https://lingpipe-blog.com/2009/07/07/welford-s-algorithm-delete-online-mean-variance-deviation/

streaming_std

private long mN = 0L;
private double mM = 0.0;
private double mS = 0.0;
public void handle(double x) {
     ++mN;
    double nextM = mM + (x – mM) / mN;
    mS += (x – mM) * (x – nextM);
    mM = nextM;
}
public void unHandle(double x) {
    if (mN == 0) {
        throw new IllegalStateException();
    } else if (mN == 1) { 
        mN = 0; mM = 0.0; mS = 0.0;
    } else {
        double mMOld = (mN * mM - x)/(mN-1);
        mS -= (x -  mM) * (x - mMOld);
        mM = mMOld;
        --mN;
    }
}
public double mean() {
    return mM;
}
public double varianceUnbiased() {
    return mN > 1 ? mS/(mN-1) : 0.0;
}