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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: http://java.sun.com/docs/books/tutorial
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Goals for Java

Simple
» easy to learn
– based on C/C++
» small

Object oriented
» single inheritance

Distributed
Robust
strongly typed
safe pointer model
Secure
Platform independent
virtual machine
Portable
no implementation
dependent data types
» libraries supply
protocols
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Goals for Java (cont.)



Compiled and interpreted
Multithreaded
Dynamic
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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
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How it Works
Java
compiler
(javac)
Java
source
Platform
independent

J-code
(JVM byte codes)
The JVM may be
»
»
»
»
Native
code
an interpreter
an applet viewer
a web browser
part of an OS
Other classes


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Java
interpreter
(a JVM)
e.g.,
AWT
Platform
dependent
Classes could be loaded from
filesystem or over a network
JVMs use an environment variable,
CLASSPATH, to find byte code
(.class files) to execute
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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
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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
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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
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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
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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
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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
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Inheritance in Java

Single inheritance hierarchy

Multiple inheritance of interfaces
» Interface specifies the operations but not the
implementations
» A class can “implement” multiple interfaces
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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
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ACONSTANT
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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
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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
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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 …”);
}
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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( );
}
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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);
}
}
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Lifetime of an Applet
leave page/stop( )
iconify/stop( )
quit/destroy( )
load/init( );start( );
running
stopped
de-iconify/start( )
reload/stop( ); destroy( ); init( )
return to page/start( )
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Part of the Java Class Hierarchy
Object
Component
Button
Container
Window
Panel
Applet
Simple
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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
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Subclassing Example
Container
Container( )
add(Component)
doLayout( )
getComponent(int)
Window
paint(Graphics)
print(Graphics)
remove(Component)
...
Panel
Panel( )
Panel(Layout)
addNotify( )
Simple
Applet
Applet( )
init( )
start( )
...
getImage(URL, String)
getParameter(String)
play(URL)
...
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init( )
start( )
stop( )
destroy ( )
paint (Graphics)
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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
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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;
}
}
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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;
public long ONE_MINUTE = 60000;
// in milliseconds
// in milliseconds
}
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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)
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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
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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
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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
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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)
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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
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II. A low level class/method handles
exceptional conditions
Exception handling class
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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
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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
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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
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A method can catch an exception and
specialize its response
handler for E2
handler for E1;
throw E2
throw E1
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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
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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
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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
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Handling Exceptions
try
{
i = s.pop( );
}
catch( EmptyStackException i);
{
system.out.println(
“Oops! The stack is empty!” );
i = 0;
}
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Handling multiple exceptions
try
{
readFromFile( “foo” );
}
catch( FileNotFoundException e);
{
system.out.println(
“Oops! The file is missing!” );
}
catch( IOException e )
{
system.out.println(
“Oops! Can’t read the file!” );
}
finally
{
readFromFile( “foo.bak”);
}
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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
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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
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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
}
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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
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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
{
}
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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
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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
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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
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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
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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
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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 ( );
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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 ( );
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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
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Life Cycle of a Thread in Java
running
waiting
new Thread ( . . . )
start ( )
new
executing
not runnable
run method terminates
stopped

Transitions to not runnable
» invokes its sleep method
» invokes a wait method for
some condition
» blocks on an IO operation
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
Transitions to running
» specified time elapses
» notify method is invoked to
signify the condition is met
» the IO operation completes
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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
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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
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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
}
}
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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;
}
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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 ( );
}
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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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