1.07 Application Program Interface (API) and Libraries

1. Reference Guide

Term	Definition / Example	College Board Requirement
Library	A collection of classes someone already wrote and tested: Java’s standard library	Unit 1, Topic 1.7, p. 39
API	The documentation that says how to use a library’s classes: the `String` API lists `length()` and `substring(int, int)`	Unit 1, Topic 1.7, p. 39
Package	A named group of related classes: `java.util` holds `ArrayList` and `Scanner`	Unit 1, Topic 1.7, p. 39
`import`	Lets you use a class from another package by its short name: `import java.util.ArrayList;`	—
Attribute	Data an object has, stored in variables: a book’s `title` and `pages`	Unit 1, Topic 1.7, p. 39
Behavior	What an object can do, defined by methods: a book’s `isLong()`	Unit 1, Topic 1.7, p. 39

Classes in java.lang, like String and Math, never need an import
An API entry gives the method’s name, parameter types, and return type. That is everything you need to call it correctly

Common Java Packages

Package	Description	Example Classes	Import needed?
`java.lang`	Core classes	`String`, `Math`, `Integer`	No
`java.util`	Utility classes	`ArrayList`, `Scanner`, `Random`	Yes
`java.io`	Input and output	`File`, `FileReader`	Yes

Reading an API Entry

API Entry	Return Type	Call It Like This
`static double sqrt(double a)`	`double`	`double r = Math.sqrt(64);`
`static double pow(double a, double b)`	`double`	`double p = Math.pow(3, 4);`
`int length()`	`int`	`int n = text.length();`
`String substring(int beginIndex, int endIndex)`	`String`	`String s = text.substring(0, 8);`
`boolean add(E e)` (ArrayList)	`boolean`	`list.add("red");`
`int size()` (ArrayList)	`int`	`int n = list.size();`

Attributes vs. Behaviors

Concept	Definition	Example
Attributes	What an object HAS: data stored in variables	`name`, `grade`, `gpa`
Behaviors	What an object DOES: actions defined by methods	`displayInfo()`, `updateGPA()`

Picking the Right Call

The entry says static → call it on the class name: Math.sqrt(64)
The entry has no static → call it on an object: text.length()
The return type is not void → store or use the result
The class is not in java.lang → add an import at the top of the file

College Board Requirements

AP CSA Unit 1, Topic 1.7 Application Program Interface (API) and Libraries. Quoted from the course and exam description (College Board, 2025, p. 39):

“Libraries are collections of classes. An application programming interface (API) specification informs the programmer how to use those classes. Documentation found in API specifications and libraries is essential to understanding the attributes and behaviors of a class defined by the API… Classes in the APIs and libraries are grouped into packages.”
“Attributes refer to the data related to the class and are stored in variables. Behaviors refer to what instances of the class can do (or what can be done with them) and are defined by methods.”

Reference: College Board, AP Computer Science A Course and Exam Description (2025), Unit 1, Topic 1.7, p. 39.

2. Real-Life Application

Eleven Lines That Broke Thousands of Projects

In March 2016, a developer removed a tiny library called left-pad from npm, the package registry for JavaScript. It had about eleven lines of code that added spaces to the left of a string. Thousands of projects, including the build tools behind big websites, used it, so their builds started failing within hours. Everyone guess: why would huge projects depend on a library that small instead of writing those eleven lines themselves?

Real Case	What Happened	Java Idea in This Lesson
`left-pad` (2016)	Projects used a tiny library through its API, so removing it broke them all at once	Using a library means depending on its API
Square roots	Nobody writes their own square root routine	`Math.sqrt(64)` is already written and tested
Lists that grow	Java’s `ArrayList` handles resizing for you	`import java.util.ArrayList;`, then call `add` and `size`
A school app	A student record has data and actions	Attributes like `gpa`, behaviors like `updateGPA()`

The takeaway: libraries save you from rewriting tested code, and the API tells you exactly how to call them. Read the documentation before you write your own version.

3. Tech Talk

Learning Objective: Use classes from Java’s libraries by reading their API documentation, import classes from packages, and name the attributes and behaviors of a class.

Success Criteria: You can call a method you have never used before from its API entry alone, add the import it needs, and label each member of a class as an attribute or a behavior.

