RFE 4286358 added a hasKey() method to the java.util.ResourceBundle class. UIDefaults has been working around the lack of this API (and thereby avoiding MissingResourceExceptions) by internally caching ResourceBundle values. It be advantageous to revisit this approach now that we have hasKey().

The newer approach may (or may not) be quicker (depending on how many ResourceBundles are specified by an applications). We should at least be able to save on memory usage, though my initial profiling indicated this would be fairly modest (3k for SwingSet). But then, there could be use cases where we would see more savings.