Properties in Java

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Properties, or instance variables, are variables that are part of an object. They can be used to store attributes of an object or other objects it owns or is keeping track of. They can be de ned like any other variable, and they have the same access control attributes as methods. Constants are special
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4: Object Oriented Programming
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properties that don t change values and are declared with the const keyword instead of var. Both constants and properties default to internal when no access control attribute is speci ed. Figure 4-7 illustrates the declaration of an instance variable.
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FIGURE 4-7
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A vivisected instance variable.
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internal var toastiness:Number = 0;
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access control attribute var or const identifier type initial value
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The access control modi er, type, and initial assignment in a declaration like that in Figure 4-7 are optional. Like methods, properties default to internal when another access control modi er is not speci ed. Just like variables, properties are nouns and adjectives and should be named as such. They can be singular or plural. They should have descriptive names and be typed in camel case. Properties should be declared at the top of a class de nition, before the constructor. The choice to use initial values or not can be one of personal style. Any assignment that can be made at the declaration time of the property may also be made inside the constructor of the class with no ill effect. If initial values are used at all, they are typically used for Boolean properties, constant numbers and strings, and other simple values.
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Accessors
When the class is a simple data type, there is no shame in using public properties. For example, a Point3D class that just collects three values is a good use of public properties:
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package { public class public var public var public var } } Point3D { x:Number; y:Number; z:Number;
In general, however, public properties should be avoided in favor of accessors. Changing the property of an object by assigning directly to the property doesn t allow that object the opportunity to react to the change, and it shifts the burden of updating the object to every other method that might need to know if the value has changed. It doesn t give the object an opportunity to ensure the value assigned to its property is valid, so invalid input or code can easily trip up a perfectly working object. Accessors x this problem by allowing you to execute code when assignments are made to and when values are retrieved from an object. You can use two kinds of accessors in ActionScript 3.0: explicit and implicit. Explicit accessors are normal functions that can retrieve and modify a property. There are no rules to explicit accessors, and they are not a special part of the language. They are typically implemented like this:
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Part I: ActionScript 3.0 Language Basics
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package com.actionscriptbible.ch4.accessors { public class BuildingExplicit { //the height of the building in feet private var _height:Number = 0; public function setHeight(newHeight:Number):void { if (!isNaN(newHeight) && newHeight >= 0 && isFinite(newHeight)) { _height = newHeight; updateFloors(); } else { throw new RangeError("Height must be finite and positive"); } } public function getHeight():Number { return _height; } private function updateFloors():void { //Make sure the number of floors //jives with the height of the building } } }
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The real, internal property is made to be private. One convention for handling properties with both a private version and public accessors is to name the private version of the property with a preceding underscore. Two normal methods are added to set and retrieve the property. These should be named setFoo() and getFoo() for the property named foo, and they are appropriately called the setter and the getter. In this example, the setter ensures that the attempted assignment is valid and only proceeds if it is. If an invalid assignment is attempted, the class throws a runtime error. For more on error handling, see 24, Errors and Exceptions. The setter also takes the opportunity to update the building s oors based on the new height. Otherwise, you always have to be on your feet, checking that the two are in sync. By handling that at the only point it might happen, you can eliminate the problem. Another thing you can do with accessors is to work with derived properties: values that aren t stored but are calculated on demand, masquerading as normal properties. For example, you could add to the Building class an accessor for the height of the building in inches:
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public function getHeightInches():Number { return getHeight() * 12; }
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When using explicit accessors, you must use method calls to manipulate the public properties of the class.
var b:BuildingExplicit = new BuildingExplicit(); b.setHeight(100); trace("The building is", b.getHeight(), "feet tall");
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The second way to use accessors enables you to use dot notation as if the properties were public, but still intercept the assignment to do whatever you require. Implicit accessors are a special language feature and use the keywords get and set before the property names to de ne accessor functions. In other words, just add a space between set and height in the example:
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