How to find correlation between two integer arrays in java

There is nothing in core Java. There are libraries out there you can use. Apache Commons has a statistical project, check PearsonCorrelation class.

Sample code:

public static void main(String[] args) {    double[] x = {1, 2, 4, 8};    double[] y = {2, 4, 8, 16};    double corr = new PearsonsCorrelation().correlation(y, x);    System.out.println(corr);}

prints out 1.0

Correlation is quite easy to compute manually:

http://en.wikipedia.org/wiki/Correlation_and_dependence

  public static double Correlation(int[] xs, int[] ys) {    //TODO: check here that arrays are not null, of the same length etc    double sx = 0.0;    double sy = 0.0;    double sxx = 0.0;    double syy = 0.0;    double sxy = 0.0;    int n = xs.length;    for(int i = 0; i < n; ++i) {      double x = xs[i];      double y = ys[i];      sx += x;      sy += y;      sxx += x * x;      syy += y * y;      sxy += x * y;    }    // covariation    double cov = sxy / n - sx * sy / n / n;    // standard error of x    double sigmax = Math.sqrt(sxx / n -  sx * sx / n / n);    // standard error of y    double sigmay = Math.sqrt(syy / n -  sy * sy / n / n);    // correlation is just a normalized covariation    return cov / sigmax / sigmay;  }