Đối tượng: Các实例 của Lớp

Classes and objects

• A class 类 is a blueprint 蓝图 — it describes what a kind of thing has and can do.
• An object 对象 is a concrete instance 实例 built from that blueprint.
• One String class; many String objects ("cat", "dog", "Ada").
• The class defines the pattern; each object is one filled-in copy.

Blueprint vs. instance

• Think cookie cutter (class) vs. cookies (objects).
• The cutter shapes them; each cookie is a separate, real thing.
• Every object has its own data, even though they share the class's design.
• Changing one object doesn't change the others.

A class diagram: private attributes and public methods

State and behavior

• An object bundles state (its data — instance variables) and behavior (its methods).
• A String object's state is its characters; its behavior includes length(), substring().
• You interact with an object by calling its methods.
• This bundling of data + methods is the heart of object-oriented programming.

Objects have reference types

• An object is stored via a reference — the variable holds its address, not the object.
• String s = "hi"; — s refers to a String object somewhere in memory.
• Two references can point to the same object.
• Reference types behave differently from primitives — a theme of the next lessons.