Class Header, main() Method, Comments and Layout Rules of a Java Program (ICSE Class 9)

Understand Class Header, main() Method, Comments and Layout Rules of a Java Program

Class Header, main() Method, Comments and Layout Rules of a Java Program: key facts

Skeleton

• •comment
• •class Name {
• •main() header {
• •statements;
• •}
• •}

Statements end with a semicolon; headers do not.

Full main header

	public static void main(String args[])
public	JVM can call it
static	no object needed
void	returns nothing

BlueJ accepts void main().

Naming rule

• •Class name: letters, digits, underscore
• •not starting with a digit
• •no spaces
• •not a keyword

Start with a capital by convention.

File name

ClassName.java | must match a public class exactly

Triangle.java for class Triangle.

Class Header, main() Method, Comments and Layout Rules of a Java Program in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Class Header, main() Method, Comments and Layout Rules of a Java Program

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Capital for classes

  Class names start with a capital, variable names with a small letter.

  Use it when: Naming conventions.

• Headers have no semicolon

  Statements end with ; but class and main() headers do not.

  Use it when: Syntax errors.

• Same name, same case

  File name = class name, letter for letter.

  Use it when: Saving a program.

Class Header, main() Method, Comments and Layout Rules of a Java Program: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

class Tally
{
public static void main(String args[])
{
int x = 8, y = 2
Int p = x * y;
System.out.println("Product = " + p);
}

// totals
int a = 12, b = 5;
System.out.print(a + b);
System.out.print(" ");
System.out.println(a - b);
System.out.println(a * b);
/* System.out.println("hidden"); */

import java.util.*;
class Fare
{
public static void main(String args[])
{
Scanner sc = new Scanner(System.in);
int adults, children;
System.out.print("Adults: ");
adults = sc.nextInt();
System.out.print("Children: ");
children = sc.nextInt();
int total = adults * 150 + children * 80;
System.out.println("Total fare = Rs " + total);
}
}

Where students lose marks in Class Header, main() Method, Comments and Layout Rules of a Java Program

The slips examiners see most often, and what to write instead.

• ✗Mistake: Semicolon after a header

  Statements and headers are confused.

  Wrong working

  class Demo;

  Right working

  class Demo

  ✓Do this: Only statements end with a semicolon.

  Usually costs: 1 mark

• ✗Mistake: Wrong letter case

  Java is case sensitive.

  Wrong working

  system.out.println("Hi");

  Right working

  System.out.println("Hi");

  ✓Do this: System and String start with capital letters.

  Usually costs: 1 mark

• ✗Mistake: Unbalanced braces

  The inner braces are not counted.

  Wrong working

  Closing brace of the class missing.

  Right working

  Every { has a matching }.

  ✓Do this: Indent and count the pairs.

  Usually costs: 1 to 2 marks

Practice questions on Class Header, main() Method, Comments and Layout Rules of a Java Program

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   A class named Triangle should be saved in a file named:
   (a) triangle.txt (b) Triangle.class (c) Triangle.java (d) Triangle.exe

   Hint

   Source files have the .java extension.

   Answer

   (c) Triangle.java

2. Q2 · 1 mark · Very short answerBoard level

   What happens if import java.util.*; is left out of a program that uses Scanner?

   Hint

   Think of the compiler's message.

   Answer

   The compiler cannot find the symbol Scanner and the program does not compile.

3. Q3 · 4 marks · Short answerHarder board level

   Write a complete program (no input) that stores length 9 and breadth 4 of a rectangle in int variables and prints the area and the perimeter on two lines. State the output.

   Hint

   Area = l * b; perimeter = 2 * (l + b).

   Answer

   class Rect
   {
   public static void main(String args[])
   {
   int l = 9, b = 4;
   System.out.println("Area = " + (l * b));
   System.out.println("Perimeter = " + 2 * (l + b));
   }
   }
   Output:
   Area = 36
   Perimeter = 26

4. Q4 · 2 marks · Short answerHarder board level

   A student writes:

   /* find the sum
   int a = 4;
   int b = 6;
   System.out.println(a + b);

   and gets a compile error. Explain the error and correct it.

   Hint

   How does a multi-line comment end?

   Answer

   The comment starts with /* but is never closed with */, so the compiler reports an unclosed comment (and everything after /* is treated as comment). Add */ at the end of the first line.

5. Q5 · 2 marks · Short answerHOTS / competency

   A student saves the file as Demo.java but declares public class Test in it. Will it compile? Explain.

   Hint

   Which name must a public class match?

   Answer

   No. A public class must be saved in a file with exactly the same name, so Test must be in Test.java; the compiler reports an error for Demo.java.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Where does execution of a Java program begin?Show answer

   At main().

2. 2.Which symbol ends a Java statement?Show answer

   A semicolon.

3. 3.Is Main() the same as main()?Show answer

   No. Java is case sensitive.

4. 4.Symbol for a single-line comment?Show answer

   //

5. 5.Symbol pair for a multi-line comment?Show answer

   /* and */

6. 6.Extension of a Java source file?Show answer

   .java

Class Header, main() Method, Comments and Layout Rules of a Java Program: frequently asked questions