sample java

8/8/2019 Sample Java

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There are a number of C++ featuresthat Java does not support. In some

cases, a specific C++ feature simplydidn't relate to the Java environment.In other cases, the designers of Javaeliminated some of the duplication offeatures that exists in C++. In still

other instances, a feature of C++ isnot supported by Java because itwas deemed too dangerous forInternet applets.

Perhaps the single biggest differencebetween Java and C++ is that Javadoes not support pointers. As a C++programmer you know that thepointer is one of C++'s most powerfuland important language features. It isalso one of its most dangerous whenused improperly. Pointers don't existin Java for two reasons:


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* Pointers are inherently insecure.

For example, using a C++-stylepointer, it is possible

to gain access to memoryaddresses outside a program's codeand data. A malicious

program could make use of thisfact to damage the system, performunauthorized

accesses (such as obtainingpasswords), or otherwise violate

security restrictions.* Even if pointers could berestricted to the confines of the Javarun-time system (which

is theoretically possible, sinceJava programs are interpreted), thedesigners of Java

believed that they were inherentlytroublesome.


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Note Since pointers don't exist in

Java, neither does the -> operator.

Here are a few more of the mostimportant "omissions":

* Java does not include structuresor unions. These were felt to beredundant since the

class encompasses them.* Java does not support operator

overloading. Operator overloading issometimes asource of ambiguity in a C++

program, and the Java design teamfelt that it causes

more trouble than benefit.* Java does not include a

preprocessor nor does it support thepreprocessor directives.


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The preprocessor plays a lessimportant role in C++ than it does in

C. The designersof Java felt that it was time to

eliminate it entirely.* Java does not perform any

automatic type conversions that

result in a loss ofprecision. For example, aconversion from long integer tointeger must be explicitly

cast.

* All the code in a Java program isencapsulated within one or moreclasses. Therefore,

Java does not have what younormally think of as global variablesor global functions.

* Java does not allow defaultarguments. In C++, you may specifya value that a


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parameter will have when there isno argument corresponding to that

parameter whenthe function is invoked. This is

not allowed in Java.* Java does not support the

inheritance of multiple superclasses

by a subclass.* Although Java supportsconstructors, it does not havedestructors. It does, however,

add the finalize( ) function.

* Java does not support typedef.* It is not possible to declareunsigned integers in Java.

* Java does not allow the goto.* Java does not have the delete

operator.* The > in Java are not

overloaded for I/O operations.* In Java, objects are passed by


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C++ is a class that contains atleast one pure virtual function.) For

example, it isimpossible to create an instance

of a C++ abstract class or a Javainterface. Both are

used to specify a consistent

interface that subclasses willimplement. The maindifference is that an interface

more cleanly represents this concept.* Java has a streamlined approach

to memory allocation. Like C++, itsupports the newkeyword. However, it does not

have delete. Instead, when the lastreference to an

object is destroyed, the object,itself, is automatically deleted thenext time that

garbage collection occurs.


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* Java "removes" the C++standard library, replacing it with its

own set of API classes.While there is substantial

functional similarity, there aresignificant differences in the

names and parameters. Also,

since all of the Java API library isobject-oriented, andonly a portion of the C++ library

is, there will be differences in the waylibrary routines

are invoked.* The break and continuestatements have been enhanced inJava to accept labels as

targets.* The char type in Java declares

16-bit-wide Unicode characters. Thismakes them

similar to C++'s wchar_t type.


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The use of Unicode helps ensureportability.

* Java adds the >>> operator,which performs an unsigned rightshift.

* In addition to supporting single-line and multiline comments, Java

adds a thirdcomment form: thedocumentation comment.Documentation comments begin witha

/** and end with a */.* Java contains a built-in stringtype called String. String issomewhat similar to the

standard string class typeprovided by C++. Of course, in C++string is only available if

you include its class declarationsin your program. It is not a built-in


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type.If you are a beginner please read

(History of Java) and (What is Java?)before proceeding.

If you are familiar with C then Javashould be very familiar to you in

terms of syntax. If you are a C++programmer the Java will be easierstill since Java is also purely anObject Oriented Programmingwithout some of the features like

Pointers in C++.

To start programming your first Javaprogram, you need to have somepre-requisites installed on yourcomputer. As with any otherprogramming language, you firstneed to install the SDK (SoftwareDeveloper Kit) in this case it is JDK


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(Java Developers Kit). Nowchoosing which JDK to download

maybe a little tricky if you are new toJava. So it will be advisable to readthe How Java is Organised? beforeyou proceed.

You have two options to get startedwith Java.

1. Manual

* If you want to do it manually, youwill need to download a J2SE fromSun website and install on yourcomputer. (The J2SE has manyversions and it is better to check atthe Sun website to get the latestJDK.)

* The J2SE download is usually anexecutable file which will install


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automatically like any windowssoftware.

* Once the installation issuccessful, open a command line(DOS Window) and type 'Java' and'Enter'. (If you see 'Bad command'then probably your PATH settings

are wrong. In which add theinstallation path to the PATH variablefrom your 'computer properties'environment variables section.)

