Java String Methods: Tracing the Output (ICSE Class 10)

Understand String Methods: Output Tracing

String Methods: Output Tracing: key facts

Positions

First index	0
Last index	length() - 1
indexOf not found	-1

Write indices under the characters.

substring

substring(b)	from b to the end
substring(b, e)	from b to e - 1
Length of result	e - b

End index is excluded.

compareTo result

First differing character	s char minus t char
One a prefix of the other	length of s minus length of t
Equal	0

Uses ASCII codes: A is 65 and a is 97.

Equality methods

equals	case sensitive
equalsIgnoreCase	ignores case
compareToIgnoreCase	ignores case

equals returns boolean; compareTo returns int.

How to answer String Methods: Output Tracing questions

1. 1Write the string with an index under every character (spaces count).
2. 2Apply each method exactly: length counts all characters; indexOf gives the first match; substring excludes the end index.
3. 3For compareTo, find the first position where the characters differ and subtract their ASCII codes; if none, subtract lengths.
4. 4Remember methods return new values; the original string stays unchanged unless it is reassigned.
5. 5Join results with + using the printed separators.

String Methods: Output Tracing 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 String Methods: Output Tracing

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

• Start in, end out

  substring(b, e) includes b and excludes e.

  Use it when: substring outputs.

• Index starts at zero

  First letter 0, last letter length - 1.

  Use it when: charAt and loops.

• Minus the first difference

  compareTo = first unequal character of s minus that of t.

  Use it when: compareTo values.

String Methods: Output Tracing: solved examples

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

String t = "  Java Rocks  ";
System.out.println("[" + t.trim() + "]");
System.out.println(t.length());
System.out.println(t.trim().length());

String a = "hi";
String b = a.concat("!");
System.out.println(b);
System.out.println(String.valueOf(25) + 5);
System.out.println(a.equals("HI") + " " + a.equalsIgnoreCase("HI"));

Where students lose marks in String Methods: Output Tracing

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

• ✗Mistake: Including the end index in substring

  substring(b, e) stops at e - 1.

  Wrong working

  "Computer".substring(2, 5) = "mput"

  Right working

  "Computer".substring(2, 5) = "mpu"

  ✓Do this: Count b up to e - 1.

  Usually costs: 1 mark

• ✗Mistake: Treating strings as changed by a method call

  Methods return new strings.

  Wrong working

  s.toUpperCase(); println(s) prints COMPUTER

  Right working

  println(s) still prints Computer; use s = s.toUpperCase();

  ✓Do this: Assign the result.

  Usually costs: 1 mark

• ✗Mistake: Writing compareTo as just -1, 0 or 1

  compareTo returns the actual difference of codes or lengths.

  Wrong working

  "apple".compareTo("apricot") = -1

  Right working

  "apple".compareTo("apricot") = -2

  ✓Do this: Subtract the first unequal codes.

  Usually costs: 1 mark

Practice questions on String Methods: Output Tracing

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

   What is the value of "hello".charAt(1)?
   (a) h (b) e (c) l (d) o

   Hint

   Index 0 is h.

   Answer

   (b) e

2. Q2 · 1 mark · Very short answerBoard level

   What is the return type of the method indexOf(char ch)?

   Hint

   It returns a position or -1.

   Answer

   int

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   String s = "banana";
   System.out.println(s.indexOf('a') + " " + s.lastIndexOf('a') + " " + s.replace('a', 'o'));

   Hint

   b0 a1 n2 a3 n4 a5.

   Answer

   1 5 bonono

4. Q4 · 3 marks · Short answerHarder board level

   Given String s = "Hello World"; write one expression using indexOf and substring that gives the part after the space, and state its value.

   Hint

   The space is at index 5.

   Answer

   s.substring(s.indexOf(' ') + 1) gives "World"; indexOf(' ') is 5, so substring(6) is taken.

5. Q5 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): "Zebra".compareTo("apple") returns a negative value.
   Reason (R): Upper-case letters have smaller ASCII codes than lower-case letters.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   Compare Z (90) with a (97).

   Answer

   (a) Both are true: Z (90) minus a (97) = -7, and the smaller code of the capital letter is the reason.

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.What does indexOf return if the character is absent?Show answer

   -1

2. 2.What is the last index of a string of length n?Show answer

   n - 1

3. 3.Which method ignores case when comparing for equality?Show answer

   equalsIgnoreCase.

4. 4.What does trim() remove?Show answer

   Spaces at the beginning and end only.

5. 5.What does substring(2, 5) take?Show answer

   Characters at positions 2, 3 and 4.

6. 6.What does compareTo return for equal strings?Show answer

   0

String Methods: Output Tracing: frequently asked questions