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CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
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Scala supports multiple assignment. If a code block or method returns a Tuple, the Tuple can be assigned to a val variable.
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val (i1: Int, s1: String) = Pair(33, "Moof")
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And the type inferencer gets it right:
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val (i2, s2) = Pair(43, "Woof")
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Code Blocks
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Method and variable definitions can be single lines:
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def meth9() = "Hello World"
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Or methods and variables can be defined in code blocks that are denoted by curly braces: { }. Code blocks may be nested. The result of a code block is the last line evaluated in the code block.
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def meth3(): String = {"Moof"} def meth4(): String = { val d = new java.util.Date() d.toString() }
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Variable definitions can be code blocks as well. This comes in handy when defining val variables, and the logic required to compute the value is non-trivial.
val x3: String = { val d = new java.util.Date() d.toString() }
Call-by-Name
In Java, all method invocations are call-by-reference or call-by-value (for primitive types). What this means is that the parameter s value, or reference in the case of an AnyRef, is placed on the stack and passed to the callee. Scala gives you an additional mechanism for passing parameters to methods (and functions): call-by-name, which passes a code block to the callee. Each time the callee accesses the parameter, the code block is executed and the value is calculated. Call-by-name allows you to pass parameters that might take a long
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
time to calculate but may not be used. For example, in a call to the logger you can use callby-name, and the express to print is only calculated if it s going to be logged. Call-by-name also allows you to create flow of control structures such as while, doWhile, and so on. We declare a nano method, which prints a message and returns the current time with nano-second resolution:
def nano() = { println("Getting nano") System.nanoTime }
Next we declare the delayed method, which takes a call-by-name parameter by putting the => symbol between the variable name and the type. delayed prints a message demonstrating that the method has been entered. Next, delayed prints a message with t s value. Finally, delayed returns t.
def delayed(t: => Long) = { println("In delayed method") println("Param: "+t) t }
Let s see what happens when we call delayed with nano as a parameter:
scala> delayed(nano())
In delayed method Getting nano Param: 4475258994017 Getting nano res3: Long = 4475259694720
This indicates that delayed is entered before the call to nano and that nano is called twice. Let s compare this to call-by-reference:
def notDelayed(t: Long) = { println("In not delayed method") println("Param: "+t) t }
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
Let s try calling notDelayed:
scala> notDelayed(nano())
Getting nano In not delayed method Param: 4513758999378 res4: Long = 4513758999378
nano is called before notDelayed is called because the parameter to notDelayed, nano(), is calculated before notDelayed is called. This is the way Java programmers expect code to work.
Method Invocation
Scala provides a number of syntactic variations for invoking methods. There s the standard Java dot notation:
instance.method()
But if a method does not take any parameters, the ending parentheses are optional:
instance.method
This allows methods without parameters methods to appear as properties or fields on the target instance. This results in more visually pleasing code. Methods that take a single parameter can be invoked just as in Java:
instance.method(param)
But methods that take a single parameter can be invoked without dots or parentheses:
instance method param
Because Scala allows method names to contain symbols such as +, -, *, and , Scala s dotless method notation creates a syntactically neutral way of invoking methods that are hard-coded operators in Java.
scala> 2.1.*(4.3)
res5: Double = 9.03
CHAPTER 2 SCALA SYNTAX, SCRIPTS, AND YOUR FIRST SCALA PROGRAMS
scala> 2.1 * 4.3
res6: Double = 9.03
Finally, you invoke multiparameter methods in Scala just as in Java:
instance.method(p1, p2)
If a Scala method takes a type parameter, typically, the type parameter will be inferred by the compiler, but you can also explicitly pass the type parameter:
instance.method[TypeParam](p1, p2)
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