Advanced Programming in Java

Sadegh AliakbarySharif University of Technology

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AgendaEnumerationsStatic ImportAnnotation

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Enumerations

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EnumerationsSuppose you have a class with a few

instancesExample:

Student Type : <BS, MS, PhD>SMS Status : <Sent, Delivered, Not Delivered,

Not Sent>Color : <Blue, Green, Black, Red>

How do you implement it?The class should not be inheritedThe instances are limited: no further instances

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An Implementationfinal class Color{

public static final Color Black = new Color(1);public static final Color Blue = new Color(2);public static final Color Green = new Color(3);public static final Color Red = new Color(4);

private int color;private Color(int i) {this.color = i;}

}

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Applications of a private constructor

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Java enumJava introduces enumerations for this

purposeEnumerated Data TypeA simple classenum keyword instead of class or interfaceComma seperated enum instancesenum instances are constantenum Color {

Black, Blue, Green, Red}

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Enumenum Color {Black, Blue, Green, Red

}

final class Color{ public static final Color Black = new Color(); public static final Color Blue = new Color(); public static final Color Green = new Color(); public static final Color Red = new Color();

}Fall 2012

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Enum Sampleenum Shape {

Rectangle, Circle, Square}

enum StudentType{BS, MS, PhD

}

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Using EnumsColor color = Color.Black;Shape shape = Shape.Circle;show(shape, color);...private static void show (Shape shape, Color color) {//show a shape with this color...

}Fall 2012

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Enum Characteristicsenum types are implicitly final

Can not be a super-classBecause they declare constants that should not

be modifiedInstances are constants

enum constants are implicitly public, static and final

No new instances can be createdobject instantiation of enum types with

operator new results in a compilation error.

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EnumEnum can be a more complex classWith many constructorsAnd fieldsAnd methods

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enum Shape {Rectangle(1), Circle(2), Square(3);

private int number;Shape(int i){ number= i;}public int getNumber(){ return number;}

}

Shape shape = Shape.Circle;print (shape.getNumber());

shape = Shape.valueOf("Rectangle");print(shape.getNumber());

Shape[] shapesArray = Shape.values();

for (Shape s : shapesArray) {print(s.name());

}

try{shape = Shape.valueOf("Pyramid");

}catch(Exception exp){print("Pyramid is not included in Shape");

}

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Static ImportIn order to access static membersQualify references with the class they came from. For example:

double r = Math.sin(Math.PI * theta);static import allows unqualified access to static members without inheriting from the type containing the static

membersimport static java.lang.Math.PI;

orimport static java.lang.Math.*;double r = sin(PI * theta);

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AnnotationAn annotation is a special form of metadata Added to Java source codeClasses, methods, variables, parameters and

packages may be annotatedUnlike Javadoc tags, Java annotations can be

reflective They can be embedded in class files (byte code)May be retained by the Java VM at run-timeExample

@Override, @Deprecated, @ SuppressWarnings, …

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Ergonomy

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