Java basic part 2 : Datatypes Keywords Features Components Security Exceptions

JAVA BASIC PART II SOUMEN SANTRA MCA, M.Tech, SCJP, MCP

Why World Needs Java ? Dynamic Web pages. Platform Independent language. Replacement of C++. A collection of more wings. The new programming language from Sun Microsystems. Allows anyone to create a web page with Java code in it. Platform Independent language Java developer James Gosling, Arthur Van , and others. Oak, The root of Java. Java is “C++ -- ++ “.

Sun : Java Features Simple and Powerful with High Performance. Object Oriented approach. Portable and scalable. Independent Architecture. Distributed based. Multi-threaded. Robust, Secure/Safe, No network connectivity. Interpreted Source code. Dynamic programming platform.

Java : OOP Simple and easily understandable. Flexible. Portable Highly effective & efficient. Object Oriented. Compatibility. High Performance. Able for Distributed Environments. Secure.

Java as Best Object Oriented Program Object is the key of program. Simple and Familiar: “C++ Lite” No Pointers overhead to the developer. Garbage Collector. Dynamic Binding. Single & Multilevel Inheritance with “Interfaces”.

Java : JVM Unicode character. Unlike other language compilers, Java complier generates code (byte codes) for Universal Machine. Java Virtual Machine (JVM): Interprets bytecodes at runtime. Independent Architecture. No Linker. No Loader. No Preprocessor. Higher Level Portable Features for GUI : AWT, Applet, Swing.

Total Platform Independence JAVA COMPILER JAVA BYTE CODE JAVA INTERPRETER Windows 95 Macintosh Solaris Windows NT (translator) (same for all platforms) (one for each different system)

Java Write Once, Run Anywhere One Source Code In Any Platform

Java : Architecture Java Compiler – Name javac, Converts Java source code to bytecode. Bytecode - Machine representation Intermediate form of Source code & native code. JVM-A virtual machine on any target platform interprets the bytecode. Porting the java system to any new platform involves writing an interpreter that supports the Java Virtual Machine. The interpreter will figure out what the equivalent machine dependent code to run.

Java : High Performance Distributed Computing JVM uses bytecodes. Small binary class files. Just-in-time Compilers. Multithreading. Native Methods. Class Loader. Lightweight Binary Class Files. Dynamic. Good communication constructs. Secure.

Java : Security Designed as safe Language. Strict compiler javac & JIT. Dynamic Runtime Loading verified by JVM . Runtime Security Manager.

12 Definition of Java ? A programming language: Object oriented (no friends, all functions are members of classes, no function libraries -- just class libraries). Simple (no pointer arithmetic, no need for programmer to deallocate memory).  Platform Independent. Dynamic. Interpreted.

13 Java Data Types Eight basic types 4 integers (byte, short, int, short) [ example : int a; ] 2 floating point (float, double) [example : double a;] 1 character (char) [example : char a; ] 1 boolean (boolean) [example : boolean a; ] Everything else is an object String s; StringBuffer, StringBuilder, StringTokenizer

14 Java : Class and Object Declaring a class class MyClass { member variables; … member functions () ; … } // end class MyClass

15 Java Program Two kinds Applications have main() method. run from the OS prompt (DOS OR SHELL through classpath). Applets have init(), start(), stop(), paint(), update(), repaint(), destroy() method. run from within a web page using HTML <applet> tag.

16 First Java Application class MyClass { public static void main(String s [ ] ) { System.out.println(“Hello World”); } } // end class MyClass

17 Declaring and creating objects Declare a reference String s; Create/Define an object s = new String (“Hello”); Object Hello StringPool

18 Java : Arrays Arrays are objects in Java. It is a derive object i.e. Integer based or double etc. A homogeneous contiguous collection of elements of specific datatype. It’s index starts with 0. Declaration of Array: int x [ ] ; // 1-dimensional int [ ] x ; // 1-dimensional int [ ] y [ ]; // 2-dimensional int y [ ][ ];// 2-dimensional Syntax of Array for allocate space x = new int [5]; y = new int [5][10];

19 Java : Arrays length It is used to retrieve the size of an array. int a [ ] = new int [7]; // 1-dimensional System.out.println(a.length); //output print ‘7’ int b [ ] [ ] = new int [7] [11]; System.out.println(a.length); //output print ‘7’ System.out.println(b.length * b[0].length); //output print ‘77’ Let int [][][][] array = new int [5][10][12[20] , then … array.length * array[3rd dimensional].length * array[][2nd dimensional].length * array[][][1st dimensional].length is 5 x 10 x 12 x 20

20 Java : Constructors All objects are created through constructors. Assign Object. They are invoked automatically. Return class type. Mainly public for access anywhere but also private (Singleton class). Two types: default & parameterize. class Matter_Weight { int lb; int oz; public Matter_Weight (int m, int n ) { lb = m; oz = n; } } parameterize

21 Java : this keyword Refer to as “this” local object (object in which it is used). use: with an instance variable or method of “this” class. as a function inside a constructor of “this” class. as “this” object, when passed as local parameter.

22 Java : this use Example as variable Refers to “this” object’s data member. class Rectangle { int length; int breath; public Weight (int length, int breath ) { this. length = length; this. breath = breath; } }

23 Java : this use Example as method Refers to another method of “this” class. class Rectangle { public int m1 (int x) { int a = this.m2(x); return a; } public int m2(int y) { return y*7 ; } }// class close

24 Java : this use Example as function It must be used with a constructor. class Rectangle { int length, breath; public Rectangle (int l, int b) { length = a; breath = b; } } public Rectangle(int m) { this( m, 0); } } Constructor is also overloaded (Java allows overloading of all methods, including constructors).

