Total designated memory, this will equal the configured -Xmx value:
Runtime.getRuntime().maxMemory();
Current allocated free memory, is the current allocated space ready for new objects. Caution this is not the total free available memory:
Runtime.getRuntime().freeMemory();
Total allocated memory, is the total allocated space reserved for the java process:
Runtime.getRuntime().totalMemory();
Used memory, has to be calculated:
usedMemory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
Total free memory, has to be calculated:
freeMemory = Runtime.getRuntime().maxMemory() - usedMemory;