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println() and strange-looking strings
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The listings in this section, and other sections in this chapter, make reference to a certain println() function. Common to all JavaFX objects, thanks to its inclusion in the javafx.lang.FX base class, println() is the JavaFX way of writing to the application s default output stream. Java programmers will be familiar with println() through Java s System.out object but may not recognize the bizarre curly braced strings being used to create the formatted output. For now accept them we ll deal with the details in a few pages.
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JavaFX Script data and variables
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As in many languages, the initializer is optional. We could have ended each declaration after its type, with just closing semicolons, resulting in each variable being initialized to a sensible default value. Listing 2.3 shows a handful of examples.
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Listing 2.3 Defining value types using defaults
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var defBool:Boolean; var defInt:Integer; var defNum:Number; var defStr:String; println("Default: B: {defBool}, " "I: {defInt}, N: {defNum}, S: {defStr}"); Default: B: false, I: 0, N: 0.0, S:
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You ll note in listing 2.3 the absence of any initial values. Despite being objects, value types cannot be null, so defaults of false, zero, or empty are used. But the initial value is not the only thing we can leave off, as listing 2.4 shows:
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Listing 2.4 Defining value types using type inference
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var infBool = true; var infFlt = 1.0; Be careful declaring non-ints var infInt = 1; var infStr = "Some text"; println("Infered: B: {infBool}, " "F: {infFlt}, I: {infInt}, S: {infStr}"); println("{infFlt.getClass()}"); Infered: B: true, F: 1.0, I: 1, S: Some text class java.lang.Float
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JavaFX Script supports type inference when declaring variables. In plain English, this means if an initializer is used and the compiler can unambiguously deduce the variable s type from it, you can omit the explicit type declaration. In listing 2.4, if we d initialized infFlt with just the value 1, it would have become an Integer instead of a Float; quoting the fractional part drops a hint as to our intended type.
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Initialize-only and reassignable variables (var, def)
So far we ve seen variables declared using the var keyword. But there s a second way of declaring variables, as listing 2.5 is about to reveal.
Listing 2.5 Declaring variables with def
var canAssign:Integer = 5; def cannotAssign:Integer = 5; canAssign = 55; //cannotAssign = 55;
Compiler error if uncommented
Using def instead of var results in variables that cannot be reassigned once created.
Data types
It s tempting to think of def variables only as constants; indeed that s how they re often used, but it s not always the case. A def variable cannot be reassigned, but the object it references can mutate (change its contents). Some types of objects are immutable (they provide no way to change their content once created, examples being String and Integer), so we might assume a def variable of an immutable type must be a constant. But again, this is not always the case. In a later section we ll investigate bound variables, revisiting def to see how a variable (even of an immutable type) can change its value without actually changing its content. So, ignoring bound variables for the moment, a valid question is when should we use var and when should we use def First, def is useful if we want to drop hints to fellow programmers as to how a given variable should be used. Second, the compiler can detect misuse of a variable if it knows how we intend to use it, but crucially, JFX can better optimize our software if given extra information about the data it s working with. For simple assignments like those in listing 2.5, it s largely a matter of choice or style. Using def helps make our intentions clear and means our code might run a shade faster.
Arithmetic on value types (+, -, etc.) Waxing lyrical about JavaFX Script s arithmetic capabilities is pointless: they re basically the same as those of most programming languages. Unlike Java, JavaFX Script s value types are objects, so they respond to both conventional operator syntax (like Java primitives) and method calls (like Java objects). Let s see how that works in practice, in listing 2.6.
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