- •Contents
- •List of Figures
- •List of Tables
- •List of Listings
- •Foreword
- •Foreword to the First Edition
- •Acknowledgments
- •Introduction
- •A Scalable Language
- •A language that grows on you
- •What makes Scala scalable?
- •Why Scala?
- •Conclusion
- •First Steps in Scala
- •Conclusion
- •Next Steps in Scala
- •Conclusion
- •Classes and Objects
- •Semicolon inference
- •Singleton objects
- •A Scala application
- •Conclusion
- •Basic Types and Operations
- •Some basic types
- •Literals
- •Operators are methods
- •Arithmetic operations
- •Relational and logical operations
- •Bitwise operations
- •Object equality
- •Operator precedence and associativity
- •Rich wrappers
- •Conclusion
- •Functional Objects
- •Checking preconditions
- •Self references
- •Auxiliary constructors
- •Method overloading
- •Implicit conversions
- •A word of caution
- •Conclusion
- •Built-in Control Structures
- •If expressions
- •While loops
- •For expressions
- •Match expressions
- •Variable scope
- •Conclusion
- •Functions and Closures
- •Methods
- •Local functions
- •Short forms of function literals
- •Placeholder syntax
- •Partially applied functions
- •Closures
- •Special function call forms
- •Tail recursion
- •Conclusion
- •Control Abstraction
- •Reducing code duplication
- •Simplifying client code
- •Currying
- •Writing new control structures
- •Conclusion
- •Composition and Inheritance
- •A two-dimensional layout library
- •Abstract classes
- •Extending classes
- •Invoking superclass constructors
- •Polymorphism and dynamic binding
- •Using composition and inheritance
- •Heighten and widen
- •Putting it all together
- •Conclusion
- •How primitives are implemented
- •Bottom types
- •Conclusion
- •Traits
- •How traits work
- •Thin versus rich interfaces
- •Example: Rectangular objects
- •The Ordered trait
- •Why not multiple inheritance?
- •To trait, or not to trait?
- •Conclusion
- •Packages and Imports
- •Putting code in packages
- •Concise access to related code
- •Imports
- •Implicit imports
- •Package objects
- •Conclusion
- •Assertions and Unit Testing
- •Assertions
- •Unit testing in Scala
- •Informative failure reports
- •Using JUnit and TestNG
- •Property-based testing
- •Organizing and running tests
- •Conclusion
- •Case Classes and Pattern Matching
- •A simple example
- •Kinds of patterns
- •Pattern guards
- •Pattern overlaps
- •Sealed classes
- •The Option type
- •Patterns everywhere
- •A larger example
- •Conclusion
- •Working with Lists
- •List literals
- •The List type
- •Constructing lists
- •Basic operations on lists
- •List patterns
- •First-order methods on class List
- •Methods of the List object
- •Processing multiple lists together
- •Conclusion
- •Collections
- •Sequences
- •Sets and maps
- •Selecting mutable versus immutable collections
- •Initializing collections
- •Tuples
- •Conclusion
- •Stateful Objects
- •What makes an object stateful?
- •Reassignable variables and properties
- •Case study: Discrete event simulation
- •A language for digital circuits
- •The Simulation API
- •Circuit Simulation
- •Conclusion
- •Type Parameterization
- •Functional queues
- •Information hiding
- •Variance annotations
- •Checking variance annotations
- •Lower bounds
- •Contravariance
- •Object private data
- •Upper bounds
- •Conclusion
- •Abstract Members
- •A quick tour of abstract members
- •Type members
- •Abstract vals
- •Abstract vars
- •Initializing abstract vals
- •Abstract types
- •Path-dependent types
- •Structural subtyping
- •Enumerations
- •Case study: Currencies
- •Conclusion
- •Implicit Conversions and Parameters
- •Implicit conversions
- •Rules for implicits
- •Implicit conversion to an expected type
- •Converting the receiver
- •Implicit parameters
- •View bounds
- •When multiple conversions apply
- •Debugging implicits
- •Conclusion
- •Implementing Lists
- •The List class in principle
- •The ListBuffer class
- •The List class in practice
- •Functional on the outside
- •Conclusion
- •For Expressions Revisited
- •For expressions
- •The n-queens problem
- •Querying with for expressions
- •Translation of for expressions
- •Going the other way
- •Conclusion
- •The Scala Collections API
- •Mutable and immutable collections
- •Collections consistency
- •Trait Traversable
- •Trait Iterable
- •Sets
- •Maps
- •Synchronized sets and maps
- •Concrete immutable collection classes
- •Concrete mutable collection classes
- •Arrays
- •Strings
- •Performance characteristics
- •Equality
- •Views
- •Iterators
- •Creating collections from scratch
- •Conversions between Java and Scala collections
- •Migrating from Scala 2.7
- •Conclusion
- •The Architecture of Scala Collections
- •Builders
- •Factoring out common operations
- •Integrating new collections
- •Conclusion
- •Extractors
- •An example: extracting email addresses
- •Extractors
- •Patterns with zero or one variables
- •Variable argument extractors
- •Extractors and sequence patterns
- •Extractors versus case classes
- •Regular expressions
- •Conclusion
- •Annotations
- •Why have annotations?
