An Introduction to Java Programming Language

1. Vishnuvardhan.M An Introduction to Java Programming Language

2. Can you make coffee with it?

3. It was meant to!! A programming language for appliances!

4. Must Run on Any Architecture debug “WRITE ONCE, RUN ANYWHERE!” pretty portable Program in Java Java Compiler Java Bytecode Java Virtual Machine Java Virtual Machine

5. Doesn’t Make Coffee Yet

6. So What’s Java Good For? Web applications! Java Applet Java Applet Server

7. Applet Java on the Web: Java Applets • Clients download applets via Web browser • Browser runs applet in a Java Virtual Machine (JVM) Interactive web, security, and client consistency Slow to download, inconsistent VMs (besides, flash won this war) Client Server

8. Servlets JSPs EJB JDBC Java on the Web: J2EE • Thin clients (minimize download) • Java all “server side” THIS IS WHAT YOU’LL BE DOING!! Client Server

9. The Java programming environment • Compared to C++: • no header files, macros, pointers and references, unions, operator overloading, templates, etc. • Object-orientation: Classes + Inheritance • Distributed: RMI, Servlet, Distributed object programming. • Robust: Strong typing + no pointer + garbage collection • Secure: Type-safety + access control • Architecture neutral: architecture neutral representation • Portable • Interpreted • High performance through Just in time compilation + runtime modification of code • Multi-threaded

10. Java Features • Well defined primitive data types: int, float, double, char, etc. • int 4 bytes [–2,147,648, 2,147,483,647] • Control statements similar to C++: if-then-else, switch, while, for • Interfaces • Exceptions • Concurrency • Packages • Name spaces • Reflection • Applet model

11. The Java programming environment • Java programming language specification • Syntax of Java programs • Defines different constructs and their semantics • Java byte code: Intermediate representation for Java programs • Java compiler: Transform Java programs into Java byte code • Java interpreter: Read programs written in Java byte code and execute them • Java virtual machine: Runtime system that provides various services to running programs • Java programming environment: Set of libraries that provide services such as GUI, data structures,etc. • Java enabled browsers: Browsers that include a JVM + ability to load programs from remote hosts

12. Java: A tiny intro • How are Java programs written? • How are variables declared? • How are expressions specified? • How are control structures defined? • How to define simple methods? • What are classes and objects? • What about exceptions?

13. How are Java programs written? • Define a class HelloWorld and store it into a file: HelloWorld.java: public class HelloWorld { public static void main (String[] args) { System.out.println(“Hello, World”); } } • Compile HelloWorld.java javac HelloWorld.java Output: HelloWorld.class • Run java HelloWorld Output: Hello, World

14. How are variables declared? Fibonacci: class Fibonacci { public static void main(String[] arg) { int lo = 1; int hi = 1; System.out.println(lo); while (hi < 50) { System.out.println(hi); hi = lo + hi; lo = hi – lo; } } }

15. How to define expressions? • Arithmetic: +, -, *,/, %, = 8 + 3 * 2 /4 Use standard precedence and associativity rules • Predicates: ==, !=, >, <, >=, <= public class Demo { public static void main (String[] argv) { boolean b; b = (2 + 2 == 4); System.out.println(b); } }

16. How are simple methods defined? Every method is defined inside a Java class definition public class Movie { public static int movieRating(int s, int a, int d) { return s+a+d; } } public class Demo { public static void main (String argv[]) { int script = 6, acting = 9, directing = 8; displayRating(script, acting, directing); } public static void displayRating(int s, int a, int d){ System.out.print(“The rating of this movie is”); System.out.println(Movie.movieRating(s, a, d)); } }

17. How are control structures specified? Typical flow of control statements: if-then-else, while, switch, do-while, and blocks class ImprovedFibo { static final int MAX_INDEX = 10; public static void main (String[] args) { int lo = 1; int hi = 1; String mark = null; for (int i = 2; i < MAX_INDEX; i++) { if ((i % 2) == 0) mark = " *"; else mark = ""; System.out.println(i+ ": " + hi + mark); hi = lo + hi; lo = hi - lo; }}}

