4: Object Oriented Programming in Java

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package com.actionscriptbible.ch4.accessors { public class BuildingImplicit { //the height of the building in feet private var _height:Number = 0; public function set height(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 get height():Number { return _height; } private function updateFloors():void { //Make sure the number of floors //jives with the height of the building } public function get heightInches():Number { return height * 12; } } }
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To access the properties, you can use dot notation and still reap the bene ts of the range checking code:
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var b:BuildingImplicit = new BuildingImplicit(); b.height = 1000; b.height = -12; //Runtime Error: Height must be finite and positive
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You can also use implicit accessors to make a property read-only. Include a public implicit getter but omit the setter. The compiler catches any attempts at setting the property from outside the class. In the Building example, the property heightInInches is, in addition to being derived, read-only.
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I just showed how to execute arbitrary code when a property assignment is made, but it s important that I issue a warning here. It s a good idea to use accessors to control access to properties and to ensure that a modi ed property has an immediate effect on anything that depends on it. However, when you call on an object to perform some task and in addition it performs something unrelated, this is known as a side effect, and although it is often convenient, you should generally avoid it. You should design your classes, methods, and properties so that they do what their names imply, and no more. For example, say you wanted to repaint your building a new color to match its height, or create new elevator banks, every time the height is changed. These would be nice, but they make changes to the Building that should probably be calculated and made explicit. In fact, one could even argue that
Part I: ActionScript 3.0 Language Basics
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adding oors is a side effect. Only change or update parts of the class that are absolutely dependent on the property that s changing. Methods as well as accessors can have side effects, so remember that setting values and performing actions should remain separate. It can be useful to use implicit accessors to beef up handling of values, but don t use it to confuse adjectives with verbs.
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In 2, ActionScript 3.0 Language Basics, you learned about scope. Local scopes are created by blocks code enclosed within curly braces within which identi ers and names are unique. When you re writing methods in classes, several scopes are open at the same time, because the package, class, and method all have their own code blocks:
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package com.actionscriptbible.ch4 { public class Camera { private var aperture:Number; private var shutterSpeed:Number; private var focalLength:Number; private var currentFrame:Number; public function takePhoto() { var image:Image = new Image(); openShutter(); accumulateImage(image, shutterSpeed); closeShutter(); currentFrame++; } } }
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The open blocks are made more obvious by the fact that each one indents its contents further. Code inside the takePhoto() method can access variables de ned in the method block like image, instance variables de ned in the class like shutterSpeed, as well as any other items de ned in the package, if there were any. This is because of the way ActionScript code searches up the scope chain from the most local open scope to the most global. Because each block de nes a namespace, you can easily have local variables inside a method with the same name as instance variables de ned on the class level. Inside the method, the local variables take precedence, since the method block is the most local. Outside of the method, those variables don t even exist.
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package com.actionscriptbible.ch4 { import com.actionscriptbible.Example; public class ScopeTest extends Example { private var a:String = "apricot"; public function ScopeTest() { testA(); testB(); } private function testA():void { var a:String = "acid"; trace(a); //acid; trace(b); //ERROR: Access of undefined property b trace(this.a); //apricot }
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