A library is a toolbox. Its API is the label on each tool: it tells you what you can call, without showing how the tool is built. Packages group related tools.


double root = Math.sqrt(64);         // java.lang: no import
String word = "Computer";
int n = word.length();               // 8

import java.util.ArrayList;          // goes at the top of the file
ArrayList<String> colors = new ArrayList<>();
colors.add("red");                   // documented as add(E e)

ArrayList is covered in depth in Unit 4. Here it is just an example of reading documentation for a class outside java.lang.

Code Examples

A. Library classes without an import. The LibraryDemo class calls Math and String from their API entries

Every call below comes from an API entry in the Reference Guide. Math methods are static, so they are called on the class name. String methods are called on a String object:


Math.sqrt(16)                 // static double sqrt(double a)
text.substring(0, 8)          // String substring(int beginIndex, int endIndex)

Terminology:

java.lang is the package Java imports for you, which is why String and Math need no import line.

Code Runner Challenge

Predict each line from its API entry, then run it

View IPYNB Source

// CODE_RUNNER: Predict each line from its API entry, then run it
public class LibraryDemo {
    public static void main(String[] args) {
        // Math: static methods, called on the class name
        System.out.println("Square root of 16: " + Math.sqrt(16));
        System.out.println("3 to the 4th: " + Math.pow(3, 4));
        System.out.println("Absolute value of -5: " + Math.abs(-5));

        // String: methods called on a String object
        String text = "Computer Science";
        System.out.println("Length: " + text.length());
        System.out.println("First 8 chars: " + text.substring(0, 8));
        System.out.println("Index of Science: " + text.indexOf("Science"));
    }
}

LibraryDemo.main(null);

Lines: 1 Characters: 0

Output

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

B. Attributes, behaviors, and an import. The StudentRecord class keeps its courses in an ArrayList

StudentRecord has three attributes and four behaviors. One attribute is an ArrayList, which lives in java.util, so the file needs an import. Every ArrayList call (add, size, get) comes from its API.


import java.util.ArrayList;   // without this line, ArrayList does not compile

Terminology:

Attributes are the variables an object keeps. Behaviors are the methods that read or change them.

Code Runner Challenge

Label each member as an attribute or a behavior, then run it

View IPYNB Source

// CODE_RUNNER: Label each member as an attribute or a behavior, then run it
import java.util.ArrayList;

public class StudentRecord {
    // Attributes: what a student record HAS
    private String name;
    private double gpa;
    private ArrayList<String> courses;

    public StudentRecord(String name, double gpa) {
        this.name = name;
        this.gpa = gpa;
        this.courses = new ArrayList<>();
    }

    // Behaviors: what a student record DOES
    public void enroll(String course) {
        courses.add(course);           // ArrayList API: boolean add(E e)
    }

    public void updateGPA(double newGPA) {
        gpa = newGPA;
    }

    public boolean isHonorRoll() {
        return gpa >= 3.5;
    }

    public void displayInfo() {
        System.out.println(name + " - GPA " + gpa + " - " + courses.size() + " courses");  // int size()
        System.out.println("First course: " + courses.get(0));                              // E get(int index)
    }

    public static void main(String[] args) {
        StudentRecord bob = new StudentRecord("Bob", 3.4);
        bob.enroll("AP CSA");
        bob.enroll("Physics");
        bob.updateGPA(3.8);
        bob.displayInfo();
        System.out.println("Honor roll: " + bob.isHonorRoll());
    }
}

StudentRecord.main(null);

Lines: 1 Characters: 0

Output

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Hacks & Practice Tasks


Lesson: CSA 1.7 Application Program Interface (API) and Libraries
MCQ 1.7: <paste the copied result line, such as 3/4 | answers: B,C,A,D>
Popcorn: Math.pow, Math.sqrt, and an imported ArrayList of 3 colors, prints and runs (yes/no); predicted 3 to the 4th = <p>; actual = <a>
Homework Hack 1: both hypotenuse checks print true (yes/no)
Homework Hack 2: Book attributes and behaviors labeled, all three checks print true (yes/no)
Homework Hack 3: playlist checks print true (yes/no); API entries cited = <the three entries you used>

Submission Safety Rules (Read First)

One class per cell, ending with ClassName.main(null);.
Label the homework hacks 1 through 3.
Use only methods that appear in the Reference Guide or in the official Java documentation.
Add an import for every class outside java.lang.
Run each Java cell and leave the output showing.
Include your MCQ score.
Use ## headings or smaller.

