Hello. Yes we have C# APi which you can use in your indicators, strategies and plug-ins. Info about this API you can find here.
So I can recommend you next: write simple plug-in with ability to send order, you can use our guide for start. For placing orders use method Send() from OrdersCollection class. Then create your java strategy and finally the most interesting - find a way to make communication between Java (your strategy) and C# code (your plug-in). According to the Google search it is not standard but possible via special bridges.
I've integrated ,NET with Java in my day job and the simplest way is to use WCF in your C# project to provide an interface for the Java app to call via http. If writing to a file isn't working out I could point you in the right direction. Feel free to send me a PM.
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