Examples of GoF Design Patterns in Java's core libraries

1. Observer pattern throughout whole swing (Observable, Observer)
2. MVC also in swing
3. Adapter pattern: InputStreamReader and OutputStreamWriter NOTE: ContainerAdapter, ComponentAdapter, FocusAdapter, KeyAdapter, MouseAdapter are not adapters; they are actually Null Objects. Poor naming choice by Sun.
4. Decorator pattern (BufferedInputStream can decorate other streams such as FilterInputStream)
5. AbstractFactory Pattern for the AWT Toolkit and the Swing pluggable look-and-feel classes
6. java.lang.Runtime#getRuntime() is Singleton
7. ButtonGroup for Mediator pattern
8. Action, AbstractAction may be used for different visual representations to execute same code -> Command pattern
9. Interned Strings or CellRender in JTable for Flyweight Pattern (Also think about various pools - Thread pools, connection pools, EJB object pools - Flyweight is really about management of shared resources)
10. The Java 1.0 event model is an example of Chain of Responsibility, as are Servlet Filters.
11. Iterator pattern in Collections Framework
12. Nested containers in AWT/Swing use the Composite pattern
13. Layout Managers in AWT/Swing are an example of Strategy

and many more I guess