Basic Structure of a Class in Java ICSE Class 9 Computer Applications

What Basic Structure of a Class in Java is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Computer Applications. Class IX is examined by your school: one 100-mark theory paper of 2 hours, plus 100 marks of internal assessment (at least 15 programming assignments and a written project on ethical computing). This is topic 5 of the 2028 syllabus.Source: CISCE ICSE 2028 Computer Applications syllabus, Class IX

Key concepts

The structure without input

// comment (non-executable: ignored by the compiler)
class ClassName
{
void main() // BlueJ form; full form: public static void main(String args[])
{
set of statements;
}
}

Parts:
1. Comment: describes the program for human readers; not executed.
2. Class header: the keyword class followed by the class name. By convention the name starts with a capital letter (Greeting, Rectangle). The class name should match the file name (Greeting.java).
3. Opening and closing braces { } of the class: everything in the program is inside them.
4. main() method: the starting point of execution. Its body is enclosed in its own braces.
5. Statements: instructions inside main(), each ending with a semicolon.

As run in this study guide:
// A class without input
class Greeting
{
public static void main(String args[])
{
String name = "Kabir";
int year = 2028;
System.out.println("Hello, " + name);
System.out.println("ICSE exam year: " + year);
}
}
Output:
Hello, Kabir
ICSE exam year: 2028

Worked example

(2 marks) Why is main() important in a Java program? What happens if it is misspelt as Main()?

Try it first, then show the solutionSolution

1. main() is the entry point: execution begins from its first statement - 1 mark.
2. Java is case sensitive; Main() is a different method, so the program compiles but cannot be run as a program (the JVM reports that the main method was not found) - 1 mark.

Answer: main() is where execution starts. Because Java is case sensitive, a method named Main() is not recognised as the entry point: the class compiles, but running it gives an error that the main method was not found.

public, static and String args[]

The full header is: public static void main(String args[])

• public: an access specifier that lets the JVM (outside the class) call main().
• static: main() belongs to the class itself, so it can run without creating an object first.
• void: main() does not return a value.
• main: the fixed name of the starting method.
• String args[]: an array of strings that can receive values typed on the command line when the program starts (command-line arguments).

The CISCE syllabus says public, static and String args[] are 'default in nature': in BlueJ you may write simply void main() and BlueJ supplies what is needed when you run it. Many other tools, and the command line in older Java versions such as JDK 17, need the full header. Writing the full header is never wrong.

Worked example

(2 marks) State the purpose of the keywords public and static in the main() header.

Try it first, then show the solutionSolution

1. public: makes main() accessible from outside the class, so the JVM can call it - 1 mark.
2. static: lets main() run without creating an object of the class - 1 mark.

Answer: public allows main() to be called from outside the class, by the JVM. static means main() belongs to the class and can be called without first creating an object.

The structure with the Scanner class

import java.util.*;
class ClassName
{
void main()
{
declaration of variables;
create an object of the Scanner class;
accept the required values using the methods of the Scanner class;
set of statements;
}
}

Step by step:
1. import java.util.*; makes the classes of the package java.util, including Scanner, available. Without it the compiler cannot find the Scanner class (unless you write java.util.Scanner in full).
2. Declare variables: int l, b, area, peri;
3. Create a Scanner object linked to the keyboard: Scanner sc = new Scanner(System.in);
4. Prompt and accept values: System.out.print("Enter length: "); l = sc.nextInt();
5. Process and output: area = l * b; System.out.println("Area = " + area);

import java.util.Scanner; imports only the Scanner class; import java.util.*; imports all classes of the package. Either works.

Worked example

(2 marks) What is the purpose of (i) import java.util.*; (ii) Scanner sc = new Scanner(System.in);?

Try it first, then show the solutionSolution

1. (i) Makes all classes of java.util, including Scanner, available to the program - 1 mark.
2. (ii) Creates a Scanner object sc that reads input from the keyboard (System.in) - 1 mark.

Answer: (i) It imports the java.util package so that its classes, such as Scanner, can be used. (ii) It creates an object sc of the Scanner class connected to the standard input (keyboard), so values can be read with methods like sc.nextInt().

