What is GOF (Gang of Four) and which patterns from this book are implemented in the standard JDK?

GOF (Gang of Four) is a group of four authors: Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, who in 1994 wrote the book "Design Patterns: Elements of Reusable Object-Oriented Software." It describes 23 classic design patterns, divided into three categories: creational, structural, and behavioral.

Several patterns from this book are implemented in the standard JDK, for example:

• Singleton — a pattern that guarantees a single instance of a class. For example, the Runtime class.
• Factory Method — a pattern for creating objects through a factory method. For example, Calendar.getInstance().
• Observer — a pattern for subscribing to events. For example, java.util.Observer and Observable (although deprecated).
• Decorator — dynamically extending functionality. For example, java.io.BufferedInputStream wraps InputStream.
• Adapter — adapting interfaces. For example, java.util.Arrays#asList() returns an adapter of an array to a list.

These patterns are widely used in the JDK to increase flexibility and reusability of code.