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JAVA Developed at SUN by James Gosling with support from Bill Joy

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1 JAVA Developed at SUN by James Gosling with support from Bill Joy
Net-based language Descended from Oak platform independent object oriented small The Java Tutorial:

2 Goals for Java Simple Object oriented Distributed Robust Secure
easy to learn based on C/C++ small Object oriented single inheritance Distributed libraries supply protocols Robust strongly typed safe pointer model Secure Platform independent virtual machine Portable no implementation dependent data types

3 Goals for Java (cont.) Compiled and interpreted Multithreaded Dynamic

4 Java Virtual Machine Compiler translates Java into J-code
Stack-based virtual machine (JVM) No undefined or platform specific data types Other languages can be translated to J-code Interpreter is lightweight, easy to implement use widely available language, like C J-code is highly optimized J-code can be compiled on the fly

5 J-code (JVM byte codes)
How it Works Java compiler (javac) Java source Native code e.g., AWT J-code (JVM byte codes) Other classes Java interpreter (a JVM) Platform dependent Platform independent The JVM may be an interpreter an applet viewer a web browser part of an OS Classes could be loaded from filesystem or over a network JVMs use an environment variable, CLASSPATH, to find byte code (.class files) to execute

6 Java combines benefits of
A strongly typed, OO language Flexibility of an interpreted language Lisp, Perl, Tcl A smalltalk virtual machine with security protection Java byte code verifier reduces runtime checks Package structure for organizing classes into subsystems

7 Other benefits of Java Exception handling Support for multi-threading
Based on Hoare’s monitors Highly optimized Easy debugging make debugging statements dependent on a constant value, which programmer sets when done debugging compiler automatically removes unexecutable statements

8 Three levels of Security
Security manager controls access to system resources highlight windows of untrusted applications Class loader restricts classes loaded from network to interact with classes from the same location Verifier checks that incoming classes can’t forge pointers, violate access permissions, over/under flow operator stack, etc. ensures type safety

9 Java doesn’t have Macros and preprocessor
mostly used for platform dependencies Operator overloading, except for + (Very many) automatic type coercions Pointer arithmetic references are a higher level type and can only point to class objects, not to class methods Explicit memory management provides automatic garbage collection

10 Java and the Web A web browser can incorporate a JVM and run Java applets as executable code Life of an applet Loaded by a web browser and asked to initialize itself Informed each time it is displayed and hidden Informed when it is no longer needed Security manager prevents applet from accessing system resources or interacting with outside applications

11 Java classes Class is the basic computation unit
encapsulates data and associated operations found and loaded dynamically as needed 22 architecture specific classes: “gateway to the real world” networking, windowing, filesystem, etc. Rest of Java is written in Java

12 Inheritance in Java Single inheritance hierarchy
Multiple inheritance of interfaces Interface specifies the operations but not the implementations A class can “implement” multiple interfaces

13 Java coding conventions
Class names begin with upper case letters Variables and method names begin with lower case letters Constants are all upper case Separate words with uppercase letter instead of underscores e.g. aVariableName AClassName aMethodName ACONSTANT

14 Classes in Java Define an abstract data type
operations called methods data called variables or fields Many class instances or objects can exist each instance may have a different state e.g., different values for variables all instances have the same methods Arranged in a hierarchy each class has a unique superclass (parent) subclass (child) can add or modify methods or variables of the superclass

15 Variables in Java Maintain state of a class instance
Belong to some class or class instance static variable -- one per class instance variable -- one per class instance All variable values are by reference point to their values, which are maintained on a heap Initial value is null access to null value raises the NullPointerException exception

16 A simple Java applet import java.applet.Applet;
import java.awt.Graphics; public class Simple extends Applet { StringBuffer buffer; public void init( ) { buffer = new StringBuffer( ); addItem(“initializing …”); } public void start( ) { addItem(“starting …”);

17 A simple Java applet (cont.)
public void stop( ) { addItem(“stopping …”); } public void destroy( ) { addItem(“starting …”); public void addItem(String newWord ) { System.out.println(newWord); buffer.append(newWord); repaint( );

18 A simple Java applet (cont.)
public void paint (Graphics g) { //Draw a rectangle around the applet’s display area g.drawRectangle(0, 0, size( ).width-1, size( ).height-1); //Draw the current string inside the rectangle g.drawString(buffer.toString( ), 5, 15); }

19 return to page/start( ) reload/stop( ); destroy( ); init( )
Lifetime of an Applet leave page/stop( ) iconify/stop( ) load/init( );start( ); quit/destroy( ) running stopped de-iconify/start( ) return to page/start( ) reload/stop( ); destroy( ); init( )

20 Part of the Java Class Hierarchy
Object Component Button Container Window Panel Applet Simple

21 Subclassing and Inheritance
A subclass extends, refines, or specializes a superclass A subclass can extend only one superclass A subclass inherits the public methods and variables from its superclass Does not inherit private members (in Java) The subclass is considered a subtype of the superclass All visible superclass operations apply to subclass

