String Handling ICSE Class 10 Computer Applications

What String Handling is about

Weightage: CISCE publishes no chapter weightage. Section A (40 marks) is compulsory: 1-mark MCQs and 2-mark short answers including output and error questions; in Section B you answer any four of six 15-mark programming questions written in Java.Source: CISCE ICSE Computer Applications (661) specimen paper structure for 2027

Key concepts

Strings, indexes and length

A String is an object of the String class (java.lang) that stores a sequence of characters: String s = "COMPUTER";. Each character has an index from 0 to length() − 1:
C O M P U T E R
0 1 2 3 4 5 6 7

int length(): number of characters: "COMPUTER".length() = 8; "".length() = 0. It never returns a negative number.
char charAt(int n): the character at index n: s.charAt(3) = 'P'; s.charAt(s.length() - 1) = 'R'. An index outside the range is a runtime error.

Strings are immutable: methods such as toUpperCase() or replace() return a new String and do not change the original unless you assign it back: s = s.toUpperCase();.

Worked example

(2 marks) Name (a) one String method that returns only a non-negative integer and (b) one that returns a character.

Try it first, then show the solutionSolution

1. (a) length(): a count, never negative (indexOf and compareTo can be negative) - 1 mark.
2. (b) charAt(int n) - 1 mark.

Answer: (a) length() (b) charAt(int)

Searching inside a string: indexOf, lastIndexOf, startsWith, endsWith

int indexOf(char ch): index of the first occurrence of ch, or −1 if absent.
int lastIndexOf(char ch): index of the last occurrence, or −1.
"MISSISSIPPI" (M0 I1 S2 S3 I4 S5 S6 I7 P8 P9 I10):
indexOf('S') = 2; lastIndexOf('S') = 6; indexOf('P') = 8; indexOf('Z') = −1.

boolean startsWith(String str) and boolean endsWith(String str) test the beginning and end:
"report2027.pdf".endsWith(".pdf") → true; "report2027.pdf".startsWith("Report") → false (case matters).

Worked example

(2 marks) String w = "BANANA"; Give w.indexOf('N') + w.lastIndexOf('A') and w.indexOf('K').

Try it first, then show the solutionSolution

1. B0 A1 N2 A3 N4 A5: indexOf('N') = 2, lastIndexOf('A') = 5; sum 7 - 1 mark.
2. 'K' is absent: −1 - 1 mark.

Answer: 7 and -1

Extracting and joining: substring, concat, trim, case, replace, valueOf

String substring(int begin): from index begin to the end. "COMPUTER".substring(3) = "PUTER".
String substring(int begin, int end): from begin up to end − 1 (end excluded); length = end − begin. "COMPUTER".substring(1, 4) = "OMP".
String concat(String str): joins: "Blue".concat("J") = "BlueJ" (same as + with strings).
String trim(): removes leading and trailing spaces (not inner ones): " Java ".trim() = "Java".
String toUpperCase() / toLowerCase(): whole string in one case.
String replace(char old, char new): replaces every occurrence: "banana".replace('a', 'o') = "bonono".
String valueOf(any type): converts a value to a String (static): String.valueOf(25) + 5 = "255"; String.valueOf('x') = "x".

Combined example: "Holiday".substring(4).concat("mobile".substring(4)) = "day" + "le" = "dayle".

Worked example

(2 marks) Give the output: System.out.println("Holiday".substring(4).concat("mobile".substring(4)));

Try it first, then show the solutionSolution

1. "Holiday": H0 o1 l2 i3 d4 a5 y6 → substring(4) = "day" - 1 mark.
2. "mobile": m0 o1 b2 i3 l4 e5 → substring(4) = "le"; joined "dayle" - 1 mark.

Answer: dayle

Comparing strings: equals, equalsIgnoreCase, compareTo, compareToIgnoreCase

boolean equals(String str): true if both have exactly the same characters (case sensitive). "Java".equals("java") → false.
boolean equalsIgnoreCase(String str): ignores case. "Java".equalsIgnoreCase("JAVA") → true.
Use equals(), not ==, to compare String contents (== checks whether they are the same object).

int compareTo(String str): compares in dictionary (Unicode) order:

• 0 if equal;
• otherwise, at the first position where they differ, (code of this char) − (code of the other char);
• if one string is a prefix of the other, (length of this) − (length of the other).

A negative result means the calling string comes first alphabetically.
"apple".compareTo("apricot"): first difference p (112) vs r (114) → −2.
"Zebra".compareTo("apple"): 'Z' (90) − 'a' (97) = −7 (capitals come before small letters).
"CAT".compareTo("CATALOG"): prefix → 3 − 7 = −4.
int compareToIgnoreCase(String str): the same, ignoring case: "Zebra".compareToIgnoreCase("apple") = 'z' − 'a' = 25.

