1.15 String Manipulation

1. Reference Guide

Term	Definition / Example	College Board Requirement
String literal	Text in double quotes, which creates a `String` object: `String s = "hello";`	Unit 1, Topic 1.15, p. 48
String constructor	Another way to create one: `String s = new String("hello");`	Unit 1, Topic 1.15, p. 48
Immutable	A `String` object never changes; methods return a new `String` instead	Unit 1, Topic 1.15, p. 48
Concatenation	`+` or `+=` joins Strings, or a String and a primitive, into a new String: `"Score: " + 10`	Unit 1, Topic 1.15, p. 48
Index	A character’s position, starting at 0: in `"code"`, `c` is at 0 and `e` is at 3	Unit 1, Topic 1.15, p. 49
`StringIndexOutOfBoundsException`	Run-time error from an index below 0 or past the end: `"code".substring(2, 5)`	Unit 1, Topic 1.15, p. 49

The last index of a String is length() - 1
Compare Strings with equals, not ==. == checks whether two references point to the same object

String Methods (AP Java Quick Reference)

Method	Returns	Example with `s = "computer"`
`int length()`	The number of characters	`s.length()` is `8`
`String substring(int from, int to)`	Characters from `from` up to, not including, `to`	`s.substring(0, 3)` is `"com"`
`String substring(int from)`	Characters from `from` to the end	`s.substring(3)` is `"puter"`
`int indexOf(String str)`	Where `str` first starts, or `-1` if it is not there	`s.indexOf("put")` is `3`
`boolean equals(Object other)`	`true` if the characters are the same	`s.equals("computer")` is `true`
`int compareTo(String other)`	Negative, `0`, or positive for before, same, or after in dictionary order	`"apple".compareTo("banana")` is negative

Reading substring

Call on `"computer"`	Indexes used	Result
`substring(2, 5)`	2, 3, 4	`"mpu"`
`substring(0, 1)`	0	`"c"` (the first letter)
`substring(7)`	7	`"r"` (the last letter)
`substring(8)`	none	`""` (an empty String)
`substring(2, 9)`	past the end	`StringIndexOutOfBoundsException`

Spotting Immutability

A method is called but its result is not stored → nothing changed; the original String is the same
You want the change to stick → assign the result: s = s.substring(1);
+ joins a String and a number → the result is a new String: "Total: " + 5 + 3 is "Total: 53"

College Board Requirements

AP CSA Unit 1, Topic 1.15 String Manipulation. Quoted from the course and exam description (College Board, 2025, pp. 48-49):

“A String object represents a sequence of characters and can be created by using a string literal or by calling the String class constructor.”
“A String object is immutable, meaning once a String object is created, its attributes cannot be changed. Methods called on a String object do not change the content of the String object.”

The Java language specification states the same rule: String objects have a constant, unchanging value (Gosling et al., 2023, section 4.3.3).

Reference: College Board, AP Computer Science A Course and Exam Description (2025), Unit 1, Topic 1.15, pp. 48–49.

2. Real-Life Application

Gene Names That Turned Into Dates

Scientists name human genes with short strings like SEPT2 and MARCH1. When researchers opened their data in a spreadsheet, the program read those strings as dates and changed them to “2-Sep” and “1-Mar”, quietly corrupting gene lists in published research. In 2020 the official gene naming committee renamed them SEPTIN2 and MARCHF1 so the text would stay text. Everyone guess: why was it easier to rename the genes than to fix every spreadsheet?

Real Case	What Happened	Java Idea in This Lesson
Gene names (renamed in 2020)	Text that looked like a date was treated as a date	A `String` is just characters; code decides what they mean
People named Null	Some systems reject or lose the surname “Null” because they treat the text as “no value”	`"Null".equals(name)` is a String comparison; `null` is no object at all
Email addresses	A sign-up form splits `ada@school.org` at the `@`	`indexOf("@")` and `substring`
Usernames	`ada` plus `lovelace` becomes `alovelace`	`first.substring(0, 1) + last`

The takeaway: treat text as text. Find positions with indexOf, cut with substring, compare with equals, and remember that each method gives back a new String.

3. Tech Talk

Learning Objective: Create and concatenate Strings, and use length, substring, indexOf, equals, and compareTo to work with them.

Success Criteria: You can predict the result of any String method in the Reference Guide, find and cut out part of a String, and explain why calling a method does not change the original String.

