AP CSA 1.7 — Application Program Interface (API) and Libraries

What Is an API?

An Application Program Interface (API) is a set of prewritten code and rules that allows programmers to use existing software components instead of writing everything from scratch. In Java, an API defines how your program can interact with built-in classes, methods, and packages provided by Java’s Standard Library.

Example in Java: When you write Math.sqrt(9), you are using the Math class API. You don’t need to know how the square root is calculated internally — only how to call it.

What Is a Library?

A library is a collection of prewritten classes and methods designed for common tasks. Java provides an extensive Standard Library that you can use in your own programs.

These built-in libraries save time and reduce bugs because they have already been tested and optimized.

How APIs and Libraries Work Together

The API is the interface: it’s the documentation that tells you what classes exist, what methods they have, what parameters to pass, and what each method returns.

The library is the implementation: it’s the actual code that runs and does the work.

When you use a method from a library, you rely on the API to answer questions like:

• What does this method do?
• What parameters do I give it?
• What type does it return?

import java.util.Scanner;

public class InputExample {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in); // Using Scanner from java.util
        System.out.print("Enter your name: ");
        String name = input.nextLine();         // Calling a method from Scanner API
        System.out.println("Hello, " + name + "!");
    }
}

Here:

• Scanner is a class from the java.util library.
• nextLine() is a method described in the API for Scanner.
• You can use it without having to write your own input-reading code.

Why This Matters in AP CSA

In AP Computer Science A, you are expected to:

• Use common Java APIs, especially String, Math, and ArrayList.
• Understand method headers: return type, method name, parameters.
• Read API documentation to figure out how to call a method correctly.

Summary