Popcorn Hack (In-Class)

3-minute challenge: use the API entries in the Reference Guide. Do not guess method names.

Use Math.pow to print 3 to the 4th power
Use Math.sqrt to print the square root of 64
Create an ArrayList of Strings (the import is already there)
Add "red", "blue", and "green"
Print the list’s size with size()
Predict the Math.pow line in the Popcorn checker under the runner before you run. When every check passes, copy its line into your submission notes

Code Runner Challenge

Complete the TODOs from the API entries, predict the pow line, then run it

View IPYNB Source

// CODE_RUNNER: Complete the TODOs from the API entries, predict the pow line, then run it
// Practice #1 - Try it yourself first!
import java.util.ArrayList;

public class UsingDocumentation {
    public static void main(String[] args) {
        // TODO: print "3 to the 4th: " followed by Math.pow(3, 4)

        // TODO: print "Square root of 64: " followed by Math.sqrt(64)

        // TODO: create an ArrayList of Strings named colors

        // TODO: add "red", "blue", and "green" to colors

        // TODO: print "Colors: " followed by colors.size()

    }
}

UsingDocumentation.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. Check the return type in the API entry.

What will the "3 to the 4th" line print after the colon?

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: Use the Math API (5 minutes)

Calling library methods. The hypotenuse of a right triangle is the square root of a squared plus b squared. Write hypotenuse using only Math.pow and Math.sqrt from the Reference Guide.

Code Runner Challenge

Write hypotenuse with Math.pow and Math.sqrt so both checks print true.

View IPYNB Source

// CODE_RUNNER: Write hypotenuse with Math.pow and Math.sqrt so both checks print true.
public class MathApiHack {
    // TODO: return the square root of (a squared + b squared), using Math.pow and Math.sqrt
    public static double hypotenuse(double a, double b) {
        return 0.0;
    }

    public static void main(String[] args) {
        System.out.println("3, 4 -> " + hypotenuse(3, 4));
        System.out.println(hypotenuse(3, 4) == 5.0);
        System.out.println(hypotenuse(5, 12) == 13.0);
    }
}
MathApiHack.main(null);

Lines: 1 Characters: 0

Output

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Hack 2: Attributes and Behaviors (5 minutes)

Designing a class. Book should have three attributes (title, author, pages) and these behaviors: a constructor that sets all three, getTitle(), isLong() (true when pages is more than 300), and displayInfo(). Finish the TODOs, and put an // attribute or // behavior comment on each member.

Code Runner Challenge

Finish Book and label each member so all three checks print true.

View IPYNB Source

// CODE_RUNNER: Finish Book and label each member so all three checks print true.
public class BookHack {
    static class Book {
        private String title;
        private String author;
        // TODO: add the third attribute, pages

        public Book(String title, String author, int pages) {
            this.title = title;
            this.author = author;
            // TODO: store pages
        }

        public String getTitle() {
            return title;
        }

        // TODO: return true when pages is more than 300
        public boolean isLong() {
            return false;
        }

        public void displayInfo() {
            System.out.println(title + " by " + author);  // TODO: also print the pages
        }
    }

    public static void main(String[] args) {
        Book big = new Book("Java Basics", "John Doe", 350);
        Book small = new Book("Quick Start", "Ana Ruiz", 120);
        big.displayInfo();

        System.out.println(big.getTitle().equals("Java Basics"));
        System.out.println(big.isLong() == true);
        System.out.println(small.isLong() == false);
    }
}
BookHack.main(null);

Lines: 1 Characters: 0

Output

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Hack 3: Follow the Documentation (10 minutes)

Combined. These ArrayList API entries are new to you. Use them to turn the playlist [Intro, Verse, Chorus] into [Opening, Verse, Bridge]. Above each call, write a comment citing the API entry you used.

API Entry	What It Does
`void add(int index, E element)`	Inserts `element` at `index` and shifts later items right
`E set(int index, E element)`	Replaces the item at `index`
`E remove(int index)`	Removes the item at `index` and shifts later items left

Code Runner Challenge

Use add(index, element), set, and remove so both checks print true.

