Variables and Data Types in Java

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5 Variables and Data Types
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You can declare a variable that refers to any instance of any object by using that object s classname as the type For example, the following code declares a variable that references an instance of the javafxstageStage class:
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import javafxstageStage; var stage:Stage = Stage { title: "Stage Title" };
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The declared type of the variable may be a JavaFX class, a Java class, or a Java interface4:
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import import import import javalangComparable; javatextDateFormat; javautilGregorianCalendar; javafxstageStage; // A JavaFX class // A Java class // A Java interface
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var stage:Stage = Stage { title: "Stage Title" }; var df:DateFormat = DateFormatgetDateInstance(); var comp:Comparable = new GregorianCalendar();
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If the type is a JavaFX or Java class, any value that is subsequently assigned to it must be either an instance of that specific class or of a subclass For example, the following code is valid because Text is a subclass of Node, both being classes in the GUI library:
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import javafxsceneNode; import javafxscenetextText; var node:Node = Text {}; // Text is a subclass of Node
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If the type is an interface, its value must be an instance of a JavaFX or Java class that implements that interface:
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// GegorianCalendar implements Comparable var comp:Comparable = new GregorianCalendar();
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The compiler checks that the type of a variable is compatible with any value assigned to it and will reject any attempt to perform an incompatible assignment As a special case, any object variable may be assigned the distinguished value null, which is typically used to mean no object Unlike Java, dereferencing an object reference that has the value null does not throw a NullPointerException Instead, the result is the default value of the data type being referenced For example, the visible instance variable of the Stage class is a Boolean, for which the default value is false, so the following code prints false:
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import javafxstageStage; var stage:Stage = null;
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This list does not contain JavaFX interface because there is no such thing!
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println(stagevisible);
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// Dereferencing null yields the default // value of the type - here, Boolean
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JavaFX classes may have state (represented by its instance variables) and functions that can be invoked to have the class perform an operation and possibly return a valueThe public functions and instance variables of a class make up its application programming interface (API)The API for the classes in the JavaFX runtime can be found in the reference documentation that is delivered with the JavaFX SDK JavaFX classes can also have instance variables and functions that are not part of its public interface and which are therefore not available to application codeWhether a function or an instance variable is part of the API is determined by its visibility and access modifiers See the section Visibility of Variables, later in this chapter, the section Visibility of Functions in 6, and the discussion of JavaFX classes in 11 for details
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Creating a JavaFX Object Instance
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To create a JavaFX object with the default values for all of its instance variables, use the following syntax:
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import javafxstageStage var stage:Stage = Stage {};
The curly brackets after the classname form an object literal, which, in this case, is emptyAn object literal is used to initialize an object s instance variables while it is being instantiatedThe following code creates a Stage and specifies initial values for its title and visible instance variables:
var stage:Stage = Stage { title: "Stage Title" visible: true };
An instance variable is initialized by quoting its name followed by a colon and then an expression that supplies the required value Instance variables for which there is no initializer are given their default value, which is taken from the class definition Object literals can also contain local variable declarations and bindings and are discussed further in 6