Data Processing in Java ICSE Class 9: Input with Scanner, Errors and Comments

What Data Processing 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 6 of the 2028 syllabus.Source: CISCE ICSE 2028 Computer Applications syllabus, Class IX

Key concepts

Three ways to supply data: initialisation, parameters, input

1. Initialisation (data before execution): values are written into the program itself when variables are declared. They are the same every time the program runs unless the code is changed.
int a = 10, b = 20;

2. Parameters (data at the time of execution): values are passed to a method when it is called. In BlueJ, if main() or any method has parameters, for example void rectangle(int l, int b), BlueJ opens a dialog box asking for the values when you run it. In code, the values are passed as arguments: obj.rectangle(6, 4); prints Area = 24 (as run in this study guide).

3. Input stream (data entry during execution): the program pauses while running and the user types values, which are read with an object of the Scanner class. The values can be different on every run.
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();

Comparison: initialisation is fixed in the code; parameters are given when the method starts; input is given while the program is running, in response to prompts.

Worked example

(2 marks) Distinguish between initialisation and input through the Scanner class.

Try it first, then show the solutionSolution

1. Initialisation: values assigned in the program before execution; fixed unless the code is edited - 1 mark.
2. Scanner input: values typed by the user during execution; can change each run - 1 mark.

Answer: In initialisation the values are written into the program (int a = 10;) before it runs, so they stay the same every time. With Scanner input the user types the values while the program runs (a = sc.nextInt();), so the same program can work on different data.

Packages

A package is a group of related classes (and interfaces) stored together, like a folder of files. Packages organise Java's large library and avoid name clashes.

Important built-in packages:

• java.lang: the basic classes of the language, such as System, String and Math. It is imported automatically (default), so no import statement is needed.
• java.util: utility classes, including Scanner.
• java.io: classes for input and output with files and streams.
• java.awt and javax.swing: classes for graphical user interfaces.
• java.net: classes for networking.

Using a package: import java.util.*; (all classes) or import java.util.Scanner; (one class). Without an import, the full name java.util.Scanner can be written each time.

The programmer can also create packages, but that is not required in Class IX.

Worked example

(2 marks) What is a package? Why do we not need to import java.lang?

Try it first, then show the solutionSolution

1. Package: a collection of related classes grouped together, e.g. java.util - 1 mark.
2. java.lang contains the basic classes (System, String, Math) and is imported by default - 1 mark.

Answer: A package is a group of related classes stored together, such as java.util, which contains Scanner. java.lang holds the fundamental classes like System, String and Math, so Java imports it automatically into every program.

Scanner methods for different data types

After Scanner sc = new Scanner(System.in);

• sc.nextShort(): reads a short.
• sc.nextInt(): reads an int.
• sc.nextLong(): reads a long.
• sc.nextFloat(): reads a float.
• sc.nextDouble(): reads a double.
• sc.next(): reads the next word (token) as a String, stopping at a space or new line.
• sc.nextLine(): reads the rest of the current line, including spaces, as a String.
• sc.next().charAt(0): reads the next word and takes its first character, giving a char (Scanner has no nextChar()).

Input items (tokens) may be separated by spaces or new lines. As run in this study guide, with the input
120 45000 9876543210 2.5 3.14159 Good morning
New Delhi India
Yes
the statements
short s = sc.nextShort(); int i = sc.nextInt(); long l = sc.nextLong();
float f = sc.nextFloat(); double d = sc.nextDouble();
String w = sc.next(); String rest = sc.nextLine(); String line = sc.nextLine();
char c = sc.next().charAt(0);
give s = 120, i = 45000, l = 9876543210, f = 2.5, d = 3.14159, w = "Good", rest = " morning" (with the leading space), line = "New Delhi India", c = 'Y'.

Worked example

(2 marks) Write statements to accept (i) a city name that may contain spaces, (ii) a grade letter.

Try it first, then show the solutionSolution

1. (i) String city = sc.nextLine(); - 1 mark.
2. (ii) char grade = sc.next().charAt(0); - 1 mark.

Answer: (i) String city = sc.nextLine();
(ii) char grade = sc.next().charAt(0);

The nextInt() then nextLine() trap

nextInt() (like nextDouble() and next()) reads the number but leaves the Enter (new-line character) in the input. A following nextLine() reads up to that new line, so it returns an empty string immediately.

As run in this study guide (input 14, Enter, Pune, Enter):
int age = sc.nextInt();
String city = sc.nextLine();
System.out.println("Age " + age + ", city [" + city + "]");
Output: Age 14, city []
The user never gets a chance to type the city.

Fixes:
1. Read strings with nextLine() before reading numbers (as in the report program below).
2. Add an extra sc.nextLine(); after nextInt() to consume the leftover new line.
3. If the string has no spaces, use next() instead of nextLine().