View IPYNB Source

// CODE_RUNNER: Use add(index, element), set, and remove so both checks print true.
import java.util.ArrayList;

public class PlaylistHack {
    public static void main(String[] args) {
        ArrayList<String> playlist = new ArrayList<>();
        playlist.add("Intro");
        playlist.add("Verse");
        playlist.add("Chorus");

        // TODO 1: replace "Intro" with "Opening" (cite the API entry in a comment)

        // TODO 2: insert "Bridge" at index 2, before "Chorus" (cite the API entry)

        // TODO 3: remove "Chorus", which is now the last item (cite the API entry)

        System.out.println(playlist);
        System.out.println(playlist.toString().equals("[Opening, Verse, Bridge]"));
        System.out.println(playlist.size() == 3);
    }
}
PlaylistHack.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	`Math.pow` and `Math.sqrt` print their results, an `ArrayList` of three colors prints its size, and the cell runs. 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	`hypotenuse` uses only `Math.pow` and `Math.sqrt`, and both checks print `true`.
Homework Hack 2	0.25	`pages` is stored, `isLong` and `displayInfo` work, every member is labeled attribute or behavior, and all three checks print `true`.
Homework Hack 3	0.2	The playlist changes using `set`, `add(index, element)`, and `remove`, each with a comment citing its API entry, and both 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.
An import for every class outside java.lang.
No hand-written code that repeats what a library method already does.
Every homework check prints true.

6. LxD Cycle Process

Empathize: Students write their own sorting, text, or random-number code from scratch, even though Java already has tested classes that do it. They treat API documentation as optional reading instead of as the main way to learn what a class can do, and they use “library,” “API,” and “package” as if they meant the same thing.

Define:

POV: CSA students need to be comfortable reading API documentation, because real Java work and the AP exam’s provided classes assume you can use a class without reading its source code.
Learning Goal: Students will call library methods from their API entries, explain how libraries, APIs, and packages relate, and tell a class’s attributes from its behaviors.

Ideate:

HMW Question: How might we teach students to trust and read documentation instead of rewriting what already exists?
HMW Question: How might we make the return type in an API entry something students check before they call a method?
Activity: Read API entries like a menu, predict a call’s output from its return type, then use new entries to finish a class and edit a list.

Prototype:

A reference guide with an API-entry table, runnable library and attributes-and-behaviors examples, a documentation Popcorn Hack with a checker, an MCQ knowledge check, three scaffolded homework hacks with self-checking output, and a grading rubric.
A trial run showed students mixing up “API,” “library,” and “package,” so the three terms sit side by side in the Reference Guide, next to the attributes and behaviors table.
Excellence means calling an unfamiliar method correctly from its documentation on the first try and citing the entry used.

Test:

Ask peers in peer review to complete the practice without additional explanation.
Observe whether peers look up the API entry before calling a method, and whether they predict 81.0 rather than 81 for Math.pow.
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

Bloch, J. (2006). How to design a good API and why it matters. In Companion to the 21st ACM SIGPLAN Symposium on Object-Oriented Programming Systems, Languages, and Applications (OOPSLA ‘06) (pp. 506–507). Association for Computing Machinery. https://doi.org/10.1145/1176617.1176622

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

Oracle. (2023). Java Platform, Standard Edition 21 API specification. https://docs.oracle.com/en/java/javase/21/docs/api/index.html

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1. Reference Guide

Common Java Packages

Reading an API Entry

Attributes vs. Behaviors

Picking the Right Call

College Board Requirements

2. Real-Life Application

Eleven Lines That Broke Thousands of Projects

3. Tech Talk

Code Examples

A. Library classes without an import. The LibraryDemo class calls Math and String from their API entries

Code Runner Challenge

B. Attributes, behaviors, and an import. The StudentRecord class keeps its courses in an ArrayList

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: Use the Math API (5 minutes)

Code Runner Challenge

Hack 2: Attributes and Behaviors (5 minutes)

Code Runner Challenge

Hack 3: Follow the Documentation (10 minutes)

Code Runner Challenge

Grading Plan (1 Point Total)

Quick Validation Checklist

6. LxD Cycle Process

7. References/Citations

Submit Assignment