• All programs consist of at least one class.

package labexe2;
public class Labexe2 {

    public static void main(String[] args) {
        int num1 =125;
        int num2=28;
        int num3=-25;
        int avg=(num1+num2+num3)/3;
        
        System.out.println("Num1 = "+ num1);
        System.out.println("Num2 = "+ num2);
        System.out.println("Num3 = "+ num3);
        System.out.println("Average = "+ avg); 
    } 
}

• Java source code file must have same name as class with .java extension.

• Identifiers are used to name classes, variables, and methods
• Identifier Rules:
  • Must start with a Character
    • A - Z, a - z, _, $, and Unicode letters
  • Can contain essentially any number of letters and digits, but no spaces
  • Case sensitive
    • Number1 and number1 are different
  • Cannot be keywords or reserved words

• Are these identifiers valid?

taxRate

• Yes.

char

• No. char is a keyword

intNumber

• Yes, The keyword int is embedded, but the identifier is not identical to a keyword.

2008Taxrate

• No. The identifier starts with a digit

public class Hello
{   
public static void main(String\[] args)
  {   
   System.out.println("Hello World!");
  }
}

public class <name>
{
    public static void main(String[] args) 
   {
        <statement(s)>;
    }
}

Every executable Java program consists of a class... that contains a method named main... that contains the statements to be executed The previous program is a class named Hello, whose main method executes one statement named System.out.println

System.out.println

• Java programs use a statement called System.out.println to instruct the computer to print a line of output on the console
• Two ways to use System.out.println :

System.out.println("<Message>");

Prints the given message as a line of text on the console.

System.out.println();

Prints a blank line on the console.

import java.util.Scanner;
public class Aaa 
{
public static void main(String[] args) 
   {
      Scanner sc=new Scanner(System.in);
      int rollno=sc.nextInt();
      System.out.println ("you inter:   "  + rollno); 
    }
}

• Performs some action
• Terminates with a semicolon (;)
• Can span multiple lines

  System.out.println( “Programming is ”  + “not a spectator sport” );

• 0, 1, or more statements
• Begins and ends with curly braces { }
• Can be used anywhere a statement is allowed

public static void main( String [ ] args )
{
   System.out.println( “Hello” );
}

• Space, tab, newline are white space characters
• At least one white space character is required between a keyword and identifier
• Any amount of white space characters are permitted between identifiers, keywords, operators, and literals

 int a
 1 + 2
 public static void main( String [] args)

• Comments explain the program to yourself and others
• Block comments
• Can span several lines
  • Begin with /*
  • End with */
• Compiler ignores all text between /* and */

   /* This program will print a message
     Anderson, Franceschi 
 */

• Line comments
  • Start with //
• Compiler ignores all text from // to the end of the line

  System.out.println( “Hello” ); // print Hello