Standard MBean

MBeanServer mbs = ...something...;
ObjectName name =
    new ObjectName("com.example:type=Memory");
MemoryUsage memoryUsage = (MemoryUsage)
     mbs.getAttribute(name, "MemoryUsage");
long used = memoryUsage.getUsed();

MXBean

MBeanServer mbs = ...something...;
ObjectName name =
    new ObjectName("com.example:type=Memory");
CompositeData memoryUsage = (CompositeData)
     mbs.getAttribute(name, "MemoryUsage");
long used = (Long) memoryUsage.get("Used");