- •Syntax of annotations
- •Standard annotations
- •Conclusion
- •Working with XML
- •Semi-structured data
- •XML overview
- •XML literals
- •Serialization
- •Taking XML apart
- •Deserialization
- •Loading and saving
- •Pattern matching on XML
- •Conclusion
- •Modular Programming Using Objects
- •The problem
- •A recipe application
- •Abstraction
- •Splitting modules into traits
- •Runtime linking
- •Tracking module instances
- •Conclusion
- •Object Equality
- •Equality in Scala
- •Writing an equality method
- •Recipes for equals and hashCode
- •Conclusion
- •Combining Scala and Java
- •Using Scala from Java
- •Annotations
- •Existential types
- •Using synchronized
- •Compiling Scala and Java together
- •Conclusion
- •Actors and Concurrency
- •Trouble in paradise
- •Actors and message passing
- •Treating native threads as actors
- •Better performance through thread reuse
- •Good actors style
- •A longer example: Parallel discrete event simulation
- •Conclusion
- •Combinator Parsing
- •Example: Arithmetic expressions
- •Running your parser
- •Basic regular expression parsers
- •Another example: JSON
- •Parser output
- •Implementing combinator parsers
- •String literals and regular expressions
- •Lexing and parsing
- •Error reporting
- •Backtracking versus LL(1)
- •Conclusion
- •GUI Programming
- •Panels and layouts
- •Handling events
- •Example: Celsius/Fahrenheit converter
- •Conclusion
- •The SCells Spreadsheet
- •The visual framework
- •Disconnecting data entry and display
- •Formulas
- •Parsing formulas
- •Evaluation
- •Operation libraries
- •Change propagation
- •Conclusion
- •Scala Scripts on Unix and Windows
- •Glossary
- •Bibliography
- •About the Authors
- •Index
Section 24.19 |
Chapter 24 · The Scala Collections API |
605 |
24.19Migrating from Scala 2.7
If you have existing applications written in Scala 2.7, porting them to use the new collections should be almost automatic. There are only a couple of possible issues to take care of.
Generally, the old functionality of Scala 2.7 collections has been left in place. Some features have been deprecated, which means they will removed in some future release. You will get a deprecation warning when you compile code that makes use of these features in Scala 2.8. In a few places deprecation was unfeasible, because the operation in question was retained in 2.8, but changed in meaning or performance characteristics. These cases will be flagged with migration warnings when compiled under 2.8. To get full deprecation and migration warnings with suggestions how to change your code, pass the -deprecation and -Xmigration flags to scalac.8 You can also pass the same options to the scala interpreter to get the warnings in an interactive session. Example:
>scala -deprecation -Xmigration Welcome to Scala version 2.8.1.
Type in expressions to have them evaluated.
Type :help for more information.
scala> val xs = List((1, 2), (3, 4))
xs: List[(Int, Int)] = List((1,2), (3,4))
scala> List.unzip(xs)
<console>:7: warning: method unzip in object List is deprecated: use xs.unzip instead of List.unzip(xs)
List.unzip(xs)
ˆ
res0: (List[Int], List[Int]) = (List(1, 3),List(2, 4))
scala> xs.unzip
res1: (List[Int], List[Int]) = (List(1, 3),List(2, 4))
scala> val m = xs.toMap
m: scala.collection.immutable.Map[Int,Int] = Map((1,2), (3,4))
scala> m.keys
<console>:8: warning: method keys in trait MapLike has
8Note that -Xmigration is an extended option, so it starts with an X.
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Section 24.20 |
Chapter 24 · The Scala Collections API |
606 |
changed semantics: As of 2.8, keys returns Iterable[A]
rather than Iterator[A].
m.keys
ˆ
res2: Iterable[Int] = Set(1, 3)
Two parts of the old libraries were replaced wholesale. For these deprecation warnings were not feasible.
1.The previous scala.collection.jcl package is gone. This package tried to mimic some of the Java collection library design in Scala, but in doing so broke many symmetries. Most people who wanted Java collections bypassed jcl and used java.util directly. Scala 2.8 offers automatic conversion mechanisms between both collection libraries in the JavaConversions object, described in Section 24.18, which replaces the jcl package.
2.Projections have been generalized and cleaned up and are now available as views. It seems that projections were used rarely, so not much code should be affected by this change.
So, if your code uses either jcl or projections there might be some minor rewriting to do.
24.20Conclusion
You’ve now seen how to use Scala’s collection in great detail. Scala’s collections take the approach of giving you powerful building blocks rather than a number of ad hoc utility methods. Putting together two or three such building blocks allows you to express an enormous number of useful computations. This style of library is especially effective due to Scala having a light syntax for function literals, and due to it providing many collection types that are persistent and immutable.
This chapter has shown collections from the point of view of a programmer using the collection library. The next chapter will show you how the collections are built and how you can add your own collection types.
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