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The next step would be to have a look at another panel implementation, so you can see how things are done. Make sure you look at the less complicated panels, as the panels with advanced features will only be confusing. All the code for building UI and such, only detracts from the essentials of what a panel needs to do. This means that you shouldn't start with UserInputPanel or ShortcutPanel. Probably HelloPanel is probably a much better choice at this stage. The source code for panels is located at: /src/lib/com/izforge/izpack/panels.
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