Why java keeps running out of memory

[Sailab]

Is there anyway to set a java heap memory to a limit where in case of more users activities it stop users from further activities until extra memory is back.

The following is one of the issue I had:

I used Java for almost over 4 years and honestly speaking it's a faster language than PHP. Most recently in one of my web application (facejar) I came across some issues which are memory exception issues. Each time when hashmap gets filled up the Java section of my app becomes disabled. If memory is filled up the PHP section performs a bit slow but never shuts down whereas the Java section unfortunately shuts down.

Let me know if anyone can solve this issue?

Thanks