I suggest you use the builtin method Double.compare(). If you need a range for double values to be equal you can use chcek for that first.
return Double.compare(p1.getY(), p2.gety());
or
if(Math.abs(p1.getY()-p2.getY()) < ERR) return 0;
return Double.compare(p1.getY(), p2.gety());
The problem with using < and > is that NaN will return false in both cases resulting in a possibly inconsistent handling. e.g. NaN is defined as not being equal to anything, even itself however in @suihock’s and @Martinho’s solutions, if either value is NaN the method will return 0 everytime, implying that NaN is equal to everything.