memory, Heap, free

   // Determine the HeapMemory:
   long sizeHeap = Runtime.getRuntime().totalMemory();
 
   // Determine the free HeapMemory:
   long freeSizeHeap = Runtime.getRuntime().freeMemory();
 
   // Determine the maximal available HeapMemory
   // (is this exceeded, then anO utOfMemoryError follows):
   long maxSizeHeap = Runtime.getRuntime().maxMemory();

See also OutOfMemoryError.