A complete program following the structure

// Program to find the area and perimeter of a rectangle
import java.util.*;
class Rectangle
{
public static void main(String args[])
{
int l, b, area, peri; // declaration of variables
Scanner sc = new Scanner(System.in); // object of the Scanner class
System.out.print("Enter length: ");
l = sc.nextInt(); // accept values
System.out.print("Enter breadth: ");
b = sc.nextInt();
area = l * b; // set of statements
peri = 2 * (l + b);
System.out.println("Area = " + area);
System.out.println("Perimeter = " + peri);
}
}

Sample run (as run in this study guide, inputs 12 and 8):
Enter length: 12
Enter breadth: 8
Area = 96
Perimeter = 40

Every part of the prescribed structure is labelled by a comment. Use this layout for every program: it earns the structure marks even if the logic has a slip.

Worked example

(2 marks) In the Rectangle program, why is print() used for the prompts and println() for the results?

Try it first, then show the solutionSolution

1. print() keeps the cursor on the same line, so the user types the value next to the prompt - 1 mark.
2. println() moves to a new line after each result, so the results appear on separate lines - 1 mark.

Answer: print() leaves the cursor on the prompt line, so the input is typed beside 'Enter length:'. println() ends each result line, so Area and Perimeter appear on separate lines.

Rules of layout: braces, semicolons, comments and case

1. Every statement ends with a semicolon (;). Headers of the class and of main() do not.
2. Braces come in pairs: the class's { } contain main(); main()'s { } contain the statements. Indent the inner lines so the pairs are easy to match.
3. Java is case sensitive: System, String and Scanner start with capitals; main, class, int, import are lower case.
4. Comments: // for a single line; /* ... */ for several lines. They explain the program and are ignored by the compiler.
5. The file name should be the same as the class name, with .java (Rectangle.java). If the class is declared public, it must match.
6. Declarations come before the variables are used.

What the compiler says (as checked in this study guide):

• Missing semicolon after int n = sc.nextInt() gives the error: ';' expected.
• Without import java.util.*; the compiler reports that it cannot find the symbol Scanner.
• Writing Main instead of main compiles, but running gives: Main method not found in class.

Worked example

(2 marks) Find two errors: class Demo { public static void main(String args[]) { int n = 5 System.Out.println(n); } }

Try it first, then show the solutionSolution

1. Missing semicolon after int n = 5 - 1 mark.
2. System.Out should be System.out (Java is case sensitive) - 1 mark.

Answer: (1) A semicolon is missing after int n = 5. (2) Out must be written in lower case: System.out.println(n);

Basic Structure of a Class in Java in real life

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

Topics in this chapter

• Class Header, main() Method, Comments and Layout Rules of a Java Program
• Writing Programs with Scanner Input and Tracing Their Outputs

Basic Structure of a Class in Java: syntax and key facts

Structure without input

// comment → class Name { void main() { statements } }

Full header: public static void main(String args[])

Structure with input

1. 1.import java.util.*;
2. 2.class
3. 3.main()
4. 4.declare
5. 5.Scanner sc = new Scanner(System.in);
6. 6.accept
7. 7.process
8. 8.output

Prescribed order.

Syntax rules

; ends statements; { } in pairs; case sensitive; // and /* */ comments

File name = class name + .java

How the school exam asks Basic Structure of a Class in Java

The question patterns to expect, and the method that works for each.

Tricks to remember Basic Structure of a Class in Java

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

• I-C-M-D-S-A-P-O

  Import, Class, Main, Declare, Scanner, Accept, Process, Output.

  Use it when: Order of a program with input.

• PSVM

  public static void main: 'Please Start Very Merrily'.

  Use it when: Full main() header.

• Capital S, small m

  System, String, Scanner start with capitals; main, class, int, import are small.

  Use it when: Case-sensitivity errors.

Basic Structure of a Class in Java: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 6-mark question split into 2 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Basic Structure of a Class in Java

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

• ✗Mistake: Forgetting import java.util.*;

  The class seems complete without it.

  ✓Do this: Write the import line first in every program that uses Scanner.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Wrong case in System, String or Scanner

  Typing fast.

  Wrong working

  system.out.println("Hi");

  Right working

  System.out.println("Hi");

  ✓Do this: Class names start with capitals; check every line.

  Usually costs: 1 mark per error.

• ✗Mistake: Unmatched braces

  No indentation.

