A pattern type, such as the upper bounded wildcard ? extends Closeable or the lower bounded wildcard ? super String.

Although this interface permits an arbitrary number of upper and lower bounds, all wildcard types of Java language programs are in one of two forms:

1. No lower bound and one upper bound. Such types are written like ? extends java.lang.Number. When the upper bound is java.lang.Object, the extends java.lang.Object suffix is optional: Set<?> is shorthand for Set<? extends java.lang.Object>.
2. One lower bound and an upper bound of java.lang.Object. Such types are written like ? super java.lang.String.

[Android Documentation]