Worked example

(2 marks) A program reads int age = sc.nextInt(); then String city = sc.nextLine(); The city is always blank. Explain and fix.

Try it first, then show the solutionSolution

1. nextInt() leaves the new-line character in the input; nextLine() reads it and returns an empty string - 1 mark.
2. Fix: add sc.nextLine(); after nextInt(), or read the city before the age - 1 mark.

Answer: nextInt() reads 14 but leaves the Enter key's new line in the buffer, which nextLine() reads at once, giving an empty string. Insert sc.nextLine(); after reading the age (or read the city first).

Types of errors: syntax, runtime and logical

1. Syntax errors (compile-time errors): violations of the grammar rules of Java, detected by the compiler; the program does not compile until they are corrected.
Examples: a missing semicolon (';' expected), a misspelt keyword (Int for int), unmatched braces, using a variable that has not been declared (cannot find symbol), a missing import for Scanner.

2. Runtime errors: the program compiles, but an error occurs while it is running and the program stops abnormally (an exception).
Examples (as run in this study guide):

• Dividing an integer by zero: java.lang.ArithmeticException: / by zero.
• Typing text when nextInt() expects a number: java.util.InputMismatchException.

3. Logical errors: the program compiles and runs without stopping but gives wrong results, because the logic or formula is wrong. The compiler cannot detect them; they are found by testing with known answers.
Example (as run): int a = 10, b = 20; System.out.println(a + b / 2); prints 20 instead of the average 15, because / is done before +. Correct: (a + b) / 2.

Debugging: correcting errors. Test programs with simple values whose answers you know.

Worked example

(2 marks) Classify each error: (i) int x = 5 (no semicolon); (ii) average = a + b / 2; (iii) entering 'ten' for nextInt().

Try it first, then show the solutionSolution

1. (i) syntax error; (ii) logical error - 1 mark.
2. (iii) runtime error (InputMismatchException) - 1 mark.

Answer: (i) Syntax (compile-time) error. (ii) Logical error: it runs but gives the wrong average. (iii) Runtime error: InputMismatchException stops the program.

Comments

Comments are notes in the program for human readers; the compiler ignores them (they are non-executable).

1. Single-line comment: begins with // and continues to the end of that line.
int total = m1 + m2 + m3; // sum of marks

2. Multi-line comment: begins with /* and ends with */; it can span several lines.
/* Program to prepare a student report
Written for ICSE Class IX */

Uses: explaining the purpose of the program and of difficult steps; recording the author and date; temporarily switching off a line while testing.

Good practice: comment what each section does (input, processing, output), not every obvious line. (Java also has documentation comments beginning /** for generating help pages; they are not required in Class IX.)

Worked example

(2 marks) Distinguish between single-line and multi-line comments with an example of each.

Try it first, then show the solutionSolution

1. Single-line: starts with //, ends at the end of the line, e.g. // sum of marks - 1 mark.
2. Multi-line: between /* and */, can cover many lines - 1 mark.

Answer: A single-line comment starts with // and lasts to the end of the line: // sum of marks. A multi-line comment is enclosed between /* and */ and can run over several lines: /* Program to prepare a student report */.

Data Processing 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

• Initialisation, Parameters and Scanner Input Methods with Input Tracing
• Packages, Types of Errors and Comments in Java Programs

Data Processing in Java: syntax and key facts

Data sources

Initialisation (before execution); parameters (at execution); Scanner input (during execution)

Syllabus wording.

Scanner methods

nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), next(), nextLine(), next().charAt(0)

Match method to variable type.

Errors

Syntax: found by compiler; runtime: program stops while running; logical: wrong output, no message

/ by zero; InputMismatchException.

Comments

// single line; /* multi-line */

Non-executable.

How the school exam asks Data Processing in Java

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

Tricks to remember Data Processing in Java

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

• Before - At - During

  Initialisation before, parameter at the start, Scanner during.

  Use it when: Three ways of supplying data.

• SRL errors

  Syntax: Spelling and grammar; Runtime: Running crash; Logical: Looks fine, wrong answer.

  Use it when: Types of errors.

• next stops at space, nextLine stops at Enter

  next() = one word; nextLine() = whole line.

  Use it when: Choosing string input.

Data Processing 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 7-mark question split into 2 + 3 + 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 Data Processing in Java

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

• ✗Mistake: Using a non-existent method

  Guessing method names.

  Wrong working

  char c = sc.nextChar();

  Right working

  char c = sc.next().charAt(0);

  ✓Do this: Learn the eight methods listed in the syllabus exactly.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Method does not match the variable type

  Copying nextInt() for every input.

  Wrong working

  double price = sc.nextInt(); (decimals rejected)

  Right working

  double price = sc.nextDouble();

  ✓Do this: Choose the method that matches the declared type.

