A straight translation from python to java; taken from  http://www.fundza.com/vectors/point2line/index.html

public double distance2(Point2D pnt)
  {
    double lineVecDx=to.x-from.x;
    double lineVecDy=to.y-from.y;
    double pntVecDx=pnt.getX()-from.x;
    double pntVecDy=pnt.getY()-from.y;
    double lineLen=dist(lineVecDx,lineVecDy);
    double pntVecLength=dist(lineVecDx,lineVecDy);
    double lineUnitvecDx=lineVecDx/pntVecLength;
    double lineUnitvecDy=lineVecDy/pntVecLength;
    double pntVecScaledDx=pntVecDx/lineLen;
    double pntVecScaledDy=pntVecDy/lineLen;
    double tx = lineUnitvecDx * pntVecScaledDx;
    double ty = lineUnitvecDy * pntVecScaledDy;
    double t = tx+ty;
    if (t<0) t=0;
    else if (t>1) t=1;
    double nearestX=lineVecDx*t;
    double nearestY=lineVecDy*t;
    return dist(nearestX-pntVecDx, nearestY-pntVecDy);
  }

double dist(double dx, double dy)
  {
    return Math.sqrt(dx*dx+dy*dy);
  }