Indexes start at 0, and the second number in substring is where it stops, not counted:


String s = "hello world";
System.out.println(s.length());          // 11
System.out.println(s.indexOf("world"));  // 6
System.out.println(s.substring(0, 5));   // hello
System.out.println(s.substring(6));      // world

A String never changes. Store the result if you want to keep it:


s.substring(6);                // the result is thrown away
System.out.println(s);         // hello world
s = s.substring(6);            // s now refers to a new String
System.out.println(s);         // world

Code Examples

A. Creating and joining Strings. The StringBasics class shows literals, the constructor, concatenation, and immutability

s1 + s2 builds a new String, so s1 is printed again afterward to show it did not change:


String s3 = s1 + s2;   // a new String
s1 += "!";             // s1 now refers to another new String

Terminology:

Immutable: no method changes a String’s characters. += does not change the old String either; it points the variable at a new one.

Code Runner Challenge

Predict every line, run it, then change s2 and predict again

View IPYNB Source

// CODE_RUNNER: Predict every line, run it, then change s2 and predict again
public class StringBasics {
    public static void main(String[] args) {
        String s1 = "hello";                // a literal
        String s2 = new String("world");    // the constructor
        System.out.println(s1);
        System.out.println(s2);

        String s3 = s1 + " " + s2;          // concatenation makes a new String
        System.out.println(s3);
        System.out.println(s1);             // s1 did not change

        String score = "Score: " + 7 + 3;   // left to right: joins 7, then 3
        String total = "Total: " + (7 + 3); // parentheses add first
        System.out.println(score);
        System.out.println(total);

        s1 += "!";                          // s1 now refers to a new String
        System.out.println(s1);
        System.out.println("She said \"hi\"");  // \" puts a quote inside a String
    }
}

StringBasics.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

B. String methods. The StringMethods class calls every method from the Reference Guide

s2 is "hihihellothere". Before you run, find where "hello" starts by counting from 0:


h i h i h e l l o t h e r e
0 1 2 3 4 5 6 7 8 9 ...

Terminology:

compareTo compares in dictionary order, letter by letter. A negative result means the first String comes first.

Code Runner Challenge

Predict each result, run it, then search s2 for a word that is not there

View IPYNB Source

// CODE_RUNNER: Predict each result, run it, then search s2 for a word that is not there
public class StringMethods {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = "hihihellothere";

        System.out.println(s2.length());          // 14
        System.out.println(s2.indexOf(s1));       // 4: where hello starts
        System.out.println(s2.indexOf("bye"));    // -1: not found

        String s3 = s2.substring(4, 9);           // indexes 4 to 8
        System.out.println(s3);
        System.out.println(s2.substring(9));      // from index 9 to the end

        System.out.println(s3.equals(s1));        // same characters
        System.out.println(s2.equals(s1));
        System.out.println(s1.compareTo(s2));     // e comes before i: negative
        System.out.println(s2.compareTo(s1));     // positive
        System.out.println(s3.compareTo(s1));     // 0: equal

        System.out.println(s2);                   // unchanged by every call above
    }
}

StringMethods.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Hacks & Practice Tasks


Lesson: CSA 1.15 String Manipulation
MCQ 1.15: <paste the copied result line, such as 3/4 | answers: B,C,A,D>
Popcorn: email split with indexOf and substring, original unchanged (yes/no); predicted index of @ = <p>; actual = <a>
Homework Hack 1: prediction = <what shortTitle printed before the fix>, all three checks print true (yes/no)
Homework Hack 2: all three username checks print true (yes/no)
Homework Hack 3: all four swapHalves and firstAlphabetically checks print true (yes/no)

Submission Safety Rules (Read First)

One class per cell, ending with ClassName.main(null);.
Label the homework hacks 1 through 3.
Write your predictions before you run.
Store the result of a String method if you want to keep it; never claim a method changed the original.
Run each Java cell and leave the output showing.
Include your MCQ score.
Use ## headings or smaller.

Popcorn Hack (In-Class)

3-minute challenge: split an email address at the @.

Find the position of "@" with indexOf
Use substring to get everything before it (the username) and everything after it (the domain)
Do not type "ada.lovelace" or "school.org" yourself: compute them from email
Predict the index of @ in the Popcorn checker under the runner before you run. When every check passes, copy its line into your submission notes

The last line prints email again to show it did not change.

