Calling Monitor.addObservedObject more than 16 times for a given Monitor (CounterMonitor, GaugeMonitor, or StringMonitor) produces an exception:

javax.management.RuntimeMBeanException: RuntimeException thrown in operation addObservedObject
at com.sun.jmx.mbeanserver.StandardMetaDataImpl.invoke(StandardMetaDataImpl.java:424)
at com.sun.jmx.mbeanserver.MetaDataImpl.invoke(MetaDataImpl.java:228)
at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:823)
at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:792)
at javax.management.MBeanServerInvocationHandler.invoke(MBeanServerInvocationHandler.java:209)
at $Proxy0.addObservedObject(Unknown Source)
at MultiMonitorTest.main(MultiMonitorTest.java:59)
Caused by: java.lang.ArrayIndexOutOfBoundsException
at java.lang.System.arraycopy(Native Method)
at javax.management.monitor.Monitor.insertintElementAt(Monitor.java:596) at javax.management.monitor.Monitor.addObservedObject(Monitor.java:363)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at com.sun.jmx.mbeanserver.StandardMetaDataImpl.invoke(StandardMetaDataImpl.java:410)
... 6 more

The problem is that the methods insertintElementAt and ensureintCapacity assume that they can resize an array that is passed as a parameter. ensureintCapacity incorrectly assigns the newly-reallocated bigger array to its array parameter, which of course has no durable effect.