18. What are classes and objects? Classes: templates for constructing instances • Fields • Instance variables • Static variables • Methods • Instance • Static class Point { public double x, y; } Point lowerleft = new Point(); Point upperRight = new Point(); Point middlePoint = new Point(); lowerLeft.x = 0.0; lowerLeft.y = 0.0; upperRight.x = 1280.0; upperRight.y = 1024.0 middlePoint.x = 640.0; middlePoint.y = 512.0

19. How are instance methods defined? Instance methods take an implicit parameter: instance on which method is invoked public class Movie { public int script, acting, directing; public int rating() { return script + acting + directing; } } public class Demo { public static void main (String argv[]) { Movie m = new Movie(); m.script = 6; m.acting = 9; m.directing = 8; System.out.print(“The rating of this movie is”); System.out.println(m.rating()); } }

20. How to extend classes? • Inheritance: mechanism for extending behavior of classes; leads to construction of hierarchy of classes [Note: no multiple inheritance] • What happens when class C extends class D: • Inherits instance variables • Inherits static variables • Inherits instance methods • Inherits static methods • C can: • Add new instance variables • Add new methods (static and dynamic) • Modify methods (only implementation) • Cannot delete anything

21. How to extend classes? public class Attraction { public int minutes; public Attraction() {minutes = 75;} public int getMinutes() {return minutes;} public void setMinutes(int d) {minutes = d;} } public class Movie extends Attraction { public int script, acting, directing; public Movie() {script = 5; acting = 5; directing = 5;} public Movie(int s, int a, int d) { script = s; acting = a; directing = d; } public int rating() {return script + acting + directing;} } public class Symphony extends Attraction { public int playing, music, conducting; public Symphony() {playing = music = conducting = 5;} public Symphony(int p, int m, int c) { playing = p; music = m; conducting = c; } public int rating() {return playing + music + conducting;} }

22. What are abstract classes? • Abstract class: Merely a place holder for class definitions; cannot be used to create instances.; public abstract class Attraction { public int minutes; public Attraction() {minutes = 75;} public int getMinutes() {return minutes;} public void setMinutes(int d) {minutes = d;} public abstract void m(); } • Following is an error: Attraction x; x = new Attraction(); • Following is not an error: public class Movie extends Attraction { … } public class Symphony extends Attraction { … } Attraction x; x = new Movie (); x = new Symphony();

23. Object extends Attraction Auxiliaries Demonstration extends Symphony Movie Packages • How do we organize above classes into a single unit? Put them in file? • However, only one public class per file (whose name is same as file’s) • Solution: Place several files (compilation units) into a package

24. Packages – cont’d. • units of organizing related Classes, Interfaces, Sub packages • Why? • Reduce name clashing • Limit visibility of names • Java programs typically organized in terms of packages and subpackages • Each package may then be divided into several packages, subpackages, and classes • Each class can then be stored in a separate file • Each source file starts with something like: package mypackage; • Code in source file is now part of mypackage

25. Packages – cont’d. package onto.java.entertainment; public class Movie extends class Attraction {…} package onto.java.entertainment; public abstract class Attraction { … } package onto.java.entertainment; import java.io.*; import java.util.*; public class Auxiliaries { … } • Where to store packages? • How does Java find packages? • Export and Import • Access control

26. Exceptions public class A { public void foo() throws MyException { if(aBadThingHappened()) { throw new MyException(); } } public void bar() { try { this.foo(); } catch (MyException e) { e.printStackTrace(); } } } public class MyException extends Exception { public MyException() {} public MyException(String message) { super(String message); } }

27. Finally public class A { public void foo() throws MyException { throw new MyException(); } } public void bar() { try { this.foo(); } catch (MyException e) { e.printStackTrace(); } catch (YourException e) { e.printStackTrace(); } finally { ... // always executed before leaving the try/catch } } }

28. Resources • http://java.sun.com/ • Java[tm] 2 Platform, Standard Edition v1.4.1 • java, javac, jar, jre, etc. • Any platform... FREE! • Online documentation and tutorials • http://www.eclipse.org/ • Integrated development environment (IDE) for nothing in particular • Java[tm] development tools (JDT) (comes with Eclips) • Project management • Editor • Incremental compiler • CVS support • C/C++ extension in progress • AspectJ support • Windows, Linux, and Mac.... FREE!

