Object Oriented Design Principles are core of OOPS programming, but I have seen most of Java programmer chasing design patterns like Singleton pattern , Decorator pattern or Observer pattern , and not putting enough attention on learning Object oriented analysis and design. It's important to learn basics of Object oriented programming like Abstraction, Encapsulation, Polymorphism and Inheritance, but same time, it's equally important to know these design principles, to create clean and modular design. I have regularly seen Java programmers and developers of various experience level, who either doesn't heard about these OOPS and SOLID design principle, or simply doesn't know what benefits a particular design principle offers, or how to use these design principle in coding.
Bottom line is, always strive for highly cohesive and loosely couple solution, code or design. Looking open source code from Apache and Sun are good examples of learning Java and OOPS design principles. They show us, how design principles should be used in coding and Java programs. Java Development Kit follows several design principle like Factory Pattern in BorderFactory class, Singleton pattern in Runtime class, Decorator pattern on various java.io classes. By the way if you really interested more on Java coding practices, read Effective Java by Joshua Bloch , a gem by the guy who wrote Java API. My another personal favorite on object oriented design pattern is, Head First Design Pattern by Kathy Sierra and others and Head First Object Oriented Analysis and Design . These books helps a lot to write better code, taking full advantage of various Object oriented and SOLID design principles.
