Download Chap1. A Quick Tour of Java

Chap1. A Quick Tour of Java
SNU-OOPSLA-Lab.
Prof. Hyoung-Joo Kim
Contents(1)
1.1 Getting Started
 1.2 Variables
 1.3 Comments in Code
 1.4 Named Constants
 1.5 Flow of Control
 1.6 Classes and Objects
 1.7 Methods and Parameters
 1.8 Arrays

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Contents(2)
1.9 String Objects
 1.10 Extending a Class
 1.11 Interfaces
 1.12 Exceptions
 1.13 Packages
 1.14 The Java Infrastructure
 1.15 Other Topics Briefly Noted

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1.1 Getting Started
 Class
and Object
 Class
 a factory with blueprints and instructions to build gadgets
 two members
 field : data and making up the state of the object or class
 method : collection of statements that operate on the fields
 Objects
 gadgets the factory makes
 instances of the class
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“Hello, World”(1)
class HelloWorld {
public static void main (String[] args) {
System.out.println(“Hello, world”);
}
}



String objects
 the main method’s only parameter
System.out.println
 println method on the System class’s out object
main
 one of a few special method in Java
 when run, main can create objects, evaluate
expressions, invoke other methods, etc.
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“Hello, World”(2)

Compilation
$ javac HelloWorld.java
$ ls HelloWorld.class
HelloWorld.class
 Java compiler(javac) compiles the source into Java
bytecodes

Execution
$ java HelloWorld
Hello, World
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1.2 Variables

Every variable must have a type
 Java

has no “default” types
Primitive data types








boolean
char
byte
short
int
long
float
double
either true or false
16-bit Unicode 1.1 character
8-bit integer ( signed )
16-bit integer ( signed )
32-bit integer ( signed )
64-bit integer ( signed )
32-bit floating point ( IEEE 754-1985 )
64-bit floating point ( IEEE 754-1985 )
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“Fibonacci” Program(1)
Class Fibonacci {
/** Print out the Fibonacci sequence for values < 50 */
public static void main(String[] args) {
int lo = 1;
int hi = 1;
System.out.println(lo);
while(hi < 50) {
System.out.println(hi);
hi = lo + hi;
// new hi
lo = hi - lo;
// new lo is (sum - old lo) I.e., the old hi
}
}
}
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“Fibonacci” Program(2)

Compilation & Execution
$ javac Fibonacci.java
$ ls Fibonacci.class
Fibonacci.class
$ java Fibonacci
1
1
2
3
5...
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1.3 Comments in Code
 Three
 //
comment


characters from // to the end of the line are ignored
/* comment */


styles of comments
character between /* and the next */ are ignored, including line
terminators \r, \n, or \r\n
/** comment */
documentation comment ( for short, doc comment )
 characters between /** and the next */ are ignored, including
line terminators
 a tool called javadoc extracts documentation comments and
generates HTML documentation

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1.4 Named Constants

Why do programmer prefer named constants?
a
form of documentation
 easy to maintain program
 Named
constants are created by
 declaring
a variable as static and final
 providing its initial value
class CircleStuff {
static final double  = 3.1416;
}
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1.4.1 Unicode Characters

You write Java code in Unicode - an international
character set standard

Unicode characters are 16 bits and provide a
character range large enough to write the major
language
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1.5 Flow of Control(1)
Decide which statements are executed
 Similar to C-derived programming language (e.g.,
C, C++…)
 Think of output of running next prograrm!

