Transcript System.out.println(“Hello World”) - DCC

Nelson Baloian
Universidad de
Chile
Goals of intensive lecture
• To learn:
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What is JAVA ?
What is JAVA good for ?
What is JAVA bad for ?
Which programming resources offers JAVA to
support internetworking programming ?
– How does JAVA programming looks like ?
Why is JAVA so popular ?
• Object oriented: the modern programming
paradigm
• Robust: avoids programming features which can
frequently cause errors, permits recovering from
errors.
• Aware of the Network: easy for programming in a
TCP/IP network programming
• Muti-platform
• It is free and has very good documentation
What is Object Orientation ?
• Data used by the program are objects
• Objects are a collection of variables and
methods
Method that modifies the
values of the object’s variables
Method that returns the
value of the object’s variables
Examples of Objects
• A clock: the variables are the position of the
pointers
Set the pointers’ value
What time is it ?
• A telephone book: the variables are a list of
names and telephone numbers
• Set a name with a telephone number
• change the telephone number
• delete a pair name-telephone
• Given a name ask the telephone number
Classes and Objects
• Every object is of a certain class
• The class define the type of the object, this means
– what variables contains an object of this class
– which methods can be applied to an object of this class
Object A of class clock
Object B of class clock
Class Clock
Object C of class clock
An Example about use of
Objects in Java (1)
• Clock A = new Clock( );
• A.setTime(3,35);
• int i;
• i = A.getHour( );
Put pointers in 3 and 35
Get the value of the hour-pointer
An Example about use of Objects
in Java (2)
• PhoneBook B = new PhoneBook( );
Creates a new Phone Book
• B.addEntry(“Eduardo Vera”,422596160);
Eduardo Vera
422596160
Adds the name and the
phone number given
• B.getNumber(“Eduardo Vera” );
Eduardo Vera
422596160
Gets the number of the person
Why Object Orientation ?
• A strategy to develop bigger programs with
few errors and at lower cost (time) by building
over existing components
Robust (compared with C)
• The basic instructions in Java are similar to the basic
instructions in C BUT it has not the instructions and
characteristics which make C less stable like:
– Pointer arithmetic
– assignation in conditions like in if (a = 9)
– garbage collection (liberating memory which is not used
any more)
• It permits to program a set of instructions within a try
and catch block
try {
instructions which may cause an error
} catch (Error e) {
instructions to react to the error
}
Network savvy
• Java appears when the internet and the WWW are
already popular and in rapid expansion (1995)
• It has a library with classes which make possible to
easily communicate 2 programs running in a TCP/IP
network (internet)
INTERNET
Examples
• Provides a library with classes which make
possible to easily communicate 2 programs
running in a TCP/IP network (internet)
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Find IP Address given the host’s name
Establish URL connections with www servers
Build TCP & UDP sockets
Implement RMI by implementing remote objects
JAVA is an interpreted
Language
P1.java
P1.class
Program’s
output
Java compiler (specific for each platform)
javac P1.java
Java interpreter (specific for each platform) also
called Java Virtual Machine
java P1 (class)
What kinds of programs can I
develop with JAVA ?
• Stand alone programs: written with a text
editor, compiled and interpreted
• Applets: programs that run inside www
page.
• Servlets: programs invoked by a www page
running on the server side
• Java script
• JSP
Java Applets
Applets are java programs which are downloaded
with the HTML page.
Html
Animator.class
Animator.class
<applet code=Animator.class >
<parameters>
</applet>
Java program
running on the client
Java Script
The code of the java program is written directly
in the HTML page
Html & Script
<script language = “JavaScript”>
the code
</script>
Java program
running on the client
Java Servlets
The code of the java program which runs on the server
and can dynamically produce HTML content according
to the particular situation (the client of a bank)
HTML from page
HTML from servlet
MyServlet.class
HTML-page with a
reference to a servlet
In JAVA Programming means
to write classes
• What components (attributes) should have an
object of this class ?
• How can I create a new Object (constructor) ?
• What methods can I perform on an object of
this class ?
• There may be a static method which can be
called from “outside”.
• Is my new class the extension of an existing
one ?
How do I write a simple stand
alone program ?
public class Classname {
public static void main(String args[ ]) {
HERE JAVA INSTRUCTIONS
} // end of the main method
} // end of the class
• Write a file with the program (the name should be
Classname.java)
• Compile it with the command javac Classname.java
• This will create a new file Classname.class
• Execute the command java Classname and the
instructions written in the main method will be executed.
