Either when the user has their cursor or mouse over a word that's either a method call or a variable name, a tooltip could pop up giving the Javadoc on that variable or method name.
Story: A user has gets a compile error that they don't understand. They move the mouse over various parts of the offending text until the see that a method they thought returned an ArrayList actually returned an explicit Array.
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The tough part would be finding the appropriate info without requiring extra setup from the user. I don't think this feature would be worth making DrJava's basic setup more complex. In theory, we could pull method signatures directly out of class files using reflection, but that might be very slow. We'll have to experiment to see how much of a penalty it would have, and what we could do to minimize it.