What Java feature(s) historically started as a pattern and is now a language feature?

They're probably looking for enums.

Before Java 1.5 introduced language support for enums, standard practice was to define a set of public static final ints as enum values.
This pattern can be seen all over Swing.

Many of these constants are defined in interfaces so that classes can implement the interface and use the constants without a qualifying typename; the SwingConstants interface is a great example.

Performance.