Programming in java Lecture-2 Java Introduction

Platform Computing platform include hardware and software framework, this combination allows software to run.

Platform problem Source code Compiler (windows) Object code

Platform problem Source code Compiler (windows) Object code Compiler (linux) Compiler (mac) Source code Object code

java Java Source code Java Source code Compiler Intermediate code Phase I Intermediate code Interpreter Object code execution Object code execution Phase II

Byte code Interpreter (windows) Object code Interpreter (linux) Interpreter (mac) Byte code Object code

Byte code Windows Operating system Windows Operating system Linux Operating system Linux Operating system Mac Operating system Mac Operating system JVM Byte Code

Byte code Java Source code Java Source code Compiler Intermediate code Phase I Intermediate code Interpreter Object code execution Object code execution Phase II 0 iload_1 1 iload_2 2 iadd 3 istore_3 A=10 B=20 C=A+B

Java Features  Simple C++ Java Object oriented Polymorphism Inheritance.... multithreading

Java’s Features  Platform Independent

Java’s Features  Pure Object Oriented Class One { Public static void main(String arg[]) { System.out.println(“Hello World”); } Class One { Public static void main(String arg[]) { System.out.println(“Hello World”); } In java everything must be inside class.

Java’s Features  Strongly typed #include Void main() { printf(“Value of I”,&i); } Every variable used in program must be declared.

Java’s Features  Secure Internet When java bytecode come from network or another machine, in that case before executing that file verifier check first.

Java Features  Multithreading This is demo for checking multithreading concept of java. Developed in early c This is demo for checking multithreading concept of java. Spelling checking operation running while used giving input Developed in early java

Java’s Features  Architectural Neutral  Distributed  Garbage Collection

Garbage collection Memory One ob1=new One(); Two ob2=new Two(); two class object one class object Ob1Ob2 Ob1=ob2;

Java Package  Java package has two parts  JDK( java development Kit)  JRE(java runtime environment)  Java version 1.7

Hello program in c #include void main() { printf(“hello world”); } 1. For including header files. 2. main function start 3. print on screen hello world #include void main(int argc, char * argv[]) { printf(“hello world”); }

Java first program  Java is pure object oriented programming language. Class demo { } main funtion(arguments) { print statement for hello world }

First Java Program import java.lang.*; Class demo { public static void main(String arg[]) { System.out.println(“Hello World”); } For including libraries in java(like include statement in c) Name of class To print message on screen(like printf function) Main method in java. With string type of argument

Commands to run  Save file with name of class, which contains main method  javac demo.java  java demo

Creating, Compiling And Running Java Programs javac Java compiler Java byte code filename.class To compile the program at the command line type "javac filename.java" To run the interpreter, at the command line type "java filename" java Java Interpreter Type it in with the text editor of your choice filename.java (java file) Java program

Compiling The Smallest Java Program public class Smallest { public static void main (String[] args) { } Smallest.java javac (Java byte code) : : Smallest.class Type “javac Smallest.java”

Running The Smallest Java Program (Java byte code) : : Smallest.class java Type “java Smallest” (Platform/Operating specific binary : :

A Java program /* Here you describe what your program does. */ public class CLASS-NAME { public static void main (String args[]) { // your program goes here } // end of main } // end of class

Compiling and Running In order to run a Java program:  First you compile it  that is, you run a program called compiler that checks whether the program follows the Java syntax  if it finds errors, it lists them  If there are no errors, it translates the program into Java bytecode  Example: assume you created a program called Hello.java prompt>javac Hello.java  If successful, this creates a file Hello.class which contains the translation (Java bytecode) of Hello.java  Then you execute it  That is, you call the Java Virtual Machine to interpret and execute the Java bytecode of your program  Example: prompt>java Hello

Hello world program When learning a new language, the first program people usually write is one that salutes the world :). Here is the Hello world program in Java. /* This program prints out “hello world!” and terminates. */ public class Hello { public static void main (String args[]) { System.out.println(“Hello world!”); } // end of main } // end of class

Notes  Comments  what follows after // on the same line is considered comment  Or, what is in between /* this is a comment */  Indentation  is for the convenience of the reader; compiler ignores all spaces and new lines ; the delimiter for the compiler is the semicolon  All instructions end by semicolon  Lower vs. upper case matters!!  Void is different than void  Main is different that main

Writing to user (output) System.out.println(variable-name); prints the value of variable to the user System.out.println(“any message “); prints the message within quotes to the user System.out.println(“hello” + “world” + a + “plus“ + b); assuming the value of a is 3 and of b is 7, it prints helloworld3plus7 Note: System.out.println() always prints on a new line.

