Debugging Errors and the Section B Class Pattern (ISC Class 12 Computer Science)

Understand Debugging Errors and the Section B Class Pattern

import java.util.Scanner;

public class PalNum
{
    int n;    // number entered
    int rev;  // its reverse

    PalNum()
    {
        n = 0;
        rev = 0;
    }

    void accept()
    {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a positive integer:");
        n = sc.nextInt();
    }

    // stores the reverse of n in rev
    void reverse()
    {
        int t = n;
        rev = 0;
        while (t > 0)
        {
            rev = rev * 10 + t % 10;
            t = t / 10;
        }
    }

    void check()
    {
        reverse();
        if (rev == n)
            System.out.println(n + " is a palindrome");
        else
            System.out.println(n + " is not a palindrome");
    }

    public static void main(String[] args)
    {
        PalNum ob = new PalNum();
        ob.accept();
        ob.check();
    }
}

Debugging Errors and the Section B Class Pattern: key facts

Types of error

Syntax	compiler rejects
Run-time	exception while running
Logical	wrong result

Logical errors need dry runs.

Section B parts

• •Data members
• •Constructor
• •Input method
• •Processing methods
• •Display or check
• •main
• •Comments

Marks are given for each.

Run-time exceptions

• •ArithmeticException
• •ArrayIndexOutOfBoundsException
• •NullPointerException
• •InputMismatchException

Know one cause each.

How to answer Debugging Errors and the Section B Class Pattern questions

1. 1Run the snippet by hand with a small input and compare with the intended result.
2. 2Classify the fault: compile failure (syntax), crash while running (run-time), wrong answer (logical).
3. 3Correct the line and re-trace to check.
4. 4For a class, use the seven-point pattern and keep names and signatures exactly as given.

Debugging Errors and the Section B Class Pattern 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 Debugging Errors and the Section B Class Pattern

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

• Compile, Crash, Confuse

  Syntax errors stop the compile; run-time errors crash; logical errors only confuse (wrong answer).

  Use it when: Classifying errors.

• Section B checklist C-I-P-D-M

  Constructor, Input, Processing, Display, Main (plus comments).

  Use it when: Writing a full class.

Debugging Errors and the Section B Class Pattern: solved examples

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

Where students lose marks in Debugging Errors and the Section B Class Pattern

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

• ✗Mistake: Calling a wrong-answer fault a syntax error

  Syntax errors stop compilation.

  Wrong working

  A loop that sums 1 to 4 instead of 1 to 5 is a syntax error.

  Right working

  It is a logical error; the program compiles and runs.

  ✓Do this: Does it compile? Does it crash? Is it wrong?

  Usually costs: 1 mark.

• ✗Mistake: Missing main( ) in a Section B answer

  main( ) carries marks.

  Wrong working

  Class with methods but no main( ).

  Right working

  main( ) creates an object and calls accept( ) and the processing methods.

  ✓Do this: Always write main( ).

  Usually costs: 1-2 marks.

• ✗Mistake: Changing the method names given in the question

  Names and signatures must match exactly.

  Wrong working

  Question asks for reverse( ); student writes rev( ).

  Right working

  Use the name reverse( ) exactly.

  ✓Do this: Copy names from the question.

  Usually costs: 1 mark.

Practice questions on Debugging Errors and the Section B Class Pattern

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 · Very short answerBoard level

   Name the exception raised when a program divides an integer by zero.

   Hint

   Arithmetic.

   Answer

   ArithmeticException.

2. Q2 · 2 marks · Short answerBoard level

   State two techniques used for debugging a program.

   Hint

   By hand or by tool.

   Answer

   Dry runs and printing intermediate values (also testing small cases, checking boundary values, the BlueJ debugger).

3. Q3 · 2 marks · Short answerHarder board level

   Why must the constructor in a Section B class have no return type?

   Hint

   Definition of a constructor.

   Answer

   A constructor is a special method with the class name that initialises the object when it is created; writing a return type would make it an ordinary method, so it would not be called automatically.

4. Q4 · 4 marks · Short answerHarder board level

   A statement 'double avg = (a + b) / 2;' with int a = 9 and int b = 4 prints 6.0. Find the correct value, the type of error and the corrected statement.

   Hint

   Integer division.

   Answer

   (9 + 4) / 2 in integers is 13 / 2 = 6, giving 6.0. Correct average = 6.5. This is a logical error from integer division. Correct statement: double avg = (a + b) / 2.0;.

5. Q5 · 4 marks · Short answerHOTS / competency

   A student writes a Section B class in which the input method also prints the result and main( ) is missing. Evaluate using the standard pattern and say what to change.

   Hint

   Separation of tasks.

   Answer

   The pattern separates input, processing and display, and requires main( ) to create an object and call the methods. Move printing into a display or check method, keep accept( ) for input only, add main( ) with an object and ordered calls, and add comments. Marks are lost for each missing component.

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.Which error stops compilation?Show answer

   A syntax error.

2. 2.Which error is found only by testing?Show answer

   A logical error.

3. 3.Does a constructor have a return type?Show answer

   No.

4. 4.What does main( ) do in Section B?Show answer

   Creates an object and calls the methods in order.

Debugging Errors and the Section B Class Pattern: frequently asked questions