Eclipse, too.
But if we're talking about "tinkering" here, I'd vote for nothing more than a simple text editor and HTML5 / JavaScript. JavaScript is a difficult language in and of itself to grasp, let alone something massive like Java, so if he / she is just messing around with stuff, I'd vote for something smaller / more easily grasped than Java...
(Just my opinion.)
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Eclipse, too. But if we're talking about "tinkering" here, I'd vote for nothing more than a simple text editor and HTML5 / JavaScript. JavaScript is a difficult language in and of itself to grasp, let alone something massive like Java, so if he / she is just messing around with stuff, I'd vote for something smaller / more easily grasped than Java... (Just my opinion.)