java01.ppt

java01.ppt

• 1.
  Introduction to Java
• 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 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
• 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) PUBLIC CLASSHELLOWORLD { PUBLIC STATIC VOID MAIN(STRING[] ARGS) { SYSTEM.OUT.PRINTLN("HELLO WORLD!"); } } NOTE THAT STRING IS BUILT IN PRINTLN IS A MEMBER FUNCTION FOR THE SYSTEM.OUT CLASS
• 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 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 ALSO INTRODUCES THE TRY STATEMENT, ABOUT WHICH MORE LATER
• 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"};
• 23.
  THANK YOU