Example /* This program illustrates the System.out.println command. */ public class Hello { public static void main (String args[]) { System.out.println(“This is my first Java program!”); System.out.print(“I like Java.”); System.out.print(“I think Java is cool.”); } // end of main } // end of class  Exercise: change the print to println above. What is the difference?

Variable declaration type variable-name; Meaning: variable will be a variable of type Where type can be:  int//integer  double//real number Example: int a, b, c; double x; int sum;

Example /* Printing ages. */ public class MyFirstJavaProgram { public static void main (String args[]) { int myAge, myFriendAge; /* declare two integer variables */ myAge = 20; myFriendAge = myAge + 1; //one year older System.out.println(“Hello, I am “ +myAge + “years old, and my friend is “ + myFriendAge + “ years old”); System.out.println(“Goodbye”); } // end of main } // end of class

If statements if (condition) { //instructions } else { //instructions } //instructions condition instructions TrueFalse instructions

Boolean conditions..are built using  Comparison operators == equal != not equal < less than > greater than <= less than or equal >= greater than or equal  Example: int x, y; //two variables //assume they have some values if (x <= y) { System.out.println(“x is smaller”); } else { System.out.println(“x is larger”); }

Java data types TypeDescription byte8 bit signed integer short16 but signed integer int32 bit signed integer long64 bit signed integer float32 bit signed real number double64 bit signed real number char16 bit Unicode character (ASCII and beyond) boolean1 bit true or false value StringA sequence of characters between double quotes ( "" )

Primitive types Primitive typeSizeMinimumMaximum boolean1-bit —— char16-bitUnicode 0Unicode byte8-bit short16-bit int32-bit long64-bit float32-bitIEEE754 double64-bitIEEE754

Default values for primitive members  When a primitive type data is a member of a class, it’s guaranteed to get a default value even if you don’t initialize it.  Not true for those local variables!!  There will be compile error if you use it without initialization Primitive typeDefault booleanfalse char ‘ \u0000 ’ (null) byte(byte)0 short(short)0 int0 long0L float0.0f double0.0d

Example class Hello{ public static void main ( String[] args ) { int x; System.out.println(x); } >javac Hello.java Hello.java:5: variable x might not have been initialized System.out.println(x); ^ 1 error

Arrays in Java  An ordered collection of something, addressed by integer index  Something can be primitive values, objects, or even other arrays. But all the values in an array must be of the same type.  Only int or char as index  long values not allowed as array index  0 based  Value indexes for array “a” with length 10  a[0] – a[9];  a.length==10  Note: length is an attribute, not method

Arrays in Java: declaration  Declaration  int[] arr;  Person[] persons;  Also support: int arr[]; Person persons[]; (confusing, should be avoided)  Creation  int[] arr = new int[1024];  int [][] arr = { {1,2,3}, {4,5,6} };  Person[] persons = new Person[50];

Arrays in Java: safety  Cannot be accessed outside of its range  ArrayIndexOutOfBoundsException  Guaranteed to be initialized  Array of primitive type will be initialized to their default value  Zeroes the memory for the array  Array of objects: actually it’s creating an array of references, and each of them is initialized to null.

Arrays in Java:  second kind of reference types in Java int[] arr = new int [5]; arr int[][] arr = new int [2][5]; arr[0] arr[1] arr

Scoping  Scope determines both the visibility and lifetime of the names defined within the scope  Scope is determined by the placement of {}, which is called block. { int x = 10; //only x available { int y = 20; //both x and y available } //only x available, y out of scope! }

Scoping  Notice, you cannot do the following, although it’s legal in C/C++. { int x = 10; { int x = 20; } Compile error Hello.java:6: x is already defined in main(java.lang.String[]) int x =20; ^ 1 error

Scope of objects  When you create an object using new, the object hangs around past the end of the scope, although the reference vanishes. { String s = new String("abc"); } Reference s vanishes, but the String object still in memory Solution: Garbage Collector!

Importing library  If you need any routines that defined by java package import java.util.*; import java.io.*;  Put at the very beginning of the java file  java.lang.* already been imported.  Check javadoc for the classes

Static keyword  Want to have only one piece of storage for a data, regardless how many objects are created, or even no objects created  Need a method that isn’t associated with any particular object of this class  static keyword apply to both fields and methods  Can be called directly by class name  Example: java.lang.Math  Non-static fields/methods must be called through an instance

main() class Hello{ int num; public static void main(String[] args) { num = 10; } >javac Hello.java Hello.java:4: non-static variable num cannot be referenced from a static context num = 10; ^ 1 error

Main() doesn’t belong in a class  Always static  Because program need a place to start, before any object been created.  Poor design decision  If you need access non-static variable of class Hello, you need to create object Hello, even if main() is in class Hello! class Hello{ int num; public static void main(String[] args){ Hello h = new Hello(); h.num = 10; }