Java String Methods and String Programs (ISC Class 11 Computer Science)

Understand The String Class: Methods and String Programs in Java

String r = "";
for (int i = s.length() - 1; i >= 0; i--)
    r = r + s.charAt(i);          // reverse
int v = 0;
for (int i = 0; i < s.length(); i++) {
    char c = Character.toLowerCase(s.charAt(i));
    if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') v++;   // vowels
}

The String Class: Methods and String Programs in Java: key facts

substring

s.substring(a, b) = characters a to b - 1 | length = b - a

End index excluded.

compareTo

s1.compareTo(s2) = ASCII difference of first mismatch | Else length difference

0 means equal.

Last index

Last index = s.length() - 1 | Loop i from 0 to length - 1

Zero-based.

How to answer The String Class: Methods and String Programs in Java questions

1. 1Write the string with indices under each character.
2. 2Apply the method exactly: watch the end index of substring.
3. 3For concatenation, evaluate left to right.
4. 4For programs, trace the loop with the first two iterations.

The String Class: Methods and String Programs in Java 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 The String Class: Methods and String Programs in Java

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

• End index is excluded

  substring(3, 8) stops before 8.

  Use it when: substring.

• Equals not ==

  Use equals() for content.

  Use it when: Comparison.

• Left to right

  1 + 2 + "a" is "3a"; "a" + 1 + 2 is "a12".

  Use it when: Concatenation.

The String Class: Methods and String Programs in Java: solved examples

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

String s = "Education";
int v = 0;
String r = "";
for (int i = 0; i < s.length(); i++) {
    char c = Character.toLowerCase(s.charAt(i));
    if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') v++;
}
for (int i = s.length() - 1; i >= 0; i--) r = r + s.charAt(i);
System.out.println(v + " " + r);

Where students lose marks in The String Class: Methods and String Programs in Java

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

• ✗Mistake: Counting the end index in substring

  Inclusive habit.

  Wrong working

  substring(3, 8) = "puters"

  Right working

  "puter"

  ✓Do this: End index is excluded.

  Usually costs: 1-2 marks.

• ✗Mistake: Using == to compare strings

  Works for numbers.

  Wrong working

  if (s1 == s2)

  Right working

  if (s1.equals(s2))

  ✓Do this: Compare content.

  Usually costs: 1-2 marks.

• ✗Mistake: Forgetting to store toUpperCase()

  Thinking it changes the string.

  Wrong working

  s.toUpperCase();

  Right working

  s = s.toUpperCase();

  ✓Do this: Assign the result.

  Usually costs: 1 mark.

Practice questions on The String Class: Methods and String Programs 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 · MCQBoard level

   The value of "Programming".substring(3, 7) is:
   (a) gram (b) gramm (c) ogra (d) ogram

   Hint

   P0 r1 o2 g3.

   Answer

   (a) gram

2. Q2 · 3 marks · Short answerBoard level

   Write the output of: String a = "Hello World"; System.out.println(a.indexOf('o') + " " + a.lastIndexOf('o') + " " + a.toUpperCase() + " " + a.replace('l', 'L')); (3 marks)

   Hint

   H0 e1 l2 l3 o4.

   Answer

   4 7 HELLO WORLD HeLLo WorLd

3. Q3 · 5 marks · Long answerHarder board level

   Write a program fragment to test whether String w = "level" is a palindrome and state its output. Also give the output for w = "Level". (5 marks)

   Hint

   Reverse and compare with equals.

   Answer

   Build r by reversing w and test w.equals(r): for "level" r = "level" and the output is "Palindrome". For "Level" r = "leveL"; equals is false (case differs), so the output is "Not a palindrome" unless equalsIgnoreCase is used.

4. Q4 · 4 marks · Short answerHarder board level

   A sentence String t = "to be or not to be" has single spaces. Write the logic to count the words and the number of characters that are not spaces; give both answers. (4 marks)

   Hint

   Count spaces first.

   Answer

   Words = number of spaces + 1 = 5 + 1 = 6. Length = 18; spaces = 5; non-space characters = 13.

5. Q5 · 3 marks · Short answerHOTS / competency

   A student writes String s = "ISC"; s.concat("Exam"); System.out.println(s + s.length()); and expects ISCExam6. Give the actual output and explain. (3 marks)

   Hint

   Immutable.

   Answer

   Output: ISC3. concat() returns a new string "ISCExam" which is not assigned, so s is unchanged and its length is 3.

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.String index starts at?Show answer

   0.

2. 2.substring(a, b) end index?Show answer

   b - 1.

3. 3.Compare content with?Show answer

   equals().

4. 4.Is String mutable?Show answer

   No, immutable.

5. 5."a" + 1 + 2 gives?Show answer

   a12.

The String Class: Methods and String Programs in Java: frequently asked questions