25 Java : this use Example as function object, when passed as parameter Refers to the object that used to call the calling method. class MyClass { int a; public static void main(String [] s ) { (new MyClass ()).Method1(); } public void Method1() { Method2(this); } public void Method2(MyClass obj) { obj.a = 75; } }

26 Java : static keyword It means “GLOBAL” for all objects refer to the same storage. It applies to variables or methods throughout the Program. use: with an instance variable of a class. with a method of a class.

27 Java : static keyword (with variables) class Order { private static int OrderCount; // shared by all objects of this class throughout the program public static void main(String [] s ) { Order obj1 = new Order(); obj1.updateOdCount(); } public void updateOdCount() { OrderCount++; } }

28 Java : static keyword (without methods) class Math { public static double sqrt(double m) { // calculate return result; } } class MyClass{ public static void main(String [] s ) { double d; d = Math.sqrt(9.34); } }

29 Java : Inheritance (subclassing) class Employee { protected String name; protected double salary; public void raise(double dd) { salary += salary * dd/100; } public Employee ( … ) { … } }

30 Manager acts as sub/derived-class of Employee class Manager extends Employee { private double bonus; public void setBonus(double bb) { bonus = salary * bb/100; } public Manager ( … ) { … } }

31 Java : Overriding (methods) class Manager extends Employee { private double bonus; public void setSalary(double bb) { …} public void cal(double dd) { salary += salary * dd/100 + bonus; } public Manager ( … ) { … } } Keyword for Inheritance

32 class First { public First() { System.out.println(“ First class “); } } public class Second extends First { public Second() { System.out.println(“Second class”); } } public class Third extends Second { public Third() {System.out.println(“Third class”);} } Java : Role of Constructors in Inheritance First class Second class Third class Topmost class constructor is invoked first (like us …grandparent-->parent-->child->)

33 Java : Access Modifiers private Same class only public Everywhere protected Same class, Same package, any subclass (default) Same class, Same package

34 Java : super keyword Refers to the superclass (base class) use: with a variable or method (most common with a method) as a function inside a constructor of the subclass

35 Java : super with a method class Manager extends Employee { private double bonus; public void setSalary(double bb) { …} public void cal(double dd) { //overrides cal() of Employee super.cal(dd); // call Employee’s cal() salary += bonus; } public Manager ( … ) { … } }

36 Java : super function inside a constructor of the subclass class Manager extends Employee { private double bonus; public void setSalary(double bb) { …} public Manager ( String name, double salary, double bonus ) { super(name, salary); this.bonus = bonus; } }

37 Java : final keyword It means “constant”. It applies to variables (makes a constant variable), or methods (makes a non-overridable or final method) or classes (makes a class non-overridable or final means “objects cannot be created”).

38 Java : final keyword with a variable class Math { public final double pi = 3.141; public static double cal(double x) { double x = pi * pi; } } note: variable pi is made “Not Overriden”

39 Java : final keyword with a method class Employee { protected String name; protected double salary; public final void cal(double dd) { salary += salary * dd/100; } public Employee ( … ) { … } } Cannot override final method cal() inside the Manager class

40 Java : final keyword with a class final class Employee { protected String name; protected double salary; public void cal(double dd) { salary += salary * dd/100; } public Employee ( … ) { … } } Not create class Manager as a subclass of class Employee (all are equal)

41 Java : abstract classes and interfaces abstract classes may have both implemented and non-implemented methods. interfaces have only non-implemented methods. (concrete classes or pure classes) have all their methods implemented.

42 Java : abstract class abstract class Figure { public abstract double area(); public abstract double perimeter(); public abstract void print(); public void setOutColor(Color cc) { // code to set the color } public void setInColor(Color cc) { // code to set the color } } KEYWORD FOR ABSTRACT CLASS

43 Java : interface interface MouseClick { public void Down(); public void Up(); public void DoubleClick(); } class PureClick implements Mouse Click { // all above methods implemented here } KEYWORD FOR INTERFACE CLASS

44 Java : Exceptions (error handling) code without exceptions: ... int x = 7, y = 0, result; if ( y != 0) { result = x/y; } else { System.out.println(“y is zero”); } ... code with exceptions: ... int x = 7, y = 0, result; try { result = x/y; } catch (ArithmeticException ex ) { System.out.println(“y is zero”); } ... A nice way to handle errors in Java programs

45 Java : Exceptions (try-catch-finally block ) import java.io.*; class Test{ public static void main(String args[]){ int x = 7, y = 0, result; try { result = x/y; /// more code .. Main operations } catch (ArithmeticException ex1 ) { System.out.println(“y is zero”); } catch (IOException ex2 ) { System.out.println(“Can’t read Format error”); } finally { System.out.println(“Closing file”); /// code to close file, release memory. } }} KEYWORD FOR Exception CLASS Package FOR Exception CLASS

46 Java : Using Throwing Exceptions public int divide (int x, int y ) throws ArithmeticException { if (y == 0 ) { throw new ArithmeticException(); } else { return x/y ; } } // end divide() KEYWORD FOR Exception CLASS

47 Java : User Defining exceptions public int divide (int x, int y ) throws MyException { if (y == 0 ) { throw new MyException(); } else { return a/b ; } } // end divide() } import java.io.*; class MyException extends ArithmeticException { KEYWORD FOR Exception CLASS Package FOR Exception CLASS inherit Exception CLASS

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