UNIT 1

Application Program Interface (API) and Libraries

UNIT 1 - TOPIC 1.7

AP Computer Science A

Learning Objective 1.7.A

Identify the attributes and behaviors of a class found in the libraries contained in an API.

What are Libraries and APIs?

        Library: A collection of pre-written classes that provide specific functionality
    

        API (Application Programming Interface): A specification that tells programmers how to use library classes
    

Key Points:

Libraries contain collections of classes
APIs provide documentation on how to use those classes
Classes in APIs and libraries are grouped into packages
We can use existing classes to create objects

Classes Define Reference Types

What is a Class?

A class defines a specific reference type
It serves as a blueprint for creating objects
Classes have two main components:

                1. Attributes (data/variables)

                2. Behaviors (methods)

Example: String Class

String message = "Hello World"; // Using String methods (behaviors) int length = message.length(); String upper = message.toUpperCase();

Attributes vs Behaviors

Attributes	Behaviors
Data related to the class Stored in variables Represent the "state" of an object What the object "has"	What instances can do Defined by methods Represent the "actions" of an object What the object "can do"

Car Class Example

// Attributes (what a car HAS): String color int year String model // Behaviors (what a car can DO): start() stop() accelerate() brake()

API Documentation is Essential

        Documentation in API specifications and libraries is essential to understanding the attributes and behaviors of a class defined by the API.
    

API Documentation tells us:

Class name and purpose - What the class represents
Constructor information - How to create objects
Method signatures - Available behaviors and how to use them
Parameter requirements - What data methods need
Return types - What methods give back
Usage examples - How to properly use the class

Classes are Organized in Packages

What are Packages?

Packages group related classes together
Help organize large libraries
Prevent naming conflicts
Make code more manageable

Java Package Examples

java.lang.* - String - Integer - Math java.util.* - ArrayList - Scanner - Random java.io.* - File - FileReader - BufferedReader

Creating Objects from Library Classes

We can utilize existing classes to create objects:

Examples of Using Library Classes

// Using Scanner class from java.util package Scanner input = new Scanner(System.in); // Using ArrayList class from java.util package ArrayList names = new ArrayList(); // Using Random class from java.util package Random generator = new Random(); // Using String class from java.lang package String greeting = new String("Hello!");

        Key Point: We don't need to write these classes ourselves - they're already provided in the Java libraries!
    

Unit 1.7 Summary

Essential Knowledge Recap:

1.7.A.1: Libraries are collections of classes. API specifications inform programmers how to use those classes. Classes define specific reference types and are grouped into packages.

1.7.A.2: Attributes refer to data stored in variables. Behaviors refer to what instances can do and are defined by methods.

Key Takeaways:

APIs provide the roadmap for using library classes
Classes have both attributes (data) and behaviors (methods)
Documentation is essential for understanding how to use classes
Existing classes save us time and provide tested functionality
Packages organize related classes together

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