private class MatrixCalculation extends RecursiveTask<Integer> {

  private int[][] matrix = null;

    public MatrixCalculation(int[][] matrix) {
      this.matrix = matrix;
    }

    @Override
    protected Integer compute() {
      if(matrix.length == 1) {
        return matrix[0][0];
      } else {
        //Particionate original matrix in four identical smaller matrix.
        MatrixCalculation matrixCalculation1 = new                       MatrixCalculation(splitMatrixForGivenQuadrant(matrix, 0,0));
        matrixCalculation1.fork();
        MatrixCalculation matrixCalculation2 = new MatrixCalculation(splitMatrixForGivenQuadrant(matrix, 1,0));
        matrixCalculation2.fork();
        MatrixCalculation matrixCalculation3 = new MatrixCalculation(splitMatrixForGivenQuadrant(matrix, 0,1));
        matrixCalculation3.fork();
        MatrixCalculation matrixCalculation4 = new MatrixCalculation(splitMatrixForGivenQuadrant(matrix, 1,1));

        return matrixCalculation4.compute() + matrixCalculation1.join() + matrixCalculation2.join() + matrixCalculation3.join() ;

        }
      }

      private int[][] splitMatrixForGivenQuadrant(int[][] matrix, int quadrantX, int quadrantY) {

        int[][] firstMatrix = new int[matrix.length/2][matrix.length/2];

        for(int i=0;i<firstMatrix.length;i++) {
          for(int j=0;j<firstMatrix[i].length;j++) {
            firstMatrix[i][j] = matrix[i+quadrantX*matrix.length/2][j+quadrantY*matrix.length/2];
          }
        }

        return firstMatrix;

      }

    }

 private static final int NUMBER_OF_PROCESSORS = 2;

 public static void main(String[] args) {
   ForkJoinTaskExample forkJoinTaskExample = new ForkJoinTaskExample();

   ForkJoinPool forkJoinPool = new ForkJoinPool(NUMBER_OF_PROCESSORS);

   int [][] matrix = new int[][]{{2,1},{3,4}};
   MatrixCalculation matrixCalculation = forkJoinTaskExample.new MatrixCalculation(matrix);

   forkJoinPool.invoke(matrixCalculation);
   System.out.println(matrixCalculation.getRawResult());
}