29. Qualifiers • public – any class* may access • (no qualifier) “package protected” – only the class* and classes* in the same package may access • protected – only the class* and decendent classes* may access • private – only the class* may access • The class or instances of the class (an object of the class)

30. Package Protected package edu.ucdavis; public class A { int x; } package edu.ucdavis; public class B { void foo(A a) { a.x; } // OK, same package } package org.omg; public class B { void foo(A a) { a.x; } // Not OK, different package } package edu.ucdavis.cs; public class B { void foo(A a) { a.x; } // Not OK, different package } package edu.ucdavis.cs; public class B { void foo(A a) { a.x; } // Not OK, different package } package edu; public class B { void foo(A a) { a.x; } // Not OK, different package }

31. Protected public class A { protected int x; } public class B extends A { void foo(A a) { this.x; a.x; } // OK, B is a decendent of A } public class C extends B { void foo(A a) { this.x; a.x; } // OK, C is a decendent of A through B } package edu; // Uh oh! public class D extends C { void foo(A a) { this.x; a.x; } // OK, D is a decendent of A } public class E { void foo(A a) { this.x; a.x; } // NOT OK, E is NOT a decendent of A }

32. Threads • Multiple “threads” of execution within the same program, share the same memory space -> “lightweight”. • Perform multiple tasks at the same time. • Work on the same task in parallel. • Heavily used in user interfaces. • Web browsers: load web pages while the user can still scroll, go back, open a new window, etc. • Web servers: serve multiple requests in parallel. • Can take advantage of multiple processors. • Threads in Java • Java manages and schedules threads • Java provides “synchronize” to help coordinate multiple threads

33. Creating a Thread in Java public class MyThread extends Thread { public MyThread(String threadName) { super(threadName); } public void run() { for(int i = 0; i < 10; i++) { System.out.println(i + “ “ + getName()); try { sleep((long)(Math.random() * 1000)); } catch(InterruptedException e) {} } } }

34. Creating a Thread in Java public class ThreadTest { public static void main(String[] args) { for(int i = 0; i < args.length; i++) { MyThread t = new MyThread(args[i]); t.start(); } } } > java ThreadTest Bob Frank 0 Bob 0 Frank 1 Bob 2 Bob 1 Frank 3 Bob 2 Frank 3 Frank 4 Frank ...

35. Creating a Thread in Java via Interface public class MyRunnable implements Runnable { String name; public MyRunnable(String name) { this.name = name; } public void run() { for(int i; i < 10; i++) { System.out.println(i + “ “ + name()); try { sleep((long)(Math.random() * 1000)); } catch(InterruptedException e) {} } } } public class ThreadTest { public static void main(String[] args) { for(int i = 0; i < args.length; i++) { Thread t = new Thread(new MyRunnable(args[i]), args[i]); t.start(); } } }

36. public class Producer extends Thread { private Share shared; public Producer(Share s) { shared = s; } public void run() { for(int i = 0; i < 10; i++){ shared.put(i); } } } shared.put(0) shared.get() // 0 gotten shared.get() // 0 gotten again!! shared.put(0) shared.put(1) shared.get() // 0 never gotten!! public class Consumer extends Thread { private Share shared; public Consumer(Share s) { shared = s; } public void run() { int value; for(int i = 0; i < 10; i++) { value = shared.get(); } } } // what about simultaneous // access?! shared.put(0) shared.get() RACE CONDITIONS! Producer Consumer Problem

37. Synchronized public class Share { private int s; public synchronized int get() { ... } public synchronized void put(int s) { ... } } • Synchronized provides mutual exclusion on an object • For any object, only one thread may execute inside any of that object’s synchronized methods Share s1 = new Share(); Share s2 = new Share(); Thread t1 = ...; Thread t2 = ...; t1 -> s1.get() // gets in t2 -> s1.put(32) // blocks t1 -> s1.get() // gets in t2 -> s2.put(4) // gets in

38. Producer Consumer Coordination public class Share { private int s; private boolean empty = true; public synchronized int get() { while (empty == true) { try { wait(); // nothing to get, wait } catch (InterruptedException e) {} } empty = true; notifyAll(); // wakeup waiting Consumers/Producers return s; } public synchronized void put(int s) { while (empty == false) { try { wait(); // no room } catch (InterruptedException e) {} } this.s = s; empty = false; notifyAll(); // wakeup waiting Consumers/Producers } }