why my program throw the OutOfMemeory

In my program(is a monitor program).
after it run 6 hours ,it throw the Error of OutOfMemory.but when i see the system memory ,there are a larger memory not used. and how can I solve this problem and use the memory that not used.let it run a longer time?thanks!!!

Even though you have free RAM, the JVM may not have access to it. You can increase the JVM heap size by giving certain parameters.
For example,
java -ms64M -mx128M example.MyClass
starts the JVM with an initial heap size of 64 megabytes and gives it permission to grow up to 128 megabytes. The JVM will not exceed the mx-value.

Oh, almost forgot.
The fact that your program slowly runs out of memory implies a "memory leak" in your code. I'd suggest double-checking all code that deals with collections of some sort and ensure that references are nullified as intended.

dear Lasse Koskela ,thanks a lot!