  Usually costs: 1 mark per input.

• ✗Mistake: Blank strings after numbers

  The leftover new line.

  ✓Do this: Read strings first, or add sc.nextLine(); after nextInt().

  Usually costs: 1 to 2 marks.

• ✗Mistake: Calling division by zero a syntax error

  Error types confused.

  ✓Do this: It compiles; the crash happens while running: a runtime error.

  Usually costs: 1 mark.

Practice questions on Data Processing 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

   Which package is imported by default in every Java program?
   (a) java.util
   (b) java.io
   (c) java.lang
   (d) java.net

   Hint

   System, String, Math.

   Answer

   (c) java.lang

2. Q2 · 1 mark · MCQExam level

   To read a char with Scanner, we use:
   (a) sc.nextChar()
   (b) sc.next().charAt(0)
   (c) sc.nextLine(0)
   (d) sc.charAt()

   Hint

   No nextChar().

   Answer

   (b) sc.next().charAt(0)

3. Q3 · 2 marks · Very short answerHarder exam level

   Write statements to accept (i) a long mobile number, (ii) a float price.

   Hint

   nextLong, nextFloat.

   Answer

   (i) long mobile = sc.nextLong();
   (ii) float price = sc.nextFloat();

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

   What are parameters? How does BlueJ obtain values for a method with parameters?

   Hint

   At execution.

   Answer

   Parameters are variables in a method's header that receive values when the method is called, so data is supplied at the time of execution. When such a method is run in BlueJ, a dialog box asks the user to enter the values.

5. Q5 · 2 marks · Very short answerHarder exam level

   Give the output: int a = 10, b = 20; System.out.println("Logical error: " + (a + b / 2)); System.out.println("Correct: " + (a + b) / 2);

   Hint

   Precedence.

   Answer

   Logical error: 20
   Correct: 15

6. Q6 · 2 marks · Short answerHOTS / competency

   Why is a syntax error easier to correct than a logical error?

   Hint

   Who finds it.

   Answer

   The compiler finds a syntax error and reports the line and the problem, so it can be located at once. A logical error produces no message; the programmer must notice the wrong output and trace the logic to find it.

7. Q7 · 3 marks · Short answerHOTS / competency

   Identify the type of error: (i) Systm.out.println(x); (ii) using sc.nextInt() and entering 4.5; (iii) area = 2 * l * b; for a rectangle's area.

   Hint

   Compiler, crash, wrong answer.

   Answer

   (i) Syntax error (cannot find symbol Systm). (ii) Runtime error (InputMismatchException). (iii) Logical error (area doubled).

8. Q8 · 15 marks · Long answerHarder exam level

   Write a program to accept the name of an item (may contain spaces), its price (double) and quantity (int), and print the item name and the amount payable.

   Hint

   Read the name first.

   Answer

   import java.util.*;
   class Purchase
   {
   public static void main(String args[])
   {
   Scanner sc = new Scanner(System.in);
   System.out.print("Item: ");
   String item = sc.nextLine();
   System.out.print("Price: ");
   double price = sc.nextDouble();
   System.out.print("Quantity: ");
   int qty = sc.nextInt();
   System.out.println(item + " : Rs " + price * qty);
   }
   }
   Sample run (as run; inputs Steel bottle, 249.5, 2):
   Item: Steel bottle
   Price: 249.5
   Quantity: 2
   Steel bottle : Rs 499.0

9. Q9 · 2 marks · Very short answerExam level

   Give two uses of comments in a program, and write one comment of each type.

   Hint

   Explain; switch off.

   Answer

   Uses: to explain what a program or a step does, and to temporarily disable a line while testing. // calculates the total /* Report program
   Class IX */

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.Scanner method for a whole line?Show answer

   nextLine()

2. 2.Exception when text is typed for nextInt()?Show answer

   InputMismatchException

3. 3.Error type found by the compiler?Show answer

   Syntax (compile-time) error.

4. 4.Package containing Scanner?Show answer

   java.util

5. 5.Symbols for a multi-line comment?Show answer

   /* ... */

6. 6.Data supplied before execution is called?Show answer

   Initialisation.

Quick revision checklist

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

• ☐Initialisation: before execution; parameters: at execution; Scanner: during execution.
• ☐Package: group of related classes; java.lang is default; Scanner is in java.util.
• ☐nextShort, nextInt, nextLong, nextFloat, nextDouble, next (word), nextLine (line), next().charAt(0) (char).
• ☐nextLine() after nextInt() reads an empty line: consume it first.
• ☐Syntax (compiler), runtime (crash: / by zero, InputMismatchException), logical (wrong answer).
• ☐Comments: // single line; /* */ multi-line; ignored by the compiler.

Data Processing in Java: frequently asked questions