Chap1. A Quick Tour of Java SNU-OOPSLA-Lab. Prof. Hyoung-Joo Kim.

Chap1. A Quick Tour of JavaChap1. A Quick Tour of Java

SNU-OOPSLA-Lab.

Prof. Hyoung-Joo Kim

Chap1. A Quick Tour of Java

SNU-OOPSLA-Lab. 2

Contents(1)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)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 1.1 Getting StartedGetting 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)Hello, World”(1)

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.

class HelloWorld { public static void main (String[] args) { System.out.println(“Hello, world”); }}


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““Hello, World”(2)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 1.2 VariablesVariables

Every variable must have a type Java has no “default” types

Primitive data types booleaneither true or false char 16-bit Unicode 1.1 character byte 8-bit integer ( signed ) short 16-bit integer ( signed ) int 32-bit integer ( signed ) long 64-bit integer ( signed ) float 32-bit floating point ( IEEE 754-1985 ) double 64-bit floating point ( IEEE 754-1985 )


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““Fibonacci” Program(1)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)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 1.3 Comments in CodeComments in Code Three styles of comments

// comment characters from // to the end of the line are ignored

/* comment */ 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 1.4 Named ConstantsNamed 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 1.4.1 Unicode CharactersUnicode 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 1.5 Flow of Control(1)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 1.5 Flow of Control(2)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 1.6 Classes and ObjectsClasses 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 1.6.1 Creating Objects(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 1.6.1 Creating Objects(2)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; lowerLeft.y = 0.0; upperRight.x = 10.0; upperRight.y = 10.0; }}


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1.6.2 1.6.2 Static or Class Fields(1)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 1.6.2 Static or Class Fields(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 = 0.0Origin.y = 0.0


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1.6.3 1.6.3 The Garbage CollectorThe 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 1.7 Methods and ParametersMethods and Parameters

Methods and parameters methods - operations of a class parameters - arguments of methods

Implementation hiding benefits of object orientation

Data encapsulation hiding data behind methods so that it is inaccessible to

other objects


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1.7.1 1.7.1 Invoking a Method(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 1.7.1 Invoking a Method(2)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; double d = lowerLeft.distance(upperRight); System.out.println("distance = " + d); }}

distance = 14.142….


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1.7.2 1.7.2 The The thisthis Reference 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 1.7.3 Static or Class MethodsStatic 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 1.8 Arrays(1)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 1.8 Arrays(2)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 1.9 String Objects(1)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 1.9 String Objects(2)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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if (oneStr.equals(twoStr))

foundDuplicate(oneStr, twoStr);

1.9 1.9 String Objects(3)String Objects(3)

equals method compare two String objects to see if they have the

same contents


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1.10 1.10 Extending a Class(1)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; }}

class Pixel extends Point {

Color color ;

public void clear() {

super.clear();

color = null; }

}


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1.10 1.10 Extending a Class(2)Extending a Class(2)Point Class

x()

y()

clear()

double xdouble y

set()

Pixel Class

Pixel extends Point

x()

y()

clear()

double xdouble y

set()

Color color


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1.10.1 1.10.1 The The Object Object ClassClass

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 1.10.2 Invoking Methods from Invoking Methods from SuperclassSuperclass

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 1.11 Interfaces(1)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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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

interface Lookup {/** Return the value associated with the name, * or null if there is no such value */ Object find(String nam); }


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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 1.12 Exceptions(1)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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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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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 1.13 Packages(1)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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When using part of a package

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);}

use fully qualified name

import all or part of the package


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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 1.14 The Java InfrastructureThe 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 1.15 Other Topics Briefly NotedOther 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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Applet examplepublic 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..."); }

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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RMI(Remote Method Invocation : At Server Part) Designing a Remote Interface

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(); ………..}

import java.rmi.*;

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){ ………………..; }}

rmiregistryrmiregistry 사용사용


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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){ ………..}

rmiregistryrmiregistry 사용사용


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Servlet Make Servlet script like CGI script

generic code are the shape of following

Object declarations (like other Java applications)Object declarations (like other Java applications)

out.println(“<HTML>”);out.println(“<HTML>”);out.println(“<HEAD><TITLE> …. </TITLE></HEAD>”);out.println(“<HEAD><TITLE> …. </TITLE></HEAD>”);

………………………………..

out.println(“</HTML>”);out.println(“</HTML>”);


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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

Chap1. A Quick Tour of Java SNU-OOPSLA-Lab. Prof. Hyoung-Joo Kim.

Documents

java u

object u class u

object u main u

gadgets u

code u

variables u

packages u

parameter u