JavaFX Script Classes in Java

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11 JavaFX Script Classes
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// This function is public, not script-private override function toString():String {
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Because the function
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toString()
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function in the base class is public, so is the overriding
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Overrides and Return Type In most cases, the return type of an overriding function will be the same as that of the function that it overrides For example, the contains() functions in the ExampleRectangle class and the ExampleShape class both explicitly state that a Boolean is returned:
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public override bound function contains(point:Point2D):Boolean {
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As a shorthand, it is permissible to omit the return type when overriding a function, so the override in the ExampleRectangle class could also be written like this:
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// Both the visibility (public) and the return type (Boolean) are // inferred from the base class declaration override bound function contains(point:Point2D) {
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The return type is not required to be the same as that of the overridden function it can instead by a subtype of the overridden function s return type (This is referred to as covariance) For example, the clone() method of javalangObject is declared like this:
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protected Object clone() throws CloneNotSupportedException
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On lines 41 through 48 of Listing 11-3, there is an implementation of clone() for the ExampleRectangle classWe could have declared the overriding clone() function like this:
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public override function clone() {
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The compiler would infer a return type of Object for this function when the class is compiledThis would be fine, but it doesn t help with type inference when this function is used by code outside the classWith the preceding declaration, the compiler would infer a type of Object for the variable newRect in the following code:
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var rect = ExampleRectangle { x: 0 y: 0 width: 100 height: 200 }; var newRect = rectclone(); // newRect is of type Object
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We can, instead, change the return type to ExampleRectangle, because this is a subclass of the return type of the overridden function:
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public override function clone():ExampleRectangle {
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With this change, the compiler can infer the correct type for the variable newRect:
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var rect = ExampleRectangle { x: 0 y: 0 width: 100 height: 200 }; var newRect = rectclone(); // newRect is of type ExampleRectangle
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Function Dispatch When a function is overridden, the selection of which version to implement is made at runtime based on the actual type of the object on which the function is called Consider the following class hierarchy, in which the toString() function is overridden in all three classes:
class A { public override function toString():String { "A" } } class B extends A { public override function toString():String { "B" } } class C extends B { public override function toString():String { "C" } }
Now suppose we create three instances of class C, but assign each of them to a variable of a different type and then invoke the toString() function of all three instances:
var a:A = C {}; println(a); var b:B = C {}; println(b); var c:C = C {}; println(c);
The output from this code is the following:
C C C
This happens because all three objects are actually instances of class C, and the variant of toString() provided by that class is executed, even though in the first two cases the variable type is not declared to be C Using an instance of class B, on the other hand, causes B s implementation of toString() to be called:
var a:A = B {}; println(a); // Prints "B" var b:B = B {}; println(b); // Prints "B"
11 JavaFX Script Classes
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You have seen that it is possible to invoke a superclass function by using the keyword super For example, we could rewrite the toString() function of the preceding class C like this:
public override function toString():String { "C + {supertoString()}" }
And the result of calling this function is as follows: