(Java language is the most popular pure OOP programming language nowadays. The average quality of programmers using java language is relatively high. These programmers are often not satisfied with the requirements of only realizing program functions. They often hope to make continuous progress and self-improvement in code structure, programming style, and even the way of thinking to solve problems. Design principles and design patterns are the preferred code structure, programming style and problem-solving thinking method after a lot of practice and theorization. Understanding and mastering patterns is a good direction for Java programmers to improve their own quality.)