  ✓Do this: Indent the body of main() inside the class; count opening and closing braces.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Integer formula for a decimal answer

  Writing 9 / 5 with int operands.

  Wrong working

  f = 9 / 5 * c + 32; (9 / 5 = 1)

  Right working

  f = c * 9 / 5 + 32; or f = 9.0 / 5 * c + 32;

  ✓Do this: Make sure a double takes part before any division.

  Usually costs: 2 to 3 marks.

Practice questions on Basic Structure of a Class in Java

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 · MCQExam level

   A single-line comment begins with:
   (a) /*
   (b) //
   (c) **
   (d) #

   Hint

   Two slashes.

   Answer

   (b) //

2. Q2 · 1 mark · MCQExam level

   The method from which execution of a Java application begins is:
   (a) start()
   (b) init()
   (c) main()
   (d) run()

   Hint

   Entry point.

   Answer

   (c) main()

3. Q3 · 1 mark · Very short answerExam level

   Write the statement to create a Scanner object named in that reads from the keyboard.

   Hint

   System.in

   Answer

   Scanner in = new Scanner(System.in);

4. Q4 · 2 marks · Very short answerHarder exam level

   What does String args[] in the main() header hold?

   Hint

   Command line.

   Answer

   An array of String values (command-line arguments) that can be passed to the program when it is started; ICSE programs usually leave it unused.

5. Q5 · 2 marks · Short answerHOTS / competency

   Give the output: class T { public static void main(String args[]) { int months = 12, fee = 1500; System.out.println("Annual fee: " + months * fee); System.out.println("Sum: " + months + fee); } }

   Hint

   Precedence of * over +.

   Answer

   Annual fee: 18000
   Sum: 121500

6. Q6 · 2 marks · Very short answerHarder exam level

   Differentiate between import java.util.*; and import java.util.Scanner;

   Hint

   All vs one.

   Answer

   import java.util.*; makes all classes of java.util available; import java.util.Scanner; makes only the Scanner class available. Both allow Scanner to be used.

7. Q7 · 15 marks · Long answerHarder exam level

   Write a program, following the prescribed structure, to accept the price of an item and the quantity bought and print the total amount.

   Hint

   double price, int qty.

   Answer

   // Bill for one item
   import java.util.*;
   class Bill
   {
   public static void main(String args[])
   {
   double price, total;
   int qty;
   Scanner sc = new Scanner(System.in);
   System.out.print("Enter price: ");
   price = sc.nextDouble();
   System.out.print("Enter quantity: ");
   qty = sc.nextInt();
   total = price * qty;
   System.out.println("Total amount = " + total);
   }
   }
   Sample run (as run, inputs 45.5 and 4):
   Enter price: 45.5
   Enter quantity: 4
   Total amount = 182.0

8. Q8 · 2 marks · Very short answerHarder exam level

   A student puts the declaration int area; after the statement area = l * b; inside main(). What happens, and why?

   Hint

   Declare before use.

   Answer

   A compile-time error (cannot find symbol area), because a variable must be declared before it is used; statements in main() are processed in order.

9. Q9 · 2 marks · Short answerHOTS / competency

   Why is it good practice to print a prompt before every input statement? What could go wrong without it?

   Hint

   The user cannot see what is expected.

   Answer

   Without a prompt the program waits silently, and the user does not know what to type or in what order; they may enter breadth before length or a decimal where an int is expected, causing wrong results or an input error.

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.Package that contains Scanner?Show answer

   java.util

2. 2.What is System.in?Show answer

   The standard input stream (keyboard).

3. 3.Which keywords does CISCE treat as default in main()?Show answer

   public, static and String args[].

4. 4.Multi-line comment symbols?Show answer

   /* and */

5. 5.What ends a Java statement?Show answer

   A semicolon (;).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐// comment; class Name { void main() { statements } }
• ☐With input: import java.util.*; → class → main() → declare → Scanner sc = new Scanner(System.in); → accept → process → output.
• ☐public: JVM can call; static: no object needed; void: no return; String args[]: command-line values.
• ☐BlueJ accepts void main(); full form public static void main(String args[]) works everywhere.
• ☐; ends statements; braces in pairs; Java is case sensitive.

Basic Structure of a Class in Java: frequently asked questions