Code Runner Challenge

Find the @ with indexOf, cut the email with substring, predict the index, then run it

View IPYNB Source

// CODE_RUNNER: Find the @ with indexOf, cut the email with substring, predict the index, then run it
// Practice #1 - Try it yourself first!
public class EmailSplitter {
    public static void main(String[] args) {
        String email = "ada.lovelace@school.org";

        int at = 0;            // TODO: where "@" is, using indexOf
        String username = "";  // TODO: everything before the @, using substring
        String domain = "";    // TODO: everything after the @, using substring

        System.out.println("Index of @: " + at);
        System.out.println("Username: " + username);
        System.out.println("Domain: " + domain);
        System.out.println("Email: " + email);
    }
}

EmailSplitter.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Popcorn checker press Run to check

Before you press Run, predict this output. Count the characters from 0.

What will email.indexOf("@") return?

Pass every check to finish the Popcorn Hack.

4. MCQ Check

4 questions, one at a time. Answer, check, then go to the next one. At the end, copy the score line into your submission notes.

5. Homework Hack

Task: Do all three hacks. Each starter compiles and runs, so you can run the initial version and compare its incorrect output with the expected output. Change only what the TODO asks for, and keep going until every check prints true.

Solution Skeleton:

Hack 1: The Change That Did Not Stick (5 minutes)

Immutability.

Part 1: Predict. Before you change anything, what does the shortTitle line print? Write it in prediction, then run it.

Part 2: Fix the bug. shortTitle should be "Java", but the result of substring is thrown away. Store it in shortTitle. title itself must not change.

Code Runner Challenge

Write your prediction, run it, then store the substring result so all three checks print true.

View IPYNB Source

// CODE_RUNNER: Write your prediction, run it, then store the substring result so all three checks print true.
public class ImmutableHack {
    public static void main(String[] args) {
        String title = "Java Programming";
        String shortTitle = title;
        title.substring(0, 4);  // TODO: the result is thrown away; store it in shortTitle instead

        // Part 1: what does the next line print BEFORE your fix?
        String prediction = "TODO: your prediction";
        System.out.println("shortTitle: " + shortTitle);

        System.out.println(prediction.equals("Java Programming"));
        System.out.println(shortTitle.equals("Java"));
        System.out.println(title.equals("Java Programming"));
    }
}
ImmutableHack.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Hack 2: Username Generator (5 minutes)

substring and concatenation. A username is the first letter of the first name followed by the whole last name: ada and lovelace become alovelace. Write makeUsername with substring and +. The names are already lowercase.

Code Runner Challenge

Write makeUsername with substring and + so all three checks print true.

View IPYNB Source

// CODE_RUNNER: Write makeUsername with substring and + so all three checks print true.
public class UsernameHack {
    // TODO: return the first letter of first, followed by all of last
    public static String makeUsername(String first, String last) {
        return "";
    }

    public static void main(String[] args) {
        System.out.println("Username: " + makeUsername("ada", "lovelace"));
        System.out.println(makeUsername("ada", "lovelace").equals("alovelace"));
        System.out.println(makeUsername("grace", "hopper").equals("ghopper"));
        System.out.println(makeUsername("alan", "turing").length() == 7);
    }
}
UsernameHack.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Hack 3: Swap and Sort (10 minutes)

Combined.

swapHalves(s) returns the second half followed by the first half. Split at s.length() / 2, so for an odd length the extra character goes in the second half: "hello" becomes "llohe".
firstAlphabetically(a, b) returns whichever String comes first in dictionary order. Use compareTo.

Code Runner Challenge

Write swapHalves and firstAlphabetically so all four checks print true.

View IPYNB Source

// CODE_RUNNER: Write swapHalves and firstAlphabetically so all four checks print true.
public class SwapAndSort {
    // TODO: split at s.length() / 2 and return the second half + the first half
    public static String swapHalves(String s) {
        return s;
    }

    // TODO: use compareTo to return the String that comes first in dictionary order
    public static String firstAlphabetically(String a, String b) {
        return a;
    }

    public static void main(String[] args) {
        System.out.println(swapHalves("abcdef") + " " + firstAlphabetically("pear", "apple"));
        System.out.println(swapHalves("abcdef").equals("defabc"));
        System.out.println(swapHalves("hello").equals("llohe"));
        System.out.println(firstAlphabetically("pear", "apple").equals("apple"));
        System.out.println(firstAlphabetically("cat", "dog").equals("cat"));
    }
}
SwapAndSort.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Grading Plan (1 Point Total)

