Teacher Guide: String Methods

Student page: String Methods IB Syllabus: B2.1.2 Estimated periods: 2 (90 min each) Prerequisites: Variables & Data Types

Contents

1. Lesson Plans
   1. Period 1: Core String Methods
   2. Period 2: String Processing with Loops
2. Differentiation
   1. Supporting Weaker Students
   2. Extending Stronger Students
3. Answer Keys
   1. Core 1: String Length
   2. Core 2: First and Last
   3. Core 3: Initials
   4. Core 4: Substring Extractor
   5. Extension 5: Word Count
   6. Extension 6: Email Parser
   7. Extension 7: Palindrome Checker
   8. Challenge 8: Caesar Cipher
   9. Challenge 9: Title Case
4. Integration Notes
   1. Additional Resources

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Lesson Plans

Period 1: Core String Methods

Phase	Time	Activity	Student Page Section
Warm-up	10 min	“Alice Johnson” on board – how to extract first/last name?	–
Teach	25 min	length(), charAt(), substring(), indexOf(), equals()	Key Concepts
Practice	40 min	Core #1-4	Practice Exercises
Wrap-up	15 min	Quick Code Check MCQs	Quick Code Check

Additional activities:

• Live coding walkthrough: concatenation, immutability, length, indexOf, substring
• Password creator exercise: build a password from substring of family name + birth year + special character
• Discussion: “Why is String manipulation important for user-facing solutions?”

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Period 2: String Processing with Loops

Phase	Time	Activity	Student Page Section
Warm-up	10 min	Parse “alice.johnson@school.edu” – extract username/domain	–
Teach	15 min	String traversal with charAt() loop, building new strings	Worked Examples
Practice	50 min	Extension #5-7, Challenge #8-9	Practice Exercises
Wrap-up	15 min	Trace exercise from student page	Trace Exercise

Additional exercises:

• Password length comparison, case conversion, file extension extraction

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Differentiation

Supporting Weaker Students

Strategy	When to use	Example
Method reference card	Can’t remember which method does what	One-page: length(), charAt(i), substring(s,e), indexOf(s), equals(s)
Index diagram	Struggles with substring indices	Draw “H-e-l-l-o” with indices 0-4 above each letter
Reduce exercise set	Overwhelmed	Focus on Core #1-3 only, skip #4
Pair programming	During Extension exercises	Stronger student explains charAt loop pattern

Extending Stronger Students

Strategy	When to use	Example
Password creator	Finishes Core early	Build a password from substring of family name + birth year + special char
Challenge exercises	Extended learners	#8 Caesar cipher, #9 Title case
String immutability investigation	After Core concepts	“Why doesn’t str.toUpperCase() change str?” – discuss immutability
Real-world parsing	After Extension #6	Parse a CSV line: “Alice,16,Grade11” into separate fields

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Answer Keys

Core 1: String Length

Show solution

import java.util.Scanner;
public class StringLength {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a word: ");
        String word = scanner.nextLine();
        System.out.println("Length: " + word.length());
        scanner.close();
    }
}

Expected output: “hello” -> Length: 5

Common mistakes: Using word.length without parentheses (that’s array syntax, not String syntax).

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Core 2: First and Last

Show solution

import java.util.Scanner;
public class FirstAndLast {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a word: ");
        String word = scanner.nextLine();
        System.out.println("First: " + word.charAt(0));
        System.out.println("Last: " + word.charAt(word.length() - 1));
        scanner.close();
    }
}

Common mistakes: Using word.charAt(word.length()) – off by one, throws StringIndexOutOfBoundsException.

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Core 3: Initials

Show solution

import java.util.Scanner;
public class Initials {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter full name: ");
        String fullName = scanner.nextLine();
        int spaceIndex = fullName.indexOf(" ");
        char firstInitial = fullName.charAt(0);
        char lastInitial = fullName.charAt(spaceIndex + 1);
        System.out.println(firstInitial + "." + lastInitial + ".");
        scanner.close();
    }
}

Expected output: “Alice Johnson” -> A.J.

Common mistakes: Hardcoding the space position instead of using indexOf(" ").

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Core 4: Substring Extractor

Show solution

import java.util.Scanner;
public class SubstringExtractor {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a word: ");
        String word = scanner.nextLine();
        System.out.print("Start index: ");
        int start = scanner.nextInt();
        System.out.print("End index: ");
        int end = scanner.nextInt();
        System.out.println("Substring: " + word.substring(start, end));
        scanner.close();
    }
}

Common mistakes: Expecting end index to be inclusive (it is exclusive).

