KEMBAR78
Java PPt.ppt
Introduction to Java
NAVNEET SHEORAN
Introduction
 Present the syntax of Java
 Introduce the Java API
 Demonstrate how to build
 stand-alone Java programs
 Java applets, which run within browsers e.g. Netscape
 Example programs
Why Java?
 It’s the current “hot” language
 It’s almost entirely object-oriented
 It has a vast library of predefined objects and operations
 It’s more platform independent
 this makes it great for Web programming
 It’s more secure
 It isn’t C++
Applets, Servlets and Applications
 An applet is designed to be embedded in a Web page,
and run by a browser
 Applets run in a sandbox with numerous restrictions; for
example, they can’t read files and then use the network
 A servlet is designed to be run by a web server
 An application is a conventional program
Building Standalone JAVA
Programs (on UNIX)
 Prepare the file foo.java using an editor
 Invoke the compiler: javac foo.java
 This creates foo.class
 Run the java interpreter: java foo
Java Virtual Machine
 The .class files generated by the compiler are not executable binaries
 so Java combines compilation and interpretation
 Instead, they contain “byte-codes” to be executed by the Java Virtual
Machine
 other languages have done this, e.g. UCSD Pascal
 This approach provides platform independence, and greater security
HelloWorld (standalone)
 Note that String is built in
 println is a member function for the System.out class
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
Comments are almost like C++
 /* This kind of comment can span
multiple lines */
 // This kind is to the end of the line
 /**
* This kind of comment is a special
* ‘javadoc’ style comment
*/
Primitive data types are like C
 Main data types are int, double,
boolean, char
 Also have byte, short, long, float
 boolean has values true and false
 Declarations look like C, for example,
 double x, y;
 int count = 0;
Expressions are like C
 Assignment statements mostly look like those in C;
you can use =, +=, *= etc.
 Arithmetic uses the familiar + - * / %
 Java also has ++ and --
 Java has boolean operators && || !
 Java has comparisons < <= == != >= >
 Java does not have pointers or pointer arithmetic
Control statements are like C
 if (x < y) smaller = x;
 if (x < y){ smaller=x;sum +=
x;}
else { smaller = y; sum += y; }
 while (x < y) { y = y - x; }
 do { y = y - x; } while (x < y)
 for (int i = 0; i < max; i++)
sum += i;
 BUT: conditions must be boolean !
Control statements II
 Java also introduces the try statement, about which more later
switch (n + 1) {
case 0: m = n - 1; break;
case 1: m = n + 1;
case 3: m = m * n; break;
default: m = -n; break;
}
Java isn't C!
 In C, almost everything is in functions
 In Java, almost everything is in classes
 There is often only one class per file
 There must be only one public class per file
 The file name must be the same as the name of that public class, but with
a .java extension
Java program layout
 A typical Java file looks like:
import java.awt.*;
import java.util.*;
public class SomethingOrOther {
// object definitions go here
. . .
}
This must be in a file named SomethingOrOther.java !
What is a class?
 Early languages had only arrays
 all elements had to be of the same type
 Then languages introduced structures
(called records, or structs)
 allowed different data types to be grouped
 Then Abstract Data Types (ADTs)
became popular
 grouped operations along with the data
So, what is a class?
 A class consists of
 a collection of fields, or variables, very much like the named
fields of a struct
 all the operations (called methods) that can be performed on
those fields
 can be instantiated
 A class describes objects and operations defined on
those objects
Name conventions
 Java is case-sensitive; maxval, maxVal, and MaxVal are three different
names
 Class names begin with a capital letter
 All other names begin with a lowercase letter
 Subsequent words are capitalized: theBigOne
 Underscores are not used in names
 These are very strong conventions!
The class hierarchy
 Classes are arranged in a hierarchy
 The root, or topmost, class is Object
 Every class but Object has at least one superclass
 A class may have subclasses
 Each class inherits all the fields and methods of its (possibly numerous)
superclasses
An example of a class
class Person {
String name;
int age;
void birthday ( ) {
age++;
System.out.println (name + ' is
now ' + age);
}
}
Another example of a class
class Driver extends Person {
long driversLicenseNumber;
Date expirationDate;
}
Creating and using an object
 Person john;
john = new Person ( );
john.name = "John Smith";
john.age = 37;
 Person mary = new Person ( );
mary.name = "Mary Brown";
mary.age = 33;
mary.birthday ( );
An array is an object
 Person mary = new Person ( );
 int myArray[ ] = new int[5];
 or:
 int myArray[ ] = {1, 4, 9, 16, 25};
 String languages [ ] = {"Prolog", "Java"};

Java PPt.ppt

  • 1.