The “Hello World” example
public class HelloWorld {
public static void main(String args[ ]) {
System.out.println(“Hello World”);
} // end of the main method
} // end of the class
• Every program should start with a class declaration:
public class ClassName (class name is given by the
programmer)
• The instruction which will be executed should be written inside a
method called main(String args[ ]) The instructions should be
written inside the brackets {}
The “Hello World” example
public class HelloWorld {
public static void main(String args[ ]) {
System.out.println(“Hello World”);
} // end of the main method
} // end of the class
• After a double slash // the rest of the line is considered
comments and are not executable instructions
• The instruction System.out.println(“Hello World”); writes
a line on the screen with everything that is inside the brackets
• This program should be written in a file named HelloWorld.java
More about programming...
public class Program2 {
public static void main(String args[ ]) {
int myNumber1 = 5;
double myNumber2 = 9.0;
System.out.println(“number is
”
+myNumber1);
System.out.println(”number plus 1 is
”+(myNumber1+1));
System.out.println(”number minus 3 is ”+(myNumber1-3));
System.out.println(”double of number is ”+(myNumber1*2));
System.out.println(”half of number is ”+(myNumber1/2));
System.out.println(”My other number is ”+myNumber2);
System.out.println(”The half is ” + (myNumber2/2));
System.out.println(“The square root is ” +
Math.sqrt(myNumber2));
}
}
Reading a line of characters from
keyboard
import java.io.*;
public class Program3 {
public static void main(String args[ ]) throws
IOException {
BufferedReader inKbd = new BufferedReader(
new InputStreamReader(System.in));
String inputLine;
System.out.print(“ please enter your name: ”);
inputLine = inKbd.readLine();
System.out.println(“Hajimemashite,”+ inputLine);
}
}
• It is only possible to read a line of characters with this method
• It is necessary to write throws IOException at the beginning of the
main method. Otherwise it will not even pass compilation
• For using input/output classes it is necessary to import the
classes from the java.io.* library
Reading a number from keyboard
import java.io.*;
public class Program4 {
public static void main(String args[ ])
IOException {
throws
BufferedReader inKbd = new BufferedReader(
new InputStreamReader(System.in));
String inputLine;
System.out.print(“I am Jalisco, enter your number ! ”);
inputLine = inKbd.readLine();
int yourNumber = Integer.parseInt(inputLine);
System.out.println(“My number is ”+ (yourNumber+1)+” I win !”);
}
} • In Java2 there is also Double.parseDouble(aString)
• If the content of the String does not correspond to a
and error will occur during the program execution
Primitive types in Java
• integer: int, long, short, byte
Const. 1, -1, 1024, 1L
• real: float, double
Const. 1.0, -3.14159, 1.5e4, 1.0f
• character: char
Const. ‘a’, ‘X’, ‘@’
• logic value: boolean
Const. true, false
String: “Hola“,“21 of Jannuary 2000”
Primitive
Not a
Declarations
int i;
int i = 1;
//declaration
//declaration and
//initialization
double pi = 3.14159;
char c = ‘a’;
boolean genki_desu = true;
• Declarations are necessary to reserve memory for
storing a value in a certain part of the memory and
referencing it with a name (not address)
• A declaration of variable can appear in any part of
the program but always before the variable is used.
Expressions & Assignation
• Aritmethics : sum + 20 * c / (mod % 3)
• Boolean: a > b, b >= c, c != 4, a == 0
• String: “Hello “+ name + “ today is the “+
day + “of”+month
• Casts: (int) pi (pi = 3.1)
(int) (Math.random()*100)+1)
• other: a == 1 ? a+1 : a-1
• Assignation: a = 1;
• Assignation as operator: a = b = c = d = 0;
Controlling the sequence of
instructions
• Conditional execution of instructions:
if (condition)
instruction;
if(condition)
instruction;
else
instruction;
• It is always possible to write more than one
instruction after the if and the else by grouping them
inside curly brackets { }
Problem
• Write a program that reads 2 numbers from the keyboard. The
first one is the amount a customer has to pay at a bookstore. The
second is the sum the customer gives to pay for the books. The
computer should respond if the sum is not enough, if it exact the
amount to pay or if then customer should receive change. In
case the customer should receive change, the computer must
give the exact amount of 5000 and 1000 bills and 500, 100, 50,
10, 5 and 5 coins.
Enter the value to pay:3561
Enter the value given by the customer :10000
give 1 of 5000
give 1 of 1000
give 4 of 100
give 3 of 10
give 1 of 5
give 4 of 1
Doing Loops
• The basic instruction for the loop is the while
while (condition)
instruction;
• There are others, but they are variations
do
instruction;
while (condition);
for (instr1; condition; instr2)
instruction;
Example Loops
• The while loop
int i = 1;
while (i <= 10) {
System.out.println(”5 X ” + i + ” = ”+(i*5))
i = i + 1;
}
• The do while loop
int i = 1;
do {
System.out.println(”5 X ” + i + ” = ”+(i*5));
i = i + 1;
}
while (i <= 10);
• The for loop
for (i = 1; i <= 10; i = i +1)
System.out.println(”5 X ” + i + ” = ”+(i*5));
Another Example: the MCD
public class MCD {
//computing of the maximum common
//divider of 15 & 24
public static void main(String args[ ]){
int x = 15, y = 24;
while (x != y) {
if (x < y)
y = y - x;
else
x = x - y;
}
System.out.println
(“The MCD of 15 & 24 is “ + x);
}
}