22 Subclassing Example Container Panel Window Simple Applet . . .
add(Component) doLayout( ) getComponent(int) paint(Graphics) print(Graphics) remove(Component) Panel Window Panel( ) Panel(Layout) addNotify( ) Simple Applet getImage(URL, String) getParameter(String) play(URL) . . . Applet( ) init( ) start( ) init( ) start( ) stop( ) destroy ( ) paint (Graphics)

23 Interfaces in Java An interface class describes a protocol of behavior
Members are constants and abstract methods Abstract methods have no implementations Can be implemented by any class anywhere in the class heirarchy Cannot be instantiated Implementing classes agree to implement all methods declared in the interface Class can implement multiple interfaces Interface can be implemented by multiple classes Does not force a class relationship

24 Interface Example Objects can register themselves with an AlarmClock object to be woken up after some specified time Objects call the letMeSleepFor method: public synchronized boolean letMeSleepFor( Sleeper theSleeper, long time) { int index = findNextSlot ( ); if (index == NOROOM) { return false; } else { sleepers[ index ] = theSleeper; sleepFor[ index ] = time; new AlarmThread( index ).start( ); return true; }

25 Interface Example (cont.)
An object that wants to use AlarmClock must implement the wakeUp method This is enforced by the type of theSleeper Public interface Sleeper { public void wakeUp ( ); public long ONE_SECOND = 1000; // in milliseconds public long ONE_MINUTE = 60000; // in milliseconds }

26 Interface Example (cont.)
Any object that implements this interface can be passed to letMeSleepFor Class GUIClock extends Applet implements Sleeper { public void wakeUp ( ) { repaint ( ); clock.letMeSleepFor( this, ONE_MINUTE); } GUIClock updates its display every minute (showing the current time)

27 Abstract class v.s. Interface class
Why not use an abstract class for Sleeper? Abstract class Sleeper { public abstract void wakeUp ( ); } Only objects that are subclasses of Sleeper would be able to use AlarmClock Conceptually, AlarmClock should not force a class relationship on its users

28 Exceptional Conditions
Handling exceptional conditions can more than double the size of the code Systems can respond to errors in many ways crash give error message and crash give error message and let user retry minimize work that must be redone allow user to decide how much work must be redone correct the error allow user to confirm that correction is valid

29 Approaches for Handling Exceptional Conditions
Each method handles the exceptional conditions that arise during its execution A low level class/method handles all exceptional conditions that may arise All methods return status information so that client methods can respond to exceptional conditions ALL OF THESE APPROACHES HAVE PROBLEMS

30 I. Each method handles its own exceptional conditions
info1 (in case an exception arises) Code to detect and handle exception info1, info2 (in case an exception arises) Code to detect and handle exception (use info1) Code to detect and handle exception (use info1, info2)

31 I. Each method handles its own exceptional conditions
No modularity or consistency changes to error handling affect all the methods May need to pass considerable information many levels to maintain context information hard to provide user friendly response w/o knowing clients context Must return status information so calling method can determine if it should proceed or terminate

32 II. A low level class/method handles exceptional conditions
Exception handling class

33 II. A low level class/method handles exceptional conditions
Error processing handled in a more consistent and modular fashion changes to error handling only affect the error handling class/method May need to pass considerable information many levels to maintain context information hard to provide user friendly response w/o knowing clients context Must return status information so calling method can determine if it should proceed or terminate

34 Calling method must always check status information
III. Methods return status information so that client methods can respond to exceptional conditions Calling method must always check status information Calling methods must be able to respond to status information call Foo(bar, status1, status2, …, statusN); if status1 then do repair1; else if status2 then do repair2; else if else normal processing using bar endif

35 Exceptions were added to languages to help with error processing
A method that detects a problem can handle the exception locally and then raise/throw/signal an exception to the methods in its calling context handler for E1 throw E1

36 A method can catch an exception and specialize its response
handler for E2 handler for E1; throw E2 throw E1

37 Exception Handling Mechanisms
Signal/raise/throw an exception predefined user defined Exception handlers local non-local propagate through call stack one level only multiple levels

38 Exception Handling Mechanisms (cont.)
Execution after handler resumption model: return to signal location termination model: terminate execution of method Java supports predefined and user defined exceptions, local and multi-level propagation, with termination

39 Exceptions in Java Indicates an unusual situation (error)
Thrown at the point in the code where the error occurs Caught by an exception handler Can be handled locally Can look back through call stack for the first handler Methods must declare the exceptions they throw

40 Handling Exceptions try { i = s.pop( ); }
catch( EmptyStackException i); system.out.println( “Oops! The stack is empty!” ); i = 0;

41 Handling multiple exceptions
try { readFromFile( “foo” ); } catch( FileNotFoundException e); system.out.println( “Oops! The file is missing!” ); catch( IOException e ) “Oops! Can’t read the file!” ); finally readFromFile( “foo.bak”);

