OBJECT-ORIENTATION, ABSTRACTION, AND DATA STRUCTURES USING SCALA S E C O N D E D I T I O N CHAPMAN & HALL/CRC TEXTBOOKS IN COMPUTING Series Editors John Impagliazzo Andrew McGettrick Professor Emeritus, Hofstra University Department of Computer and Information Sciences University of Strathclyde Aims and Scope This series covers traditional areas of computing, as well as related technical areas, such as software engineering, artificial intelligence, computer engineering, information systems, and information technology. The series will accommodate textbooks for undergraduate and gradu- ate students, generally adhering to worldwide curriculum standards from professional societ- ies. The editors wish to encourage new and imaginative ideas and proposals, and are keen to help and encourage new authors. The editors welcome proposals that: provide groundbreaking and imaginative perspectives on aspects of computing; present topics in a new and exciting context; open up opportunities for emerging areas, such as multi-media, security, and mobile systems; capture new developments and applications in emerging fields of computing; and address topics that provide support for computing, such as mathematics, statistics, life and physical sciences, and business. Published Titles Paul Anderson, Web 2.0 and Beyond: Principles and Technologies Henrik Bærbak Christensen, Flexible, Reliable Software: Using Patterns and Agile Development John S. Conery, Explorations in Computing: An Introduction to Computer Science John S. Conery, Explorations in Computing: An Introduction to Computer Science and Python Programming Iztok Fajfar, Start Programming Using HTML, CSS, and JavaScript Jessen Havill, Discovering Computer Science: Interdisciplinary Problems, Principles, and Python Programming Ted Herman, A Functional Start to Computing with Python Pascal Hitzler, Markus Krötzsch, and Sebastian Rudolph, Foundations of Semantic Web Technologies Mark J. Johnson, A Concise Introduction to Data Structures using Java Mark J. Johnson, A Concise Introduction to Programming in Python Lisa C. Kaczmarczyk, Computers and Society: Computing for Good Mark C. Lewis, Introduction to the Art of Programming Using Scala Mark C. Lewis and Lisa L. Lacher, Introduction to Programming and Problem- Solving Using Scala, Second Edition Published Titles Continued Mark C. Lewis and Lisa L. Lacher, Object-Orientation, Abstraction, and Data Structures Using Scala, Second Edition Efrem G. Mallach, Information Systems: What Every Business Student Needs to Know Bill Manaris and Andrew R. Brown, Making Music with Computers: Creative Programming in Python Uvais Qidwai and C.H. Chen, Digital Image Processing: An Algorithmic Approach with MATLAB® David D. Riley and Kenny A. Hunt, Computational Thinking for the Modern Problem Solver Henry M. Walker, The Tao of Computing, Second Edition Aharon Yadin, Computer Systems Architecture Chapman & Hall/CRC TEXTBOOKS IN COMPUTING OBJECT-ORIENTATION, ABSTRACTION, AND DATA STRUCTURES USING SCALA S E C O N D E D I T I O N Mark C. Lewis Lisa L. 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Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Contents List of Figures xvii List of Tables xxiii Preface xxv 1 Scala Language Basics 1 1.1 First Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Variables and Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Statements and Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4.1 Lambda Expressions/Closures . . . . . . . . . . . . . . . . . . . . 9 1.5 Control Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5.1 Conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5.2 Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.5.3 Error Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.6 Declarations and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.7 Essential Libraries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.7.1 Standard Input and import Statements . . . . . . . . . . . . . . . 19 1.7.2 Basic Collections . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1.7.2.1 Array and List . . . . . . . . . . . . . . . . . . . . . . . 21 1.7.2.2 Creating Collections Using fill . . . . . . . . . . . . . . 22 1.7.2.3 Creating Collections Using tabulate . . . . . . . . . . . 22 1.7.2.4 Creating Arrays Using new . . . . . . . . . . . . . . . . . 23 1.7.2.5 Creating Lists Using Cons . . . . . . . . . . . . . . . . . 24 1.7.2.6 Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 1.7.2.7 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 1.7.2.8 Higher-Order Methods . . . . . . . . . . . . . . . . . . . 29 1.7.3 The Option Type . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1.7.4 Text Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 1.8 Other Language Features . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 1.8.1 Unfinished Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 1.8.2 Named Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 1.8.3 Default Parameter Values . . . . . . . . . . . . . . . . . . . . . . . 38 1.8.4 Curried Functions/Methods . . . . . . . . . . . . . . . . . . . . . . 38 1.8.5 Pass-by-Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 1.9 The Read, Evaluate, Print Loop (REPL) . . . . . . . . . . . . . . . . . . 41 1.10 Putting It Together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 1.11 End of Chapter Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 1.11.1 Summary of Concepts . . . . . . . . . . . . . . . . . . . . . . . . . 43 1.11.2 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 vii viii Contents 2 Basics of Object-Orientation and Software Development 51 2.1 The Meaning of Object-Orientation . . . . . . . . . . . . . . . . . . . . . 51 2.2 What Are classes? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 2.3 Software Development Stages . . . . . . . . . . . . . . . . . . . . . . . . . 59 2.4 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 2.5 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.6 Bank Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 2.6.1 Bank Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 2.6.2 Bank Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 2.6.3 Bank Implementation . . . . . . . . . . . . . . . . . . . . . . . . . 