Part	Points	What earns the points
Popcorn	0.2	The index is predicted before running, and the username and domain come from `indexOf` and `substring`, with the original email unchanged. The Popcorn checker line is in the notes.
MCQ	0.2	4 correct. 0.15 for 2 or 3, 0.1 if every question was answered.
Homework Hack 1	0.15	The prediction was written before the fix, the `substring` result is stored, and all three checks print `true`.
Homework Hack 2	0.2	`makeUsername` uses `substring` and `+`, and all three checks print `true`.
Homework Hack 3	0.25	`swapHalves` splits at `length() / 2`, `firstAlphabetically` uses `compareTo`, and all four checks print `true`.
Total	1.0

Quick Validation Checklist

Each cell ends with ClassName.main(null); and shows output.
Popcorn checker line in the notes.
MCQ score in the notes.
Every String result you need is stored in a variable.
Strings compared with equals or compareTo, not ==.
Every homework check prints true.

6. LxD Cycle Process

Empathize: Students write s1.substring(1) and expect s1 itself to change. They treat String methods like ArrayList methods, which really do change the object, and they lose track of indexes because counting starts at 0 and substring stops before its second number.

Define:

POV: CSA students need to see that a String never changes, because the AP exam often tests code that wrongly assumes a String method changed the original variable.
Learning Goal: Students will create and concatenate Strings, predict the results of length, substring, indexOf, equals, and compareTo, and store results instead of expecting the original to change.

Ideate:

HMW Question: How might we make immutability visible, something students see fail rather than just hear about?
HMW Question: How might we make index counting automatic before students reach loops over Strings in Unit 2?
Activity: Print the original String after each method call, split an email at the @, then fix a change that did not stick and build usernames.

Prototype:

A reference guide with the AP String methods and a substring table, runnable creation and methods examples, an email-splitting Popcorn Hack with a checker, an MCQ knowledge check, three scaffolded homework hacks with self-checking output, and a grading rubric.
The earlier version spent most of its examples on Dog objects, Arrays.sort, Math, and wrapper classes, which belong to Topics 1.11 to 1.14. It now stays on Strings, the subject of Topic 1.15.
Excellence means predicting every index and result exactly, and storing each String result that needs to be kept.

Test:

Ask peers in peer review to complete the practice without additional explanation.
Observe whether peers count indexes from 0, and whether they expect a method call to change the original String.
Compare your lesson with another that is posted.
Use the findings to revise any instruction or rubric criterion that did not guide students clearly.
On submission, collect evidence from runner output, MCQ results, AI grading and student explanations.
After teaching, grading and analysis, come back and revise lesson to complete teaching cycle for continuous improvement.

7. References/Citations

Bruford, E. A., Braschi, B., Denny, P., Jones, T. E. M., Seal, R. L., & Tweedie, S. (2020). Guidelines for human gene nomenclature. Nature Genetics, 52(8), 754–758. https://doi.org/10.1038/s41588-020-0669-3

College Board. (2025). AP Computer Science A course and exam description [Effective fall 2025]. https://apcentral.collegeboard.org/media/pdf/ap-computer-science-a-course-and-exam-description-effective-fall-2025.pdf

Gosling, J., Joy, B., Steele, G., Bracha, G., Buckley, A., Smith, D., & Bierman, G. (2023). The Java® language specification (Java SE 21 ed., section 4.3.3, The class String). Oracle. https://docs.oracle.com/javase/specs/jls/se21/html/jls-4.html

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1. Reference Guide

String Methods (AP Java Quick Reference)

Reading substring

Spotting Immutability

College Board Requirements

2. Real-Life Application

Gene Names That Turned Into Dates

3. Tech Talk

Code Examples

A. Creating and joining Strings. The StringBasics class shows literals, the constructor, concatenation, and immutability

Code Runner Challenge

B. String methods. The StringMethods class calls every method from the Reference Guide

Code Runner Challenge

Hacks & Practice Tasks

Submission Safety Rules (Read First)

Popcorn Hack (In-Class)

Code Runner Challenge

4. MCQ Check

5. Homework Hack

Hack 1: The Change That Did Not Stick (5 minutes)

Code Runner Challenge

Hack 2: Username Generator (5 minutes)

Code Runner Challenge

Hack 3: Swap and Sort (10 minutes)

Code Runner Challenge

Grading Plan (1 Point Total)

Quick Validation Checklist

6. LxD Cycle Process

7. References/Citations

Submit Assignment