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

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

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

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

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

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

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

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

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

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

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