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A couple of times I have seen people have a the same performance problem - they use URL objects as model objects and add them to a HashSet or similar - they don't realize that URL.equals() will make a socket connection to try to determine if two URL objects point to the same IP address. It seems like a common enough and non-obvious bug that it would be nice if we showed an error mark and a warning if a java.net.URL or subclass is added to a java.util.Collection, java.util.Map or subclass thereof.
Max volunteered :)