APIとライブラリ

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.