If it is under around 25 degrees or so our pure javas get heat. They have indoor/outdoor pens. If it is windy, snowing, or icy we will not let them outdoors in temps that are under 30. They will lose toes or worse.
If it is a windstill day with lots of sunshine we will let them outdoors at 30. They have access to indoors and will usually come inside after sunbathing and sit on their feet on the perch which is heated. After the sun drops they are generally perched.
We are at 17 degrees right now with wind chills of around 0 and they currently have the heat.
I have a pair of Java that have done fine in short Texas freezes. We'll have several nights below freezing but it does warm up during the day (it's 28 now). Last year we had a 10 degree night followed by a few in the teens with the freeze lingering through a day or 2. They did ok. My pens have a shed 20' wide and 10' deep with only the wall open into the rest of the pen overall 30' deep. The roof covers 20' of the 30' depth which keeps rain from reaching two 10' roosts in each pen so they stay dry from rain and other moisture falling like fog/dew. I'm now in the process of purchasing a Java peahen with friend buying the other 9 available. I don't have room for them. They have been housed in a huge pen in a field where winter winds can be strong. There is a small sheltered area but only walled on one side. My roosts are large cedar logs that we've used the chain saw to flatten the side they stand on so when roosting their feet are covered. I realize some of you have really harsh weather but here in the warmer parts of Texas people I know with Java have not had any problems to my knowledge.