Expressions, Functions, and Object Literals in Java

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6 Expressions, Functions, and Object Literals
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Anonymous Functions and Function Variables It is possible to define a function variable and directly assign an anonymous function to it, as this example demonstrates:
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var fn1 = function():Number { return Mathrandom(); }; println("A random number: {fn1()}");
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As with all uses of anonymous functions, the prime benefit of this is that there is no need to invent a name for the function being assigned to the variable Returning an Anonymous Function from Another Function Functions that return a function can return an anonymous function:
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function getRandomSource():function(:Number):Number { return function (max:Number):Number { return Mathrandom() * max; } }
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This (rather artificial) example shows a function that, when you call it, returns to you another function from which you can obtain a random number Here s how you would use it:
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println("Random from anonymous function: {getRandomSource()(4)}");
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An Anonymous Function as an Initializer in an Object Literal One of the most common uses of an anonymous function is to initialize an instance variable, such as the onClose instance variable of the Stage class that you saw earlier in this section Recall that using a named function to initialize this instance variable resulted in some clumsy code Here s how you could do the same thing with an anonymous function:
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def stage2:Stage = Stage { title: "Close Action #2" width: 400 height: 400 x: 500 y: 10 visible:true onClose: function() { println("'{stage2title}' closing"); } }
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As you can see, all the code is now contained within the initialization of the stage It is clear that the code in the anonymous function is intended only to be called in response to the closing of the stage, which makes understanding easier and maintenance simpler This is a pattern that you will use a lot when writing JavaFX applications When an instance variable requires a function that has one or more arguments, it is not necessary to restate the argument types when initializing that instance variable in an
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Object Literals
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object literal For example, the Rectangle class in the javafxsceneshape package has several function-valued instance variables (inherited from its parent class, javafxsceneNode) that allow application code to be called in response to various events, such as those generated when the mouse is over the part of the screen occupied by the RectangleThe onMouseMoved instance variable, which is a typical example, is defined like this:
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public var onMouseMoved: function(e:MouseEvent):Void
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Here s one way to assign a handler to this instance variable:
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// Restates "MouseEvent " as the argument type onMouseMoved: onMouseMoved: function(evt:MouseEvent) { println("Mouse at {evtx}, {evty}"); }
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Because the declaration of the instance variable onMouseMoved includes the information that the function argument is of type MouseEvent, it is not necessary to respecify this when assigning a handler to itAs a result, the preceding code could also be written more succinctly like this:
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// No need to restate the argument type - it is implicit onMouseMoved: function(evt) { println("Mouse at {evtx}, {evty}"); }
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Visibility of Functions
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Like variables, a function declaration can have an associated visibility modifierThe following modifiers can be applied to function declarations:
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No modifier:The function is visible only in the script in which it is declared public:The function is visible everywhere package:The function is visible in all scripts that are in the same package as the declaring script function is visible in all scripts that are in the same package as the declaring script and in any subclasses of the class that owns it See 11 for more about this
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Object Literals
As you have already seen, object literals are used to create and initialize an instance of a JavaFX class In this section, we look more closely at the syntax of an object literal