String Methods

IB Syllabus: B2.1.2 — Extract and manipulate substrings using String methods.

Table of Contents

  1. Key Concepts
    1. Quick Reference
  2. Worked Examples
    1. Example 1: Basic String Methods
    2. Example 2: Parsing a Full Name
    3. Example 3: Comparing Strings
    4. Example 4: Building Strings Character by Character
    5. Example 5: Counting Characters
  3. Quick Code Check
  4. Trace Exercise
  5. Trace Exercise 2 — Parsing
  6. Code Completion
  7. Spot the Bug
  8. Output Prediction
  9. Practice Exercises
    1. Core
    2. Extension
    3. Challenge
  10. GitHub Classroom
  11. Connections

Key Concepts

String is not a primitive type — it is an object with built-in methods. Strings are immutable: every method returns a new String without changing the original.

Quick Reference

Method What it returns Example
.length() Number of characters "Hi".length() → 2
.charAt(i) The char at index i "Hi".charAt(1) → 'i'
.substring(s) From index s to end "Hello".substring(2) → "llo"
.substring(s, e) From s up to (not incl.) e "Hello".substring(1, 4) → "ell"
.indexOf(str) Start index of str, or -1 "Hello".indexOf("ll") → 2
.toUpperCase() All uppercase "hi".toUpperCase() → "HI"
.toLowerCase() All lowercase "HI".toLowerCase() → "hi"
.equals(str) True if content matches "a".equals("a") → true
.equalsIgnoreCase(str) Matches ignoring case "A".equalsIgnoreCase("a") → true
.compareTo(str) Negative / 0 / positive "a".compareTo("b") → negative

Common exam trap: substring(start, end) returns characters from start up to but not including end. "Hello".substring(1, 4) gives "ell" (indices 1, 2, 3) — index 4 is excluded.

Always compare Strings with .equals(), never ==. The == operator compares memory references, not content.

Worked Examples

Example 1: Basic String Methods 

String name = "Hello, World!";

System.out.println(name.length());          // 13
System.out.println(name.charAt(0));         // 'H'
System.out.println(name.charAt(7));         // 'W'
System.out.println(name.substring(7));      // "World!"
System.out.println(name.substring(7, 12)); // "World"
System.out.println(name.indexOf("World")); // 7
System.out.println(name.indexOf("Java"));  // -1
System.out.println(name.toUpperCase());     // "HELLO, WORLD!"
System.out.println(name.toLowerCase());     // "hello, world!"

Key points:

  • Indices start at 0, not 1
  • .length() uses parentheses (it’s a method call) — compare with array .length which has no parentheses
  • .indexOf() returns -1 if the substring is not found

Example 2: Parsing a Full Name 

String fullName = "Alice Johnson";

int spaceIndex = fullName.indexOf(" ");                    // 5
String firstName = fullName.substring(0, spaceIndex);      // "Alice"
String lastName = fullName.substring(spaceIndex + 1);      // "Johnson"

System.out.println("First: " + firstName);
System.out.println("Last:  " + lastName);
System.out.println("Total characters: " + fullName.length());  // 13

Pattern: Find a delimiter with indexOf(), then split with substring().

Example 3: Comparing Strings 

String s1 = "apple";
String s2 = "apple";
String s3 = "APPLE";

System.out.println(s1.equals(s2));                // true
System.out.println(s1.equals(s3));                // false (case matters)
System.out.println(s1.equalsIgnoreCase(s3));      // true

System.out.println("apple".compareTo("banana"));  // negative (a < b)
System.out.println("cat".compareTo("cat"));        // 0 (equal)
System.out.println("dog".compareTo("cat"));        // positive (d > c)

compareTo() returns:

  • Negative → first string comes before alphabetically
  • Zero → strings are equal
  • Positive → first string comes after alphabetically

Example 4: Building Strings Character by Character 

String word = "Hello";
String reversed = "";

for (int i = word.length() - 1; i >= 0; i--) {
    reversed = reversed + word.charAt(i);
}

System.out.println(reversed);  // "olleH"

This pattern — iterating through characters with charAt() — is very common in IB Paper 2 questions.

