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A practical description of the software design patterns as they are mentioned in the 1994 book "Design Patterns - Elements of Reusable ObjectOriented Software" by the author group Gamma, Helm, Johnson and Vlissides (also called "Gang of Four", GoF for short). All patterns are explained in detail by means of examples and also critically appreciated. Furthermore, design principles of object-oriented programming are described and considered. All examples are commented in detail in the source code and are executable under Java 16. In part, newer features of Java up to and including version 16 are also explained and used.
Patterns describe language-independent abstract solution approaches. This means that each generation of developers is challenged to develop their own implementations. Gamma et al. wrote their exemplary implementations in Smalltalk, among other languages. Implementations in Java may look quite different. Many patterns are already anchored in Java’s class library. It is decidedly exciting to look for and identify the patterns there. By studying design patterns in Java, you will learn a lot about Java along the way.
At this point I would like to address a blur. The question of whether or not you are dealing with a design pattern depends on your point of view. For a procedurally minded programmer, concepts like inheritance, encapsulation, or polymorphism would be design patterns. However, if you are native to object-oriented programming languages, you will consider the aforementioned concepts as fixed language components.
Patterns can inspire and motivate a developer. He might try to implement as many patterns as possible in his programs. However, this can lead to difficulties. Many patterns require the developer to introduce an additional layer of abstraction. This can cause a design to become unnecessarily complicated or performance to suffer. “A design pattern should only be applied when the flexibility provided is really needed,” read Gang of Four on this. Just because you can use patterns doesn’t mean they always have to. They are tools to be used judiciously. Years ago I once heard a British colleague say “A fool with a tool is still a fool”, which should be heeded here too. Not everywhere where you could use a tool (in this case Design Patterns) it makes sense to do so. I will go into this again in the final chapter on combining patterns.
A few programming tricks from Java practice will also be mentioned, but I would like to assume a basic knowledge of the Java programming language and object-oriented programming (OOP). At the time of writing, I created the examples with NetBeans 12.6 and Java 16 on a PC with Windows 10, and there they work as described.
1. The Term “Design Pattern”
2. Object-Oriented Programming and Design Principles
3. Singleton
4. Template Method
5. Observer
6. Chain of Responsibility
7. Mediator
8. State
9. Command
10. Strategy
11. Iterator
12. Composite
13. Flyweight
14. Interpreter
15. Abstract Factory (Abstract Factory)
16. Factory Method
17. Prototype
18. Builder
19. Visitor
20. Memento
21. Facade
22. Adapter
23. Proxy
24. Decorator
25. Bridge
26. Combine Patterns