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Extension 5: Word Count

Show solution

import java.util.Scanner;
public class WordCount {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a sentence: ");
        String sentence = scanner.nextLine();
        int spaces = 0;
        for (int i = 0; i < sentence.length(); i++) {
            if (sentence.charAt(i) == ' ') {
                spaces++;
            }
        }
        System.out.println("Word count: " + (spaces + 1));
        scanner.close();
    }
}

Common mistakes: Off-by-one: forgetting to add 1 to space count; not handling empty strings.

Marking notes: Accept alternative approaches (split not taught yet, but counting spaces + 1 is expected).

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Extension 6: Email Parser

Show solution

import java.util.Scanner;
public class EmailParser {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter email: ");
        String email = scanner.nextLine();
        int atIndex = email.indexOf("@");
        String username = email.substring(0, atIndex);
        String domain = email.substring(atIndex + 1);
        System.out.println("Username: " + username);
        System.out.println("Domain: " + domain);
        scanner.close();
    }
}

Expected output: “alice@school.edu” -> Username: alice, Domain: school.edu

Common mistakes: Off-by-one with substring around the @ symbol.

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Extension 7: Palindrome Checker

Show solution

import java.util.Scanner;
public class PalindromeChecker {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a word: ");
        String word = scanner.nextLine();
        boolean isPalindrome = true;
        for (int i = 0; i < word.length() / 2; i++) {
            if (word.charAt(i) != word.charAt(word.length() - 1 - i)) {
                isPalindrome = false;
            }
        }
        if (isPalindrome) {
            System.out.println(word + " is a palindrome");
        } else {
            System.out.println(word + " is not a palindrome");
        }
        scanner.close();
    }
}

Common mistakes: Comparing wrong indices; forgetting length() - 1 - i; comparing with .equals() instead of charAt() (charAt returns char, use == for char comparison).

Marking notes: Accept break/flag approaches. Using .equals() to compare characters (chars) is incorrect – use == for char.

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Challenge 8: Caesar Cipher

Show solution

import java.util.Scanner;
public class CaesarCipher {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a word (uppercase): ");
        String word = scanner.nextLine();
        System.out.print("Enter shift: ");
        int shift = scanner.nextInt();
        String result = "";
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            char shifted = (char)('A' + (c - 'A' + shift) % 26);
            result += shifted;
        }
        System.out.println("Encrypted: " + result);
        scanner.close();
    }
}

Common mistakes: Not wrapping around Z (forgetting modulo 26); not handling lowercase/uppercase separately.

Marking notes: Accept uppercase-only solution. Handling both cases is bonus.

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Challenge 9: Title Case

Show solution

import java.util.Scanner;
public class TitleCase {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a sentence: ");
        String sentence = scanner.nextLine().toLowerCase();
        String result = "";
        boolean capitalizeNext = true;
        for (int i = 0; i < sentence.length(); i++) {
            char c = sentence.charAt(i);
            if (capitalizeNext && c != ' ') {
                result += Character.toUpperCase(c);
                capitalizeNext = false;
            } else {
                result += c;
            }
            if (c == ' ') {
                capitalizeNext = true;
            }
        }
        System.out.println(result);
        scanner.close();
    }
}

Common mistakes: Not lowercasing the input first; capitalize logic off-by-one.

Marking notes: Accept approach using indexOf(" ") and substring to build word-by-word.

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Integration Notes

Item	In Class	Homework	Link
Worked Examples 1-2	Walk through together	–	Worked Examples
Quick Code Check MCQs	End of Period 1	–	Quick Code Check
Core #1-4	Period 1 practice	–	Practice Exercises
Extension #5-7	Start in Period 2	Complete for homework	Practice Exercises
Challenge #8-9	–	Optional extension	Practice Exercises
Trace Exercise	Period 2 wrap-up	–	Trace Exercise
GitHub Classroom	–	Due end of week	GitHub Classroom

Additional Resources

• Live coding walkthrough: demonstrate concatenation, immutability, length, indexOf, substring
• Password creator activity: combine substring + charAt + concatenation
• Discussion prompt: “Why is String manipulation important for user-facing solutions?”