* Open a Notepad, write your first

sample program (you will find manyjava samples here) and save it as.java file (the name of the file shouldbe same as the name of the CLASSin the program)

* from the command window,change current directory to whereyou saved your .java file and compilethe program ( Use 'javac' command


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followed by the name of the file tocompile)

* Once it is compiled a .class filewill be created in the same folder.Then run the program using 'Java'command (Use 'Java' commandfollowed by the name of the class)

2. Automated

* As with any other programminglanguage, Java too has many free

IDE (Integrated DevelopmentEnvironments) You can choose anyof them. Most of them comesbundled with the required JDKs.

* One of the widely used Javadevelopment tool is the Sun Studio.You can download it from the Sunwebsite.

* Or you can download and use


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the net beansFor the most part, it is quite easy to

convert a C++ function that usespointer parameters into its equivalentJava method. Since Java passes allobjects by reference, sometimes theconversion simply requires the

removal of C++'s pointer operators.For example, consider this C++program that reverses the signs of aCoord object, which stores a pair ofCartesian coordinates. The function

reverseSign( ) is passed a pointer tothe Coord object that will bereversed. As you can see, C++'s *, &,and -> pointer operators are used toperform the operation.

// Reverse the signs of acoordinate - C++ version.

#include


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using namespace std;class Coord {

public:int x;int y;};

// Reverse the sign of thecoordinates.void reverseSign(Coord *ob) {ob->x = -ob->x;ob->y = -ob->y;

}int main(){Coord ob;ob.x = 10;ob.y = 20;cout


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reverseSign(&ob);cout


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int y;};

class DropPointers {// Reverse the sign of the

coordinates.static void reverseCoord(Coord

ob) {ob.x = -ob.x;ob.y = -ob.y;}

public static void main(Stringargs[]) {Coord ob = new Coord();ob.x = 10;ob.y = 20;System.out.println("Original values

for ob: " +ob.x + ", " + ob.y);reverseCoord(ob);


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System.out.println("Sign reversedvalues for ob: " +

ob.x + ", " + ob.y);}}

The output from both of these

programs is the same and is shownhere:

Original values for ob: 10, 20Sign reversed values for ob: -10, -20

*, &, and -> pointer operators areused to perform the operation.

Internationalization involves manyaspects of application development.Practically speaking, the primary goal


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behind all of these facets ofdevelopment is to engineer a user

interface -and its supportinginfrastructure - that presents all UIinformation in a comprehensible wayto local users. At a minimum, thiseffort involves supporting the

following aspects of an application'sexecution:

* Messaging presentation of allvisible text (message text, error text,

UI component titles,prompts, and so forth) in thelanguage of the appropriate runtimelocale context.

* Formatting policy use of thecorrect locale-specific formats for alldate, time, and

numeric quantities.* Calendar and time zone policy


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use of the correct calendar for theapplication's runtime

locale.* String collation policy use of an

appropriate policy for string collationbased on the

locale's language.

* General UI features, locale-sensitive images, icons, and colorsusing images and colors

that represent meaningfulmnemonics to local users.

To support the foregoing features, aninternationalized application mustperform some dynamic configurationand information retrieval. Typically,an application will determine itslocale context dynamically uponstartup. Then, it will configure all thenecessary runtime components


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such as calendar objects, stringcollators, format objects and

messaging componentsthat itneeds to conform to the localecontext requirements.

Messaging: Messaging is the

presentation of all text data to theuser in a language appropriate forthe application's runtime localecontext. It's the most visible area ofi18n. Messaging involves the

execution of the following steps atruntime:

determination of the device localeenvironment loading of the application's localizedresources dynamic lookup and retrieval oflocalized resources for UI display


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display of localized resources

Messaging is the area that besthighlights the close relationshipbetween i18n and l10n. To make ani18n implementation usable, anapplication must be localized. For

each locale supported, the l10nprocess must produce a set oftranslated message strings that theapplication can access at runtime.

String Collation: String collation, alsoknown as lexicographic sorting, isdifferent from messaging.Nevertheless, the two areas arerelated in the sense that collationfunctions manipulate message texttext that the user sees.

Different languages define different


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Date, Time, Numeric, and MonetaryValue Formatting: Different locales

use different formats for writingdates, times, and numbers. Forinstance, in Europe, people writedates, times and numbers differentlyfrom people in the United States. A

French user writes date, time, andnumeric quantities using thefollowing forms.

25 dcembre 2002

2002/12/2525/12/200208.3014.4520.000,45 (twenty thousand, andforty-five hundredths)

In the United States, however, thesesame quantities would normally be


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written as follows.

December 25, 200212/25/20028:30 am2:45 pm20,000.45 (twenty thousand, and

forty-five hundredths)

An internationalized program needsto format and display dates, times,and numbers appropriately for the

runtime locale. Programs don't fetchthese formatted quantities from somedatabase; they calculate themdynamically in the same way thatstrings are collated dynamically.

Calendar and Time Zone Support:Calendars, although related to datesand times, define different


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characteristics and functionality. Thedifference is that calendars perform

calculations that involve dates andtimes, whereas date and time objectssupport the formatting and display ofthose quantities.

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