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1.5 Flow of Control(2)
class ImprovedFibonacci {
/** Print out the first few Fibonacci numbers, marking events with a ‘*’
*/
static final int MAX_INDEX = 10;
public static void main(String[] args) {
int lo =1, hi =1; String mark;
System.out.println(“1: “ + lo);
for(int i=2; i < MAX_IINDEX; i++) {
if(hi%2 == 0)
mark = “ *”;
else
mark =“”;
System.out.println(I + “: “ + hi + mark);
hi = lo + hi; // new hi
lo = hi - lo; /* new lo is (sum - old lo) I.e., the old hi */
} } }
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1.6 Classes and Objects

Relationship between classes and objects
 an
object have a type
 that type is the object’s class
 Each
class has two kinds of members
 fields
- data variables associated with a class and its
objects
 methods - contain executable code of a class
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1.6.1 Creating Objects(1)

Creating objects (= instantiations)
 objects
are created using new keyword
 newly created objects are allocated within heap
 objects are accessed via object references

instantiation, instance, instance variable
 creating
an object from a class definition is instantiation
 created objects are called instances
 the fields in objects are instance variables
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1.6.1 Creating Objects(2)
class Point {
public double x, y;
public static void main(String[] args) {
Point lowerLeft
= new Point(); // creating objects
Point upperRight = new Point();
lowerLeft.x = 0.0;
upperRight.x = 10.0;
lowerLeft.y = 0.0;
upperRight.y = 10.0;
}
}
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1.6.2 Static or Class Fields(1)

Static(class) fields
 known
as class variables
 shared among all objects of a class
 declared by static keyword

Non-static (per-object) fields
 each
object has distinct fields from other object
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1.6.2 Static or Class Fields(2)
class Point {
public double x, y;
public static Point origin = new Point();
public static void main(String[] args) {
System.out.println("Origin.x = " + Point.origin.x);
System.out.println("Origin.y = " + Point.origin.y);
}
}
Origin.x = 0.0
Origin.y = 0.0
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1.6.3 The Garbage Collector
Unreferenced Java objects are automatically
reclaimed by a garbage collector
 The garbage collector runs in the background and
tracks object references

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1.7 Methods and Parameters

Methods and parameters
 methods
- operations of a class
 parameters - arguments of methods
 Implementation
 benefits

hiding
of object orientation
Data encapsulation
 hiding
data behind methods so that it is inaccessible to
other objects
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1.7.1 Invoking a Method(1)

To invoke a method,
 provide
an object reference and the method name,
separated by a dot(.)
 To
return more than one value,
 create
an object to hold return values and return that
object

receiving object ( for short, receiver)
 the
object on which the method is invoked
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1.7.1 Invoking a Method(2)
class Point {
public double x, y;
public double distance(Point that){
double xdiff, ydiff;
xdiff = x - that.x;
ydiff = y - that.y;
return Math.sqrt(xdiff *xdiff + ydiff*ydiff); }
public static void main(String[] args) {
Point lowerLeft
= new Point();
Point upperRight = new Point();
lowerLeft.x = 0.0;
lowerLeft.y = 0.0;
upperRight.x = 10.0; upperRight.y = 10.0;
distance = 14.142….
double d = lowerLeft.distance(upperRight);
System.out.println("distance = " + d);
}
}
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1.7.2 The this Reference

Implicit reference named “this”
a
reference to the current(receiving) object
class Point {
public double x, y;
public void move(double x, double y) {
this.x = x;
this.y = y;
}
}
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1.7.3 Static or Class Methods
 Static(class)
methods
 declared
using static keyword
 shared among on static fields of all instance object
 can’t directly access non-static members

Non-static(per-object) methods
 each
object has distinct method from other instance
class AnIntegerNamedX {
static private int x;
static public int getX() { return x; }
static public void setX(int newX) { x = newX; }
}
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1.8 Arrays(1)

Array
a
collection of variables all of the same type
 Array size is fixed and provided from the length field of
array object
 an IndexOutOfBoundsException is thrown

in case of using an index outside the bounds of the array
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1.8 Arrays(2)
class Deck {
final int DECK_SIZE = 52;
card[] cards = new Card[DECK_SIZE];
public void print() {
for (int i=0; i< cards.length; i++)
System.out.println( cards[i] );
}
}
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1.9 String Objects(1)