Shortcut: count letters apart instead of using codes: 't' − 'g' = 13 because t is 13 letters after g.

Worked example

(2 marks) String a = "KING", b = "KINGDOM"; boolean x = a.compareTo(b); gives 'incompatible types: int cannot be converted to boolean'. Correct the statement and give the output of printing the corrected value.

Try it first, then show the solutionSolution

1. compareTo returns int, so store it in an int: int x = a.compareTo(b); - 1 mark.
2. KING is a prefix of KINGDOM: 4 − 7 = −3 - 1 mark.

Answer: int x = a.compareTo(b); prints -3 (or use boolean x = a.equals(b); which prints false).

String programs: extracting characters, sorting and searching

Character-by-character processing:
for (int i = 0; i < s.length(); i++)
{
char ch = s.charAt(i);
... test or change ch ...
}
Building a new string: res = res + ch; (start with res = "").

Alphabetical order of a String array uses compareTo() inside bubble or selection sort:
if (names[j].compareTo(names[j + 1]) > 0) swap them;

Linear search of a String array uses equals() (or equalsIgnoreCase()):
for (int i = 0; i < n; i++)
if (names[i].equalsIgnoreCase(key)) { pos = i; break; }

Other common tasks: counting vowels, words (count spaces + 1 after trim), reversing a word, checking a palindrome (compare with its reverse using equalsIgnoreCase), capitalising the first letter of each word.

Worked example

(2 marks) Give the output and the number of times the loop runs:
String s = "BLUEJ";
for (int i = 1; i < s.length(); i += 2)
System.out.println(s.substring(0, i));

Try it first, then show the solutionSolution

1. i = 1: substring(0, 1) = "B"; i = 3: substring(0, 3) = "BLU" - 1 mark.
2. i = 5 is not < 5: loop runs 2 times - 1 mark.

Answer: B
BLU
(the loop runs 2 times)

String Handling 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

• String Methods: Output Tracing
• String Programs: Palindrome, Words and Sorting Names

String Handling: syntax and key facts

Return types

int: length, indexOf, lastIndexOf, compareTo, compareToIgnoreCase; char: charAt; boolean: equals, equalsIgnoreCase, startsWith, endsWith; String: trim, toLowerCase, toUpperCase, concat, replace, substring, valueOf

Only length() is never negative.

substring

substring(b) = from b to end; substring(b, e) = b to e − 1; length e − b

End index excluded.

compareTo

first mismatch: code(this) − code(other); prefix: length(this) − length(other); equal: 0

'A' 65, 'a' 97; capitals sort first.

Sorting strings

bubble: if (a[j].compareTo(a[j+1]) > 0) swap

Search with equals()/equalsIgnoreCase().

How the board asks String Handling

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

Tricks to remember String Handling

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

• Zero-start, end-excluded

  Indexes start at 0; substring's end index is not included.

  Use it when: charAt and substring outputs.

• First mismatch decides

  compareTo looks only at the first different letter (or the lengths if one is a prefix).

  Use it when: compareTo outputs.

• Strings never change

  String methods return a new String; assign it if you want to keep it.

  Use it when: toUpperCase, replace, trim.

String Handling: board-level solved questions

Ordered from board 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 the board paper this is a 10-mark question split into 2 + 3 + 5 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 String Handling

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

• ✗Mistake: Counting index from 1

  The 2024 examiners reported candidates chose 7 for "CONCENTRATION".indexOf('T') by counting from 1.

  ✓Do this: Write 0, 1, 2 ... above the letters before answering.

  Usually costs: 1 mark.

• ✗Mistake: Not knowing how compareTo() works

  In 2024 candidates were confused by the sign and wrote out all ASCII codes.

  ✓Do this: Find the first mismatch; subtract (this − other); count letters apart.

  Usually costs: 1 mark.

• ✗Mistake: Naming compareTo or indexOf as 'positive only'

  In 2025 candidates gave these for a method returning only a positive integer, and Character methods for a String method returning a char.

  ✓Do this: length() is never negative; charAt() returns a char.

  Usually costs: 1 mark.

• ✗Mistake: Copying the code instead of correcting it

  In 2024 candidates copied the compareTo-in-boolean question without studying it.

  ✓Do this: Read the error message: int cannot be converted to boolean → store compareTo() in an int.

  Usually costs: 2 marks.

• ✗Mistake: Ad hoc logic for validation

  In the 2024 gmail program candidates used varied, unsuitable logic.

