What design patterns are implemented in the Java standard library?

The Java standard library implements several design patterns:

• Singleton: for example, the Runtime class implements the singleton pattern, providing a single instance through the getRuntime() method.

• Factory Method: in classes that create objects, for example, Calendar.getInstance() returns an object of a specific calendar implementation.

• Observer: in java.util, there are interfaces Observer and the class Observable for implementing the observer pattern.

• Decorator: wrapper classes for input-output streams (BufferedInputStream, DataInputStream, etc.) implement the decorator pattern, adding functionality to basic streams.

• Iterator: the Iterator interface and its implementations in collections allow sequential traversal of elements.

• Adapter: for example, the InputStreamReader class adapts a byte stream into a character stream.

These patterns help create flexible, extensible, and maintainable applications using proven solutions.