1. Week 6, Class 1 & 2: Decorators Return Exam Questions about lab due tomorrow in class? Threads Locking on null object invokeLater & the squares example Decorator & Java IO continued Decorator vs. List of Add-ons SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 1

2. Decorator Pattern context You want to attach additional functionality to an (existing) class dynamically… …without having to resort to sub-classing the existing class We don’t want a class explosion We want to allow classes to be easily “extended” to incorporate new behavior without modifying existing code. 2

3. SE-2811 Dr. Mark L. Hornick 3 + doMoreStuff()

4. How are decorators useful? Decorators have the same super-type as the objects they decorate. Helps organize: e.g., can put all beverage objects in the same data-structure Used when initializing super-class One or more decorators can be used to wrap an object. Why is this useful? Can still get to specific & useful method When creating the object By casting back to cream (leaves open question: Is it cream?) Test using instanceof Can get more & more specific about the object with multiple decos. Don’t need all the classes for all combinations

5. The java.io package contains dozens of classes OutputStream, FileOutputStream, PipedOutputStream, DataOutputStream, ObjectOutputStream, PrintStream, PrintWriter, … Understanding the associations between them just by reading the Javadoc API is difficult SE-2811 Dr. Mark L. Hornick 5

6. Knowing that the input stream classes are based on the Decorator pattern can make things easier SE-2811 Dr. Mark L. Hornick 6 >

7. The Decorator pattern applied to output streams SE-2811 Dr. Mark L. Hornick 7 Only for error propagation in my Java version

8. Decorator vs. Array of Add-ons SE-2811 Dr. Mark L. Hornick 8

9. Design Principles T? Reduce coupling Abstract classes are unliked T Increase cohesion T Encapsulate what varies E.g. mocha class only cares about chocolate Favor composition over inheritance T Program to interfaces, not implementations T Classes should be open for extension but closed for modification Which of these are met? SE-2811 Dr. Mark L. Hornick 9

10. Downsides What are the disadvantages of decorators? Need to pay attention to abstract classes & type SE-2811 Dr. Mark L. Hornick 10

11. SE-2811 Dr. Mark L. Hornick 11