I’ve seen this question a quite few times on the forums. For users new to Eclipse and/or Flash Builder, it’s not immediately obvious. The short story is that since Flash Builder is based on Eclipse, users get the benefit of the Eclipse plug-in ecosystem. The classic Eclipse distribution is widely known for it’s Java IDE.
Here’s the step-by-step guide to installing the Java Development Tools (JDT) provided by Eclipse:
1) Open Flash Builder
2) Help > Install New Software…
3) At the top of the dialog, click “Add…” to add an update site URL
4) Specify any name and this URL http://download.eclipse.org/releases/galileo/. Press OK.
5) In the table of features, expand “Programming Languages” and select “Eclipse Java Development Tools”
6) Finish the wizard
You’ll be prompted to restart Flash Builder when installation is complete. From here, if you want to jump right into Java development, I recommend you switch perspectives (Window > Perspective > Other > Java). This configures the workbench window so that Java-related actions are the primary focus.