  ✓Do this: Use indexOf, substring, startsWith and endsWith to test each required part.

  Usually costs: Up to 10 marks.

Practice questions on String Handling

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

   "PROGRAM".lastIndexOf('R') returns:
   (a) 1
   (b) 4
   (c) 5
   (d) -1

   Hint

   P0 R1 O2 G3 R4.

   Answer

   (b) 4

2. Q2 · 1 mark · MCQHarder board level

   "apple".compareTo("apricot") returns:
   (a) 2
   (b) -2
   (c) 0
   (d) -3

   Hint

   Third letters p and r.

   Answer

   (b) -2

3. Q3 · 1 mark · MCQBoard level

   Which method returns a boolean?
   (a) indexOf()
   (b) concat()
   (c) endsWith()
   (d) charAt()

   Hint

   Yes or no.

   Answer

   (c) endsWith()

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

   Give the output: System.out.println("COMPUTER".substring(1, 4) + String.valueOf(25) + 5);

   Hint

   End excluded; string joins.

   Answer

   OMP255

5. Q5 · 2 marks · Very short answerBoard level

   Distinguish between equals() and compareTo().

   Hint

   Return types.

   Answer

   equals() returns a boolean telling whether two strings are exactly the same; compareTo() returns an int: 0 if equal, negative if the calling string comes first in dictionary order, positive if it comes after.

6. Q6 · 2 marks · Short answerHOTS / competency

   Give the output:
   String s = "level";
   String r = "";
   for (int i = s.length() - 1; i >= 0; i--)
   r = r + s.charAt(i);
   System.out.println(r + " " + r.equals(s));

   Hint

   Reverse.

   Answer

   level true

7. Q7 · 15 marks · Long answerHarder board level

   Write a program to accept a word and check whether it is a palindrome, ignoring case (e.g. "Madam").

   Hint

   Build the reverse.

   Answer

   import java.util.Scanner;
   class Palin
   {
   public static void main(String args[])
   {
   Scanner sc = new Scanner(System.in);
   System.out.print("Word: ");
   String w = sc.next();
   String rev = "";
   for (int i = w.length() - 1; i >= 0; i--)
   rev = rev + w.charAt(i);
   if (rev.equalsIgnoreCase(w))
   System.out.println("Palindrome");
   else
   System.out.println("Not a palindrome");
   }
   }
   Trace "Madam": rev = "madaM"; equalsIgnoreCase → Palindrome. "Java": rev "avaJ" → Not a palindrome.

8. Q8 · 15 marks · Long answerHOTS / competency

   Write a program to accept a sentence and count the number of words and the number of times the letter 'e' (either case) occurs.

   Hint

   Words = spaces + 1 after trim (single spaces).

   Answer

   import java.util.Scanner;
   class Count
   {
   public static void main(String args[])
   {
   Scanner sc = new Scanner(System.in);
   System.out.print("Sentence: ");
   String s = sc.nextLine().trim();
   int words = 1, e = 0;
   for (int i = 0; i < s.length(); i++)
   {
   char ch = s.charAt(i);
   if (ch == ' ') words++;
   if (ch == 'e' || ch == 'E') e++;
   }
   System.out.println("Words = " + words + ", e = " + e);
   }
   }
   Trace "Every exam needs preparation": 3 spaces → 4 words; e's: E, e (Every 2: E, e), exam 1, needs 2, preparation 1 → 6. Output: Words = 4, e = 6. (Assumes single spaces between words.)

9. Q9 · 2 marks · Short answerHOTS / competency

   Why does "Java" == new String("Java") give false while "Java".equals(new String("Java")) gives true?

   Hint

   Objects vs contents.

   Answer

   == checks whether both refer to the same object; new String creates a different object, so it is false. equals() compares the characters, which are the same, so it is true.

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."HELLO".charAt(1)?Show answer

   'E'

2. 2."HELLO".substring(1, 3)?Show answer

   "EL"

3. 3.indexOf when the character is absent?Show answer

   -1

4. 4.Which compares ignoring case and returns boolean?Show answer

   equalsIgnoreCase()

5. 5.Does toUpperCase() change the original String?Show answer

   No; it returns a new String.

Quick revision checklist

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

• ☐Index 0 to length() − 1; charAt(i).
• ☐indexOf/lastIndexOf: −1 if absent.
• ☐substring(b, e): e excluded.
• ☐compareTo: first mismatch difference, or length difference; 0 if equal.
• ☐equals: case sensitive; use it, not ==.
• ☐Strings are immutable: assign the result.

String Handling: frequently asked questions