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scala> s.toString
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res70: String = Stuff(David,45)
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Stuff s equals method does a deep comparison: scala> s == Stuff("David", 45)
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res72: Boolean = true
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scala> s == Stuff("David", 43)
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res73: Boolean = false
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CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
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And the instance has properties:
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scala> s.name
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res74: String = David
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scala> s.age
res75: Int = 45
If you want to write your own class that does the same thing as a case class, it would look like the following:
class Stuff(val name: String, val age: Int) { override def toString = "Stuff("+name+","+age+")" override def hashCode = name.hashCode + age override def equals(other: AnyRef) = other match { case s: Stuff => this.name == s.name && this.age == s.age case _ => false } } object Stuff { def apply(name: String, age: Int) = new Stuff(name, age) def unapply(s: Stuff) = Some((s.name, s.age)) }
Case classes also come in handy for pattern matching, a topic we ll explore in the next subsection.
Basic Pattern Matching
Scala s pattern matching allows you to construct very complex tests in very little code. Pattern matching is like Java s switch statement, but you can test against almost anything, and you can even assign pieces of the matched value to variables. Like everything in Scala, pattern matching is an expression, so it results in a value that may be assigned or returned. The most basic pattern matching is like Java s switch, except there is no break in each case
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
as the cases do not fall through to each other. This example matches the number against a constant, but with a default:
44 match { case 44 => true // if we match 44, the result is true case _ => false // otherwise the result is false }
Like C#, you can match against a String:
"David" match { case "David" => 45 // the result is 45 if we match "David" case "Elwood" => 77 case _ => 0 }
You can pattern match against case classes. Case classes provide a particularly good set of pattern-matching semantics. In this case, we are matching against a Stuff instance with name == David and age == 45 in a declarative form:
Stuff("David", 45) match { case Stuff("David", 45) => true case _ => false }
We can test the name but accept any age:
Stuff("David", 45) match { case Stuff("David", _) => "David" case _ => "Other" }
And we can extract the age field into the howOld variable:
Stuff("David", 45) match { case Stuff("David", howOld) => "David, age: "+howOld case _ => "Other" }
We can place a guard between the pattern and the => that adds further testing that cannot be described declaratively. In this case, we ll extract the age, and if it s less than 30, the result will be young David , otherwise the result will be old David :
Stuff("David", 45) match { case Stuff("David", age) if age < 30 => "young David" case Stuff("David", _) => "old David" case _ => "Other" }
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
Pattern matching can also test whether the input is an instance of a given class and do the casting if it is:
x match { case d: java.util.Date => "The date in milliseconds is "+d.getTime case u: java.net.URL => "The URL path: "+u.getPath case s: String => "String: "+s case _ => "Something else" }
The previous code replaces the following Java code:
if (x instanceOf Date) return "The date in milliseconds is "+((Date) x).getTime(); if (x instanceOf URL) return "The URL path: "+((URL) x).getPath(); if (x instanceOf String) return "String "+((String) x); return "Something else";
if/else and while
It may seem strange that we re covering simple flow of control statements late in this section. It turns out that while is used very rarely in Scala code. if/else is used more frequently, a bit more frequently than the ternary operator is used in Java. The result of if and while expressions is always Unit. The result of if/else is based on the type of each part of the expression. This will print yes if exp is true:
if (exp) println("yes")
Like Java, an if expression may have a multiline code block:
if (exp) { println("Line one") println("Line two") } if/else behaves like the ternary operator in Java: val i: Int = if (exp) 1 else 3
and either (or both) parts of the expression may have multiline code blocks:
val i: Int = if (exp) 1 else { val j = System.currentTimeMillis (j % 100L).toInt }
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