Design patterns, as name suggest, are solutions for most commonly (and frequently) occurred problems while designing a software.

These patterns provide various object creation mechanisms, which increase flexibility and reuse of existing code.

These patterns explain how to assemble objects and classes into larger structures while keeping these structures flexible and efficient.

These patterns are concerned with algorithms and the assignment of responsibilities between objects.

factory

Depend upon abstractions. Do not depend upon concrete classes.
Define an interface for creating an object, but let subclasses decide which class to instantiate.

it is one of the "Gang of Four" design patterns.The Simple Factory isn’t actually a Design Pattern; it’s more of a programming idiom. it Hides the object creation process from the client. the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. Defining a simple factory as a static method is a common technique and is often called a static factory.
Remember it also has the disadvantage that you can’t subclass and change the behavior of the factory method.

java.sql.DriverManager.getConnection() uses factory pattern.

Head First Design Patterns.

decorator

Decorator Design Pattern

Structural design pattern.

Classes should be open for extension but closed for modification.

Decorator pattern allows adding additional behaviors to objects dynamically at runtime without affecting other object of same type by placing them inside wrapper objects. Basically, Attach additional responsibilities to an object dynamically and provide a flexible alternative to subclassing for extending functionality.

InputStream is a Base abstract Class and FilterInputStream is a decorator class extended by child classes like BufferedInputStream.

Head First Design Patterns.

observer

Observer Design Pattern

Behavioral design pattern.

Favor composition over inheritance.
program to interface not implementation.

Observer pattern defines one to many dependency between a set of objects so that when object(Subject) changes state,all of its dependent(Observer) are notified and update automatically.

java.util.Observable
Model-View-Controller (MVC) frameworks also use Observer pattern.
Java Message Service (JMS) uses Observer pattern.

Head First Design Patterns.