Example 5: Counting Characters 

String sentence = "The quick brown fox";
int spaceCount = 0;

for (int i = 0; i < sentence.length(); i++) {
    if (sentence.charAt(i) == ' ') {
        spaceCount++;
    }
}

System.out.println("Spaces: " + spaceCount);  // 3

Notice == ' ' with single quotes — we are comparing char values (primitives), so == is correct here. Only use .equals() for String objects.

Quick Code Check

Q1. What does "Hello".substring(1, 4) return?

Q2. What does "Hello".indexOf("lo") return?

Q3. What does "Hello".indexOf("xyz") return?

Q4. After running this code, what is word?
String word = "hello"; word.toUpperCase();

Q5. Which is correct for getting the length of a String?

Trace Exercise

Trace this code — fill in the value of each variable.

String word = "Computer";
int len = word.length();
char first = word.charAt(0);
String sub = word.substring(3, 7);
String upper = word.toUpperCase();

Hint: C=0, o=1, m=2, p=3, u=4, t=5, e=6, r=7

LineVariableValue
word.length() len
word.charAt(0) first
word.substring(3, 7) sub
word.toUpperCase() upper

Trace Exercise 2 — Parsing

Trace this code — work out the values step by step.

String email = "alice@school.edu";
int atIndex = email.indexOf("@");
String username = email.substring(0, atIndex);
String domain = email.substring(atIndex + 1);
int dotIndex = domain.indexOf(".");
String extension = domain.substring(dotIndex + 1);
VariableValue
atIndex
username
domain
dotIndex
extension

Code Completion

This exercise practises the common pattern of splitting a string at a delimiter — use indexOf() to find the space, then substring() to extract the parts before and after it. Complete the code to extract the first name and last name from a full name.

String fullName = "John Smith";
int space = fullName.;
String first = fullName.;
String last = fullName.substring();
System.out.println("First: " + first);   // John
System.out.println("Last: " + last);     // Smith

Spot the Bug

This code should check if two strings have the same content using an if statement, but it uses the wrong comparison operator. This is one of the most common Java mistakes on IB exams. Find the bug.

String input = "hello";
String expected = "hello";
if (input == expected) {
    System.out.println("Match!");
}

What is wrong and how do you fix it?

This code should use charAt() to print the last character of a word, but it crashes with a StringIndexOutOfBoundsException. The bug tests your understanding of zero-based indexing. Find it.

String word = "Java";
char last = word.charAt(word.length());
System.out.println("Last char: " + last);

What is wrong and how do you fix it?

Output Prediction

This code demonstrates three core String methods — substring(), charAt(), and indexOf() — applied to the same string. Trace through each call carefully, remembering that indices start at 0 and substring(start, end) excludes the end index.

What does this code print?

String s = "Programming";
System.out.println(s.substring(0, 7));
System.out.println(s.charAt(3));
System.out.println(s.indexOf("gram"));
Expected: Program
g
3

This code builds a new string by iterating through the characters of the original string in reverse order using charAt(). This is a classic string-reversal algorithm that combines loop traversal with string concatenation.

What does this code print?

String word = "Hello";
String result = "";
for (int i = word.length() - 1; i >= 0; i--) {
    result = result + word.charAt(i);
}
System.out.println(result);
Expected: olleH

Practice Exercises

Core

  1. String length — Read a word from the user and print how many characters it has.

  2. First and last — Read a word and print its first and last characters.

  3. Initials — Read a first name and last name (separated by a space). Print the initials (e.g., “Alice Johnson” → “A.J.”).

  4. Substring extractor — Read a word, a start index, and an end index. Print the substring.

Extension

  1. Word count — Read a sentence and count how many words it contains (hint: count spaces and add 1).

  2. Email parser — Read an email address like "alice@school.edu" and print the username and domain separately.

  3. Palindrome checker — Read a word and check if it reads the same forwards and backwards (e.g., “racecar”). Use a loop with charAt().

Challenge

  1. Caesar cipher — Read a word and a shift value. Shift each character by that many positions in the alphabet (e.g., ‘A’ with shift 3 → ‘D’). Use charAt() and type casting.

  2. Title case — Read a sentence and convert it to title case (first letter of each word uppercase, rest lowercase).

GitHub Classroom

String Methods
Practice using length(), charAt(), substring(), indexOf(), and String comparison.
Core + Extension Open in GitHub

Connections

Prerequisites:

Related Topics:

  • Iteration — loops for traversing Strings character by character
  • 1D Arrays — similar index-based access pattern

What’s Next:

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