Can you name the main design patterns implemented in the Java standard library?

The Java standard library implements several common design patterns, including:

• Singleton — ensures that a class has only one instance and provides a global point of access to it. For example, the Runtime class.
• Factory Method — provides an interface for creating objects, allowing subclasses to alter the type of objects that will be created. For example, Calendar.getInstance().
• Observer — allows objects to subscribe to events of another object and receive notifications of changes. Example — java.util.Observer and Observable (although deprecated).
• Decorator — dynamically adds new responsibilities to objects. For example, input-output classes in java.io (BufferedReader, InputStreamReader).
• Iterator — provides a way to access elements of a collection sequentially without exposing its internal structure. For example, Iterator in collections.

These patterns help create flexible and extensible code.