String class
 provide
language-level support for initialization
 provide a variety of methods
 String objects are immutable
str = “redwood”;
// ….. Do do something with str….
str = “oak”; /* give a new value to object reference str, not to
the contents of the string */

StringBuffer class
 provide
for mutable strings
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1.9 String Objects(2)
class BetterStringsDemo {
public static void main(String[] args) {
String myName = "Petronius";
String occupation = "Reorganization Specialist";
myName = myName + " Arbeiter";
myName += " ";
myName += "(" + occupation + ")";
System.out.println("Name = " + myName); }
}
Name = Petronius Arbeiter (Reorganization Specialist)
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1.9 String Objects(3)

equals method
 compare
two String objects to see if they have the
same contents
if (oneStr.equals(twoStr))
foundDuplicate(oneStr, twoStr);
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1.10 Extending a Class(1)

Subclass
 inherit
all the fields and methods of superclass
 if providing new implementation of inherited methods,
then overrides the behavior of superclass
class Point {
public double x, y;
public void clear(){
x = 0.0;
y = 0.0; }
}
Chap1. A Quick Tour of Java
class Pixel extends Point {
Color color ;
public void clear() {
super.clear();
color = null;
}
}
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1.10 Extending a Class(2)
Point Class
clear()
x()
double x
double y
Pixel Class
y()
set()
clear()
x()
Pixel extends Point
double x
double y
y() Color color
set()
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1.10.1 The Object Class
Classes that do not explicitly extend any other
class implicitly extend the Object class
 All object references are polymorphically of Object
class, so Object class is the generic class for
references that can refer to objects of any class
 The Object class defines several methods

Object oref = new Pixel();
oref = “Some String”
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1.10.2 Invoking Methods from
Superclass

super vs this
 super
- reference things from superclass
 this - reference things from the current object
 To
invoke a method
 uses
the actual type of the object, not the type of the
object reference
Point point = new Pixel();
point.clear(); // uses Pixel’s clear(), not Point’s clear()
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1.11 Interfaces(1)

Interfaces
 similar
to a class, but with only declarations of its
methods
 implementation details of the methods are irrelevant
 the class that implements the interface is responsible
for the specific implementation

Class’s supertypes are
 superclass
that it extends
 interfaces that it implements
 all the supertypes of those classes and interfaces
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1.11 Interfaces(2)
interface Lookup {
/** Return the value associated with the name,
* or null if there is no such value */
Object find(String nam);
}
Void processValues(String[] names, Lookup table) {
for(int i=0; i<names.length; i++) {
Object value = table.find(names[i]);
if(value != null)
processVaule(names[i], value);
} }
Code that uses references to Lookup objects and get the
expected results, no matter the actual type of the object
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1.11 Interfaces(3)
class SimpleLookup implements Lookup {
private String[] Names;
private Object[] Values;
public Object find(String name) {
for( int i=0; i<names.length; i++ ) {
if( Names[i].equals(name) )
return Values[i];
}
return null;
// not found
}
// ….
}
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1.12 Exceptions(1)
Java uses checked exceptions to manage error
handling
 Checked exceptions

 force
you to consider what to do with errors where they
may occur in the code
 exception is an object, with type, methods,and data
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1.12 Exceptions(2)

Exception object
 generally
derived from the Exception class, which
provides a string field to describe the error
 all exceptions is extensions of Throwable class, which
is the superclass of Exception

Exception Handling
 try-catch-finally
sequence
 finally - clean up from either the normal code path or
the exception code path
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1.12 Exceptions(3)
class IllegalAverageException extends Exception { }
class MyUtilities {
public double averageOf ( double[] vals, int i, int j )
throws IllegalAverageException {
try{
return ( vals[i] + val[j] ) / 2;
} catch ( IndexOutOfBoundsException e ){
throw new illegalAverageException();
}
}
}
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1.13 Packages(1)

