code: public static long getSizeOfFreeHeapMemory() {
  		Runtime r = Runtime.getRuntime();
  		return getMaxJvmHeapMemory() - r.totalMemory() + r.freeMemory();
  	}
docs: Gets an estimate of the size of the free heap memory. The estimate may vary, depending on the current
  level of memory fragmentation and the number of dead objects. For a better (but more heavy-weight)
  estimate, use {@link #getSizeOfFreeHeapMemoryWithDefrag()}.
  
  @return An estimate of the size of the free heap memory, in bytes.