APIs and Libraries

Vocabulary

English	Meaning
library/ˈlaɪbrəri/	A collection of reusable code, such as ready-made classes and methods, available for programs to use.
API/ˌeɪ piː ˈaɪ/	A published set of classes, methods and rules through which a program uses a library or service.

Standing on others' code

You don't write everything from scratch — Java ships with huge amounts of ready-made code.
A library 库 is a collection of pre-written classes you can use.
Java's standard library includes String, Math, Scanner, ArrayList, and thousands more.
Reusing tested library code is faster and safer than reinventing it.

What an API tells you

An API (Application Program Interface) 应用程序接口 is the documented list of what a class offers.
It tells you each method's name, what parameters it takes, and what it returns.
You program to the API — you use a method by its documented behavior, not its inner code.
The API is a contract: "give me these inputs, I'll give you this output."

A library groups classes; each class documents the methods you can call

Importing a class

Some classes need an import statement before you use them.
import java.util.Scanner; makes Scanner available.
Core classes like String and Math are in java.lang — imported automatically.
The import goes at the top of the file, above the class.

Use it without seeing inside

You can call Math.sqrt(x) without knowing how it computes a square root.
This is abstraction: the API hides the implementation behind a simple interface.
You only need the what (the documented behavior), not the how.
Good libraries let you build fast by treating complex code as a black box.

Explore

Use a library, or write it yourself?

Sort each task by whether a standard library already does it.

Watch out

You program to the API's documented behavior, not its internal code. Read what a method takes and returns — you don't need (or want) to see how it works inside. Remember most classes need an import, but java.lang classes (String, Math, System, Integer) are imported automatically, so Scanner needs import java.util.Scanner; while Math does not.

Example

Using the Math library:

No import needed — Math is in java.lang.
double r = Math.sqrt(16); → r is 4.0, and you never see the algorithm.
The API just promised: "give me a number, I return its square root."

Key idea

A library is reusable pre-written code; the API documents each class's methods — their names, parameters, and return values — so you program to the documented behavior. Most classes need an import (e.g. Scanner), but java.lang classes (String, Math) are imported automatically. The API is abstraction: use it without seeing inside.

Practice

An API documents a class's methods by telling you their...