In Java, wildcards are a powerful feature of generics that allow for more flexible and reusable code. They are represented by the “?” symbol and can be used in various contexts, particularly when dealing with collections. There are three main types of wildcards: unbounded, upper-bounded, and lower-bounded.

1. Unbounded Wildcards: Represented by “?”, this wildcard allows a collection to hold elements of any type. It is useful when the specific type of elements is unknown. For example, a method can accept a List<?> to work with any type of list.

2. Upper-Bounded Wildcards: Denoted as “? extends Type”, this wildcard restricts the collection to hold elements of a specific type or its subtypes. For instance, List<? extends Number> can hold a list of Number or any subclass like Integer or Double. This is useful when you want to read items from a collection.

3. Lower-Bounded Wildcards: Represented as “? super Type”, this wildcard allows a collection to hold elements of a specific type or its supertypes. For example, List<? super Integer> can hold Integer and any of its superclasses, which is useful for adding items to a collection.

These wildcards enhance type safety and flexibility in Java generics, allowing developers to write more generic and reusable code ¹²⁵.