Say you are writing a high speed trading program. At some point you get to the "if(wouldMakeMoneyOnTrade())...". However for every time this passes and you trade there are 1000 where it fails and you don't. However time is critical because your competitors are using similar algorithms and so you need to beat them - thus you put in a likely and ensure that the CPU jumps to the trade code the fastest possible. Your competitors that use Java pay a CPU branch prediction penalty here because the hotspot optimizer has noticed that the if almost always fails and optimized for the false case. Thus you can make money by beating your competitor to the trade. (Note, this doesn't mean you can't use Java for your code, but if you do you need to manage that risk somehow)
Many HFT firms use C# or Java. Quite a few matching engines are written in those languages as well.
The trick is to remember that garbage collection is not some sort of werewolf that wakes up and decides to mess with your stuff whenever it feels like it; it (can) happen only at certain, documented points. So you have to write your code in such a way that it does not do memory allocations in the critical path (which you would not be doing in C++ anyway, for a similar reason; malloc/free is expensive).
Generally if you're doing HFT your critical path is not going to being doing much anyway. A few reads, comparisons, maybe some trivial arithmetic and that should be it.
For what is worth, at least in London, all the HFT firms I have worked with or interviewed for use C++. As I'm a C++ programmer there is bias of course, so my sample is probably not representative. Still I suspect that those that use anything other than C++ are a small minority.
Say you are writing a high speed trading program. At some point you get to the "if(wouldMakeMoneyOnTrade())...". However for every time this passes and you trade there are 1000 where it fails and you don't. However time is critical because your competitors are using similar algorithms and so you need to beat them - thus you put in a likely and ensure that the CPU jumps to the trade code the fastest possible. Your competitors that use Java pay a CPU branch prediction penalty here because the hotspot optimizer has noticed that the if almost always fails and optimized for the false case. Thus you can make money by beating your competitor to the trade. (Note, this doesn't mean you can't use Java for your code, but if you do you need to manage that risk somehow)