultiplyMatrix(double[][] a, double[][] b, double[][] c, int x1, int x2, int y1, int y2) {
   this.a = a;
   this.b = b;
   this.c = c;
   this.x1 = x1;
   this.x2 = x2;
   this.y1 = y1;   
   this.y2 = y2;   
  }
  
  @Override
  protected void compute() {
   if (((x2 - x1) < THRESHOLD) || ((y2 - y1) < THRESHOLD)) {
    for (int i = x1; i < x2; i++) {
     for (int j = y1; j < y2; j++) {
      c[i][j] = a[i][0] * b[0][j];
      for (int k = 1; k < a.length; k++) {
       c[i][j] += a[i][k] * b[k][j]; 
      } 
     }
    }
   } else {
    int midX = (x1 + x2) / 2;
    int midY = (y1 + y2) / 2;
    
    invokeAll(
      new MultiplyMatrix(a, b, c, x1, midX, y1, midY),
      new MultiplyMatrix(a, b, c, midX, x2, y1, midY),
      new MultiplyMatrix(a, b, c, x1, midX, midY, y2),
      new MultiplyMatrix(a, b, c, midX, x2, midY, y2));
   }
  }
 }
 
 public static double[][] multiplyMatrix(double[][] a, double[][] b) {
  double[][] result = new double[a.length][a[0].length];
  for (int i = 0; i < a.length; i++) {
   for (int j = 0; j < a[i].length; j++) {
    result[i][j] = a[i][0] * b[0][j];
    for (int k = 1; k < a.length; k++) {
     result[i][j] += a[i][k] * b[k][j]; 
    }    
   }
  }
  return result;
 }
}