66 2.6.4 Making an Application . . . . . . . . . . . . . . . . . . . . . . . . 72 2.7 Making Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 2.8 End of Chapter Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 2.8.1 Summary of Concepts . . . . . . . . . . . . . . . . . . . . . . . . . 77 2.8.2 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.8.3 Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 3 Details of Object-Orientation in Scala 81 3.1 The 2D Vector classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 3.1.1 this Keyword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 3.2 Special Methods and Symbolic Names . . . . . . . . . . . . . . . . . . . . 84 3.2.1 Scala Naming Rules/Symbolic Names . . . . . . . . . . . . . . . . 84 3.2.1.1 Precedence and Associativity . . . . . . . . . . . . . . . . 88 3.2.2 Unary Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.2.3 Property Assignment Methods . . . . . . . . . . . . . . . . . . . . 89 3.2.4 The apply Method. . . . . . . . . . . . . . . . . . . . . . . . . . . 91 3.2.5 The update Method . . . . . . . . . . . . . . . . . . . . . . . . . . 92 3.3 object Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 3.3.1 Companion Objects . . . . . . . . . . . . . . . . . . . . . . . . . . 94 3.4 Final Versions of 2D Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.5 case classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.6 Encapsulation/Separating Interface from Implementation . . . . . . . . . 102 3.7 Revisiting the API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 3.8 import Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 3.9 End of Chapter Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 3.9.1 Summary of Concepts . . . . . . . . . . . . . . . . . . . . . . . . . 106 3.9.2 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 3.9.3 Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 4 Abstraction and Polymorphism 115 4.1 Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 4.2 Inclusion Polymorphism (Inheritance and Subtyping) . . . . . . . . . . . . 118 4.2.1 Inheritance in the Scala Type System . . . . . . . . . . . . . . . . 122 4.2.2 Inheritance in Scala Code . . . . . . . . . . . . . . . . . . . . . . . 122 4.2.3 private Visibility and Inheritance . . . . . . . . . . . . . . . . . . 126 4.2.4 protected Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . 127 4.2.5 Calling Methods on the Supertype . . . . . . . . . . . . . . . . . . 127 4.2.6 Abstract Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 4.2.7 traits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 4.2.8 final . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 4.2.9 Method Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Contents ix 4.2.10 Inheriting from Function Types. . . . . . . . . . . . . . . . . . . . 134 4.2.11 Enumerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 4.3 Parametric Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 4.3.1 Parametric Types . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 4.3.2 Parametric Functions and Methods . . . . . . . . . . . . . . . . . 140 4.3.2.1 Parametric Sorts . . . . . . . . . . . . . . . . . . . . . . . 142 4.3.3 Type Bounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 4.3.3.1 Type Bounds and Sorting. . . . . . . . . . . . . . . . . . 145 4.4 End of Chapter Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 4.4.1 Summary of Concepts . . . . . . . . . . . . . . . . . . . . . . . . . 147 4.4.2 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 4.4.3 Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 5 GUIs and Graphics 149 5.1 Project Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 5.2 ScalaFX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 5.3 Drawing Program Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 5.3.1 Menus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 5.3.2 Panes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 5.3.3 Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 5.4 Drawing Program Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 5.5 Interactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 5.5.1 Properties, Observables, and Bindings . . . . . . . . . . . . . . . . 166 5.6 Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 5.6.1 Shapes and Scene Graph . . . . . . . . . . . . . . . . . . . . . . . 167 5.6.2 Canvas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 5.7 Keyboard and Mouse Input . . . . . . . . . . . . . . . . . . . . . . . . . . 174 5.8 Animation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 5.9 Other Elements of ScalaFX . . . . . . . . . . . . . . . . . . . . . . . . . . 179 5.10 End of Chapter Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 5.10.1 Summary of Concepts . . . . . . . . . . . . . . . . . . . . . . . . . 179 5.10.2 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 5.10.3 Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 6 Other Collection Types 185 6.1 The scala.collection Packages . . . . . . . . . . . . . . . . . . . . . . . . . 185 6.1.1 scala.collection.immutable . . . . . . . . . . . . . . . . . . . . . . . 186 6.1.2 scala.collection.mutable . . . . . . . . . . . . . . . . . . . . . . . . 188 6.2 Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 6.2.1 Running through Sets . . . . . . . . . . . . . . . . . . . . . . . . . 190 6.2.2 Mutable vs. Immutable . . . . . . . . . . . . . . . . . . . . . . . . 191 6.2.3 Using a Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 6.3 Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 6.3.1 Looping through a Map . . . . . . . . . . . . . . . . . . . . . . . . 195 6.3.2 Using Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 6.4 Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 6.5 Collections as Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 6.6 Project Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 6.6.1 Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 6.6.2 Adding Drawables . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 6.7 End of Chapter Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208