In JDK there are many places where Decorator design pattern is

used. Some of these are as follows:

1. In java.io package many classes use Decorator pattern.

Subclasses of java.io.InputStream, OutputStream, Reader

and Writer have a constructor that can take the instance of

same type and decorate it with additional behavior.

2. In java.util.Collections, there are methods like

checkedCollection(), checkedList(), checkedMap(),

synchronizedList(), synchronizedMap(),

synchronizedSet(), unmodifiableSet(), unmodifiableMap()

and unmodifiableList() methods that can decorate an object

and return the same type.

3. In javax.servlet package, there are classes like

javax.servlet.http.HttpServletRequestWrapper and

HttpServletResponseWrapper that are based on Decorator

design pattern

https://www.digitalocean.com/community/tutorials/decorator-design-pattern-in-java-example