String and StringBuffer Methods with Programs (ISC Class 12 Computer Science)

Understand String and StringBuffer Methods with Programs

String and StringBuffer Methods with Programs: key facts

Indexing

• •First index 0
• •Last index s.length() - 1
• •substring(a, b) gives indices a to b - 1
• •substring(a) runs to the end

indexOf returns -1 when absent.

Comparison methods

• •equals compares content
• •== compares references
• •compareTo gives the difference of first unequal chars
• •Equal strings give 0

Prefix case: the length difference.

StringBuffer changes

• •append(x) adds at the end
• •insert(i, x) inserts at i
• •reverse() reverses
• •setCharAt(i, ch) replaces one char
• •delete(a, b) removes a to b - 1

These change the same object.

Immutability

• •String methods return a new String
• •s.toUpperCase() alone leaves s unchanged
• •Assign the result: s = s.toUpperCase()

StringBuffer methods change the object itself.

How to answer String and StringBuffer Methods with Programs questions

1. 1Write the string with its index numbers below each character.
2. 2Apply each method to the original string, remembering that String methods do not change it.
3. 3For substring(a, b), take indices a to b - 1; for indexOf, find the first match from the left.
4. 4For StringBuffer, update the buffer after every call and write its new content.
5. 5For a program, loop with charAt(i) and build a result with +, or use a StringBuffer.
6. 6Print with the exact spacing asked.

String and StringBuffer Methods with Programs 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 and StringBuffer Methods with Programs

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

• Index from zero, end is exclusive

  substring(3, 8) takes 3, 4, 5, 6, 7.

  Use it when: Substring questions.

• equals is for text

  == asks "same object?", equals asks "same letters?".

  Use it when: Comparison questions.

• String stays, buffer changes

  String methods give a new string; buffer methods rub out and rewrite.

  Use it when: Immutability questions.

• compareTo = first difference

  Subtract the first unequal characters; if a prefix, subtract lengths.

  Use it when: compareTo values.

String and StringBuffer Methods with Programs: solved examples

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

String s = "Digital Library";
System.out.println(s.length() + " " + s.indexOf('i') + " " + s.lastIndexOf('i')
    + " " + s.substring(2, 7) + " " + s.charAt(8));

String a = "apple", b = "apricot";
System.out.println(a.compareTo(b));
System.out.println(a.equals("Apple") + " " + a.equalsIgnoreCase("Apple")
    + " " + "abc".compareTo("abcde"));

StringBuffer sb = new StringBuffer("ISC");
sb.append(" 2027");
sb.insert(3, "-12");
sb.reverse();
sb.setCharAt(0, '9');
System.out.println(sb + " " + sb.length());

static boolean isPal(String s) {
    String r = "";
    for (int i = s.length() - 1; i >= 0; i--)
        r += s.charAt(i);
    return r.equalsIgnoreCase(s);
}

Where students lose marks in String and StringBuffer Methods with Programs

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

• ✗Mistake: Taking substring(a, b) up to b

  The end index is excluded.

  Wrong working

  "Computer".substring(0, 3) gives Comp

  Right working

  "Computer".substring(0, 3) gives Com

  ✓Do this: Count indices a to b - 1.

  Usually costs: 1 mark

• ✗Mistake: Using == to compare string content

  == compares references.

  Wrong working

  if (s == "yes")

  Right working

  if (s.equals("yes"))

  ✓Do this: Use equals or equalsIgnoreCase.

  Usually costs: 1 mark

• ✗Mistake: Not assigning the result of a String method

  Strings are immutable, so the original is unchanged.

  Wrong working

  s.toUpperCase(); then use s

  Right working

  s = s.toUpperCase();

  ✓Do this: Store the returned String.

  Usually costs: 1 mark

Practice questions on String and StringBuffer Methods with Programs

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

   Which StringBuffer method reverses the characters in place?
   (a) reverse() (b) invert() (c) flip() (d) turn()

   Hint

   It is spelt as the word.

   Answer

   (a) reverse()

2. Q2 · 2 marks · Short answerBoard level

   Distinguish between String and StringBuffer.

   Hint

   One can be changed, the other cannot.

   Answer

   A String object is immutable: its characters cannot be changed after creation and its methods return new Strings. A StringBuffer is mutable: append, insert, reverse and similar methods change the same object, so it suits heavy modification.

3. Q3 · 3 marks · Short answerHarder board level

   Write the output:

   String s = "  Java Programming  ";
   String t = s.trim();
   System.out.println(t.length() + " " + t.substring(5) + " " + t.replace('a', 'o')
       + " " + t.toUpperCase().indexOf("PRO"));

   Hint

   Trim first, then use t.

   Answer

   trim gives "Java Programming" (4 + 1 + 11 = 16 characters). substring(5) is Programming. Replacing a by o gives Jovo Progromming. In upper case the text is JAVA PROGRAMMING and PRO starts at index 5. Output: 16 Programming Jovo Progromming 5

4. Q4 · 4 marks · Short answerHarder board level

   Write a program segment to count the vowels in the string "Education" (any case). Give the count.

   Hint

   Convert to lower case first.

   Answer

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

   The vowels of education are e, u, a, i, o: the count is 5.

5. Q5 · 4 marks · Case-basedHOTS / competency

   Trace the program and state the output:

   String s = "to be or not", w = "", r = "";
   for (int i = 0; i < s.length(); i++) {
       char c = s.charAt(i);
       if (c != ' ')
           w += c;
       else {
           r = w + " " + r;
           w = "";
       }
   }
   r = w + " " + r;
   System.out.println(r.trim());

   Also state what the program does and why trim() is needed.

   Hint

   Each finished word is put in front of the text built so far.

   Answer

   Words are collected in w. At each space the word is placed before r: "to ", then "be to ", then "or be to ". At the end w = "not", so r becomes "not or be to " and trim removes the trailing space. Output: not or be to. The program reverses the order of the words. trim() is needed because r ends with a space left by the last concatenation.

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 when the character is absent?Show answer

   -1.

2. 2.Is String mutable?Show answer

   No, it is immutable.

3. 3.Which method removes leading and trailing spaces?Show answer

   trim().

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

   0.

5. 5.Which StringBuffer method adds text at the end?Show answer

   append().

6. 6.What does substring(3) give?Show answer

   The characters from index 3 to the end.

String and StringBuffer Methods with Programs: frequently asked questions