Solution for name-conflicts
 use

a “package prefix” at the front of every class
it isn’t a complete solution
 Packages
 have
a set of types and subpackages as members
 package names are hierarchical and separated by dots
 in case of using a package
use its fully qualified name
 import all or part of the package

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1.13 Packages(2)

When using part of a package
use fully qualified
name
import all or part
of the package
Chap1. A Quick Tour of Java
class Date1 {
public static void main(String[] args) {
java.util.Date now = new java.util.Date();
System.out.println(now);
}
import java.util.Date;
class Date2 {
public static void main(String[] rgs){
Date now = new Date();
System.out.println(now);
}
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1.13 Packages(3)

Convention of naming packages
 complete
solution for name collision
 use reversed Internet domain name of the organization
to prefix the package name.

e.g., COM.acme.package, KR.ac.snu.oopsla.package
package com.sun.games;
class Card {
// …………
}
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1.14 The Java Infrastructure
Java is designed to maximize portability
 Java virtual machine

 assign

each application its own runtime
runtime - isolate applications from each other and provide a
security model
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1.15 Other Topics Briefly Noted

Applet
 Java
program that runs on the browser of client
platform
 Make a class that extends the Applet class
 Make methods named as init, start, stop, destroy
 There is no main method
 Use other classes and utilities as the Java application
program
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1.15 Other Topics Briefly Noted

Applet example
public class Simple extends Applet {
StringBuffer buffer;
public void init() {
buffer = new StringBuffer();
addItem("initializing... ");
}
public void start() {
addItem("starting... ");
}
public void stop() {
addItem("stopping... ");
}
public void destroy() {
addItem("preparing for unloading...");
}
Chap1. A Quick Tour of Java
void addItem(String newWord) {
System.out.println(newWord);
buffer.append(newWord);
repaint();
}
public void paint(Graphics g) {
g.drawRect(0, 0, size().width - 1, size().height - 1);
g.drawString(buffer.toString(), 5, 15);
}
}
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1.15 Other Topics Briefly Noted
 RMI(Remote
 Designing
Method Invocation : At Server Part)
a Remote Interface
import java.rmi.*;
import java.rmi.*;
import java.rmi.server.*;
import java.io.Serializable;
public interface Task extends Remote {
TaskObject getTaskObjcet() throws
RemoteException;
}
public interface TaskObject extends Serializable {
type1 task1();
type2 task2();
………..
}
Chap1. A Quick Tour of Java
public class Server{
public static void main(String args[]){
if(System.getSecurityManager() == null){
System.setSecurityManager(new
RMISecurityManager());
}
try{
TaskImpl task = new TaskImpl();
Naming.rebind(“Task”, task);
}catch(Exception e){
………………..;
}
}
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1.15 Other Topics Briefly Noted

RMI(Remote Method Invocation : At Client Part)
 Use
the same interface as the server
 Use following code to invoke remote object
……………..
if(System.getSecurityManager() == null)
System.setSecurityManager()(new RMI…());
String url = “rmi://server_address”;
try{
Task t = (Task)Naming.lookup(“Task”);
TaskObject to = t.getTaskObject();
}catch(Exception e){
………..
}
rmiregistry 사용
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1.15 Other Topics Briefly Noted

Servlet
 Make

Servlet script like CGI script
generic code are the shape of following
Object declarations (like other Java applications)
out.println(“<HTML>”);
out.println(“<HEAD><TITLE> …. </TITLE></HEAD>”);
……………….
out.println(“</HTML>”);
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1.15 Other Topics Briefly Noted

Java & XML
 Many
XML parsers are implemented by Java
 DOM : Use Object Model
Makes a model (like tree structure)
 Provide traversal methods

 SAX
: Event Driven XML Parser
Makes a event handler class
 Makes the methods to be invoked when an event occur
 SAX parser invokes an appropriate method when an event
occur

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