42 Try/Catch Statement Exceptions raised in the try body are handled in the catch statements Catch statements are evaluated in order first match leads to execution of the catch body Usually list exceptions from most specific to least specific If there is a finally clause then it is always executed May not execute all statements in try body could be interrupted by an exception

43 Finding Exception Handlers
Look in enclosing blocks Look in calling methods If no exception handler is found in call stack, program crashes handler for E1 propagate E1 throw E1

44 Multiple Levels of Propagation
getContent( _) { try { openConection( ); readData ( ); } catch (IOException e) { //handle IO error openConnection ( ) throws IOException openSocket( ); sendRequest( ); …. sendRequest ( ) throws IOException write (body); //write error

45 Explanation Write throws the exception
sendRequest doesn’t handle the exception but must indicate that it propagates the exception Same for openConnection getContent catch statement handles the exception May never execute the readData( ) statement in getContent

46 Throwing Exceptions int Dequeue (Queue q) throws QueueEmpty {
if ( q.head == q.tail ) throw new QueueEmpty ( ); } else q.head = q.head + 1; return q.contents [q.head - 1]; class QueueEmpty extends Exception

47 Types of Java Exceptions
General exceptions Must be explicitly thrown Should be handled by all programs Runtime exceptions Frequent runtime problems No need to explicitly state that such an exception might be thrown Runtime message generated if they are not caught

48 Summary of Exception Handling
Exceptions allow the programmer to separate the handling of unusual cases from expected ones Program should catch predefined exceptions and throw more specific exceptions when possible Exception handling is difficult, even with exception handlers Exception handling is an important part of most programs

49 Concurrent System Multiple threads of execution
Logically concurrent: share a single processor Physically concurrent: multiple processors Run independently, for the most part Typically, need to communicate Share data Pass messages Typically, need to synchronize their activities

50 Threads in Java A thread is a sequential flow of control within a program has a beginning, an execution sequence, and an end cannot be run on its own, but exists only within a larger program A program with three threads

51 Defining the behavior of a thread
The behavior of a thread in Java is given by a special method the run method Two techniques for providing a run method for a thread Subclass Thread and override the run method Implement the Runnable interface

52 Defining thread behavior: Subclassing Thread
A thread that computes names larger than a given value class PrimeThread extends Thread { long minPrime; PrimeThread ( long minPrime ) { this.minPrime = minPrime; } public void run ( ) { // compute primes larger than minPrime Code to create a PrimeThread thread and start running it: PrimeThread p = new PrimeThread(143); p.start ( );

53 Defining thread behavior: Implementing runnable
A thread that computes names larger than a given value class PrimeRun implements Runnable { long minPrime; PrimeRun ( long minPrime ) { this.minPrime = minPrime; } public void run ( ) { // compute primes larger than minPrime Code to create a PrimeThread thread and start running it: PrimeRun p = new PrimeRun(143); new Thread( p ).start ( );

54 When should you implement Runnable?
If a class needs its own thread and must subclass some other class, you should implement Runnable Example: Suppose you want an applet that displays the time, updating it every second It has to subclass Applet to run in a browser It needs its own thread in order to continuously update its display without taking over the process that its running in

55 Life Cycle of a Thread in Java
running waiting new Thread ( ) start ( ) not runnable new executing run method terminates stopped Transitions to not runnable invokes its sleep method invokes a wait method for some condition blocks on an IO operation Transitions to running specified time elapses notify method is invoked to signify the condition is met the IO operation completes

56 Synchronizing Threads in Java
A lock is associated with objects containing synchronized methods The object is locked when a thread enters one of its synchronized methods Other threads cannot enter a synchronized method on the same object until the object is unlocked Lock is acquired and released automatically by the Java Runtime System

57 Synchronizing Threads in Java (cont.)
Threads that use synchronized methods are coordinated using wait and notify (or notifyAll) Invoking the wait method blocks the thread and releases the lock An object invokes notify to wake up a thread that is waiting on it

58 Producer/Consumer Example
public class CubbyHole { private int contents; private boolean available = false; public synchronized int put ( ) { //CubbyHole is locked by the Producer //CubbyHole is unlocked by the Producer } public synchronized int get ( ) { //CubbyHole is locked by the Consumer //CubbyHole is unlocked by the Consumer

59 Producer/Consumer Example (cont.)
public synchronized int get ( ) { while ( available == false ) { try { // wait for Producer to put value wait ( ); } catch ( InterruptedException e) { } available = false; // notify Producer that value has been retrieved notifyAll ( ); return contents;

60 Producer/Consumer Example (cont.)
public synchronized void put ( int value ) { while ( available == true ) { try { // wait for Consumer to get value wait ( ); } catch ( InterruptedException e) { } contents = value; available = true; // notify Consumer that value has been set notifyAll ( );

61 Concurrency Summary A thread is a sequential flow of control
Multiple threads execute concurrently within the same program Objects with synchronized methods implement monitors monitors use the wait and notify methods to coordinate the activities of the threads that they serve


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