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;