  • 2.
    Introduction  Present thesyntax of Java  Introduce the Java API  Demonstrate how to build  stand-alone Java programs  Java applets, which run within browsers e.g. Netscape  Example programs
  • 3.
    Why Java?  It’sthe current “hot” language  It’s almost entirely object-oriented  It has a vast library of predefined objects and operations  It’s more platform independent  this makes it great for Web programming  It’s more secure  It isn’t C++
  • 4.
    Applets, Servlets andApplications  An applet is designed to be embedded in a Web page, and run by a browser  Applets run in a sandbox with numerous restrictions; for example, they can’t read files and then use the network  A servlet is designed to be run by a web server  An application is a conventional program
  • 5.
    Building Standalone JAVA Programs(on UNIX)  Prepare the file foo.java using an editor  Invoke the compiler: javac foo.java  This creates foo.class  Run the java interpreter: java foo
  • 6.
    Java Virtual Machine The .class files generated by the compiler are not executable binaries  so Java combines compilation and interpretation  Instead, they contain “byte-codes” to be executed by the Java Virtual Machine  other languages have done this, e.g. UCSD Pascal  This approach provides platform independence, and greater security
  • 7.
    HelloWorld (standalone)  Notethat String is built in  println is a member function for the System.out class public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); } }
  • 8.
    Comments are almostlike C++  /* This kind of comment can span multiple lines */  // This kind is to the end of the line  /** * This kind of comment is a special * ‘javadoc’ style comment */
  • 9.
    Primitive data typesare like C  Main data types are int, double, boolean, char  Also have byte, short, long, float  boolean has values true and false  Declarations look like C, for example,  double x, y;  int count = 0;
  • 10.
    Expressions are likeC  Assignment statements mostly look like those in C; you can use =, +=, *= etc.  Arithmetic uses the familiar + - * / %  Java also has ++ and --  Java has boolean operators && || !  Java has comparisons < <= == != >= >  Java does not have pointers or pointer arithmetic
  • 11.
    Control statements arelike C  if (x < y) smaller = x;  if (x < y){ smaller=x;sum += x;} else { smaller = y; sum += y; }  while (x < y) { y = y - x; }  do { y = y - x; } while (x < y)  for (int i = 0; i < max; i++) sum += i;  BUT: conditions must be boolean !
  • 12.
    Control statements II Java also introduces the try statement, about which more later switch (n + 1) { case 0: m = n - 1; break; case 1: m = n + 1; case 3: m = m * n; break; default: m = -n; break; }
  • 13.
    Java isn't C! In C, almost everything is in functions  In Java, almost everything is in classes  There is often only one class per file  There must be only one public class per file  The file name must be the same as the name of that public class, but with a .java extension
  • 14.
    Java program layout A typical Java file looks like: import java.awt.*; import java.util.*; public class SomethingOrOther { // object definitions go here . . . } This must be in a file named SomethingOrOther.java !
  • 15.
    What is aclass?  Early languages had only arrays  all elements had to be of the same type  Then languages introduced structures (called records, or structs)  allowed different data types to be grouped  Then Abstract Data Types (ADTs) became popular  grouped operations along with the data
  • 16.
    So, what isa class?  A class consists of  a collection of fields, or variables, very much like the named fields of a struct  all the operations (called methods) that can be performed on those fields  can be instantiated  A class describes objects and operations defined on those objects
  • 17.
    Name conventions  Javais case-sensitive; maxval, maxVal, and MaxVal are three different names  Class names begin with a capital letter  All other names begin with a lowercase letter  Subsequent words are capitalized: theBigOne  Underscores are not used in names  These are very strong conventions!
  • 18.
    The class hierarchy Classes are arranged in a hierarchy  The root, or topmost, class is Object  Every class but Object has at least one superclass  A class may have subclasses  Each class inherits all the fields and methods of its (possibly numerous) superclasses
  • 19.
    An example ofa class class Person { String name; int age; void birthday ( ) { age++; System.out.println (name + ' is now ' + age); } }
  • 20.
    Another example ofa class class Driver extends Person { long driversLicenseNumber; Date expirationDate; }
  • 21.
    Creating and usingan object  Person john; john = new Person ( ); john.name = "John Smith"; john.age = 37;  Person mary = new Person ( ); mary.name = "Mary Brown"; mary.age = 33; mary.birthday ( );
  • 22.
    An array isan object  Person mary = new Person ( );  int myArray[ ] = new int[5];  or:  int myArray[ ] = {1, 4, 9, 16, 25};  String languages [ ] = {"Prolog", "Java"};