CS221

1. Week 1 - Wednesday CS221

2. Last time • What did we talk about last time? • Course overview • Policies • Schedule

3. Questions?

4. OOP

5. What is an object? • Members • Methods • Why are they useful?

6. What is a class? • A template or prototype for an object

7. Object Oriented Programming • Encapsulation • Dynamic dispatch • Polymorphism • Inheritance • Self-reference

8. Encapsulation • Information hiding • We want to bind operations and data tightly together • Consequently, we don't want you to touch our privates • Encapsulation in Java is provided by the private and protected keywords (and also by default, package level access) • Hardcore OOP people think that all data should be private and most methods should be public

9. Encapsulation example public class A { privateint a; public intgetA() { return a; } public voidsetA(int value) { a = value; } }

10. Inheritance • Allows code reuse • Is thought of as an is-a relationship • Java does not allow multiple inheritance, but some languages do • Deriving a subclass usually means creating a "refined" or "more specific" version of a superclass

11. Inheritance example public class B extends A { //has member and methods from A } public class C extends A { //has A stuff and more private intc; public intgetC(){ return c; } void increment() { c++; } }

12. Polymorphism • A confusing word whose underlying concept many programmers misunderstand • Polymorphism is when code is designed for a superclass but can be used with a subclass • If AudiRS5 is a subtype of Car, then you can use an AudiRS5 anywhere you could use a Car

13. Polymorphism example //defined somewhere public void drive( Car car ) { … } public class AudiRS5 extends Car { … } Car car = new Car(); AudiRS5 audi = new AudiRS5(); drive( audi ); //okay drive( car ); //okay

14. Dynamic dispatch • Polymorphism can be used to extend the functionality of an existing method using dynamic dispatch • In dynamic dispatch, the method that is actually called is not known until run time

15. Dynamic dispatch example public class A { publicvoid print() { System.out.println("A"); } } public class B extends A { publicvoid print() { System.out.println("B"); } }

16. Dynamic dispatch example A a = new A(); B b = new B(); // B extends A A c; a.print(); // A b.print(); // B c = a; c.print(); // A c = b; c.print(); // B

17. Self-reference • Objects are able to refer to themselves • This can be used to explicitly reference variables in the class • Or, it can be used to provide the object itself as an argument to other methods

18. Self reference example public class Stuff { privateint things; public voidsetThings(intthings) { this.things = things; } }

19. Self reference example public classSelfAdder { publicvoidaddToList(List list) { list.add(this); } }

20. Constructor syntax • Java provides syntax that allows you to call another constructor from the current class or specify which superclass constructor you want to call • The first line of a constructor is a call to the superclass constructor • If neither a this() or a super() constructor are the first line, an implicit default super() constructor is called

21. Constructor example public class A { private double half; public A(int value) { half = value / 2.0; } } public class B extends A { publicB(int input) { super(input); // calls super constructor } publicB() { this(5); // calls other constructor } }

22. Interfaces

23. Interface basics • An interface is a set of methods which a class must have • Implementingan interface means making a promise to define each of the listed methods • It can do what it wants inside the body of each method, but it must have them to compile • Unlike superclasses, a class can implement as many interfaces as it wants

24. Interface definition • An interface looks a lot like a class, but all its methods are empty • Interfaces have no members except for (static final) constants public interface Guitarist { voidstrumChord(Chord chord); voidplayMelody(Melody notes); }

25. Interface use public class RockGuitaristextendsRockMusicianimplements Guitarist { public voidstrumChord( Chord chord ) { System.out.print( "Totally wails on that "+ chord.getName() + " chord!"); } public void playMelody( Melody notes ) { System.out.print( "Burns through the notes " + notes.toString() + " like Jimmy Page!"); } }

26. Usefulness • A class has an is-a relationship with interfaces it implements, just like a superclass it extends • Code that specifies a particular interface can use any class that implements it public static void perform(Guitarist guitarist, Chord chord, Melody notes) { System.out.println("Give it up " +"for the next guitarist!"); guitarist.strumChord( chord ); guitarist.playMelody( notes ); }

27. Exceptions

28. Exceptions • Java handles errors with exceptions • Code that goes wrong throws an exception • The exception propagates back up through the call stack until it is caught • If the exception is never caught, it crashes the thread it's running on, often crashing the program

29. Kinds of exceptions • There are two kinds of exceptions: • Checked • Unchecked • Checked exceptions can only be thrown if the throwing code is: • Surrounded by a try block with a catch block matching the kind of exception, or • Inside a method that is marked with the keyword throws as throwing the exception in question • Unchecked exceptions can be thrown at any time (and usually indicate unrecoverable runtime errors)

30. Examples of exceptions • Checked exceptions • FileNotFoundException • IOException • InterruptedException • Any exception you write that extends Exception • Unchecked exceptions • ArrayIndexOutOfBoundsException • NullPointerException • ArithmeticException • Any exception you write that extends Error or RuntimeException

31. Exceptions in code Scanner in = null; try { in = new Scanner( file ); while( in.hasNextInt() ) process( in.nextInt() ); } catch( FileNotFoundException e ) { System.out.println ("File " + file.getName() + " not found !"); } finally { if( in != null ) in.close(); }

32. Threads

33. Threads • In Java, concurrency and parallelism are achieved primarily through threads • A thread is a path of code execution that runs independently of others • But threads can share memory with each other • Some other languages use message passing semantics and no direct memory sharing

34. Creating threads in Java • To create a customized thread in Java, there are two routes: • Create a class which is a subclass of Thread • Create a class which implements the Runnable interface • If you subclass Thread, you can't subclass some other object • If you implement Runnable, there's an extra bit of syntax to create a new thread

35. Sample thread to sum things public class Summer extends Thread { private double[] array; private intstart; private intend; private double result = 0.0; public Summer(double[] array, intstart, intend) { this.array = array; this.start = start; this.end = end; } public void run() { for( inti = start; i < end; i++ ) result += array[i]; } public double getResult() { return result; } }

36. Using Summer public double findSum( double[] array, intthreads ) throwsInterruptedException { Summer[] summers = new Summer[threads]; double result = 0.0; intstep = array.length / threads; if( array.length % threads > 0 ) step++; for( inti = 0; i < threads; i++ ) { summers[i] = new Summer(array, i*step, Math.min((i+1)*step, array.length)); summers[i].start(); } for( inti = 0; i < threads; i++ ) { summers[i].join(); result += summers[i].getResult(); } return result; }

37. Tips for concurrency • Keep it simple • Do not share data via static variables • Be careful with load balancing • If the work is not evenly divisible by n (the number of threads), give the first n – 1 threads one extra piece of work • Why? • Be aware that threading out a program will not necessarily make it faster

38. Upcoming

39. Next time… • Inner classes • Generics • Java Collection Framework

40. Reminders • Continue to read Chapter 1 • Keeping brushing up on Java if you are rusty • Project 1 will be assigned Friday • Send me an e-mail with your team members in it by Friday, August 29