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Code reuse

Code reuse. Using someone else's classes Aggregation and Composition Inheritance The briefest of Unified Modeling Language (UML) Choosing between composition and inheritance "Casting" from one type to another. Packages. Package is a Java construct for sharing code

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Code reuse

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  1. Code reuse • Using someone else's classes • Aggregation and Composition • Inheritance • The briefest of Unified Modeling Language (UML) • Choosing between composition and inheritance • "Casting" from one type to another ME 135 - Spring 2005 - Java object/syntax intro - 1

  2. Packages • Package is a Java construct for sharing code • A single package is typically a body of related code • Allow use of the objects in a package from your own .java file by using the Java keyword "import". ME 135 - Spring 2005 - Java object/syntax intro - 2

  3. Code slide import java.io.*; // Makes all of java.io available public class ConsoleIO { public static void main(String[] args) { BufferedReader input; input = new BufferedReader(new InputStreamReader(System.in)); while(true){ try { String s = input.readLine(); System.out.println("Input: " + s); } catch(Exception e){} } } } ME 135 - Spring 2005 - Java object/syntax intro - 3

  4. Exception handling (brief) • An exception is "thrown" when an exceptional condition (usually an error condition) is detected • When you write code using a method from a library that throws an exception, you must "catch" the exception (or your code won't compile). ME 135 - Spring 2005 - Java object/syntax intro - 4

  5. Exception handling example public class Except { public static double rnd() throws Exception { double d = java.lang.Math.random(); // Alt. to import if(d < 0.01)throw new Exception(); return d; } public static void main(String[] args) { int notcaught = 0; int caught = 0; while(true){ try{ rnd(); notcaught++; } catch(Exception e){ caught++; System.out.println((float)caught/notcaught); } } } } ME 135 - Spring 2005 - Java object/syntax intro - 5

  6. Aggregation • Aggregation is relevant when someone else has developed a class that, as it is, does something useful for you • One class "uses" another • If the "using" class goes away, the "used" class does not go away; that is, the lifetime of the used class does not depend on the lifetime of the using class ME 135 - Spring 2005 - Java object/syntax intro - 6

  7. Aggregation example class Professor { Professor(short i){id = i;} short id; } class Student { Student(int i, Professor a){advisor = a; id = i;}; Professor advisor; int id; } public class Aggregation { public static Professor p; public static void main(String[] args) { p = new Professor((short)123); func(); System.out.println("Professor: " + p); // Professor still exists here! } private static void func(){ Student s1 = new Student(23456, p); Student s2 = new Student(54321, p); } } ME 135 - Spring 2005 - Java object/syntax intro - 7

  8. Composition • One class "has" another • Distinct from aggregation because the lifetime of the two classes are the same. ME 135 - Spring 2005 - Java object/syntax intro - 8

  9. Composition example class Heart{ Heart(double m){mass = m;} private double mass; } class Head { Head(double m){mass = m;} private double mass; } class HumanBody { HumanBody(double headmass, double heartmass){ head = new Head(headmass); heart = new Heart(heartmass); } private Head head; private Heart heart; } // Continued next page……. ME 135 - Spring 2005 - Java object/syntax intro - 9

  10. Composition example public class Composition { public static void main(String[] args) { func(); // Human body, including heart and head, is gone. } private static void func() { HumanBody body = new HumanBody(2.0, 1.0); System.out.println("Head: " + body.head.mass + ", heart: " + body.heart.mass); } } ME 135 - Spring 2005 - Java object/syntax intro - 10

  11. Inheritance • Inheritance is relevant when someone has developed a class that is a generalized version of what you need • When one class "inherits" from another, the inheriting class is said to have an "is-a" relationship with the inherited class • The inheriting class is often called a subclass of the inherited class • The inherited class is often called the superclass of the inheriting class. ME 135 - Spring 2005 - Java object/syntax intro - 11

  12. Inheritance example class Bike { int rearGear; int frontGear; double gearRatio(){return (double)frontGear / rearGear;} } class MountainBike extends Bike { // Additional behavior for triple chainrings in front. } class TandemBike extends Bike { // Additional behavior for dual seats and pedals. } public class Inheritance { public static void main(String[] args) { MountainBike mb = new MountainBike(); // Subclass uses the superclass method. System.out.println("Gear ratio: " + mb.gearRatio()); } } ME 135 - Spring 2005 - Java object/syntax intro - 12

  13. Another inheritance example See this Java file.... ME 135 - Spring 2005 - Java object/syntax intro - 13

  14. UML very briefly • What is UML, and what is it used for? • We will use the static diagram and state diagrams • We will use the static diagram to show inheritance, composition, aggregation • See pages 4, 5, 6, 7 of Introduction to the UML ME 135 - Spring 2005 - Java object/syntax intro - 14

  15. Choosing between composition and inheritance • The default should be composition or aggregation • Inheritance used only when there is a clear "is-a" relationship. ME 135 - Spring 2005 - Java object/syntax intro - 15

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