Tạo và Lưu trữ Đối tượng

Từ vựng

English	Ý nghĩa
constructor/kənˈstrʌktə/	Hàm khởi tạo
instantiation/ˌɪnstænʃɪˈeɪʃn/	Khởi tạo thực thể

English

Making a new object

To create an object, use the new keyword and a constructor 构造函数.
Scanner sc = new Scanner(System.in); builds a fresh Scanner object.
Instantiation 实例化 is the act of creating an object from a class.
The constructor sets up the object's starting state.

English

Constructor arguments

A constructor can take arguments to configure the new object.
new Random(42) seeds a random generator; new String("hi") copies text.
The arguments match the constructor's parameters, by position and type.
Different constructors (overloaded) let you build the same class different ways.

A primitive variable holds its value; a reference variable holds an arrow to an object

English

Storing the reference

new returns a reference to the object; store it in a variable of the right type.
Scanner sc = new Scanner(System.in); — sc now refers to that object.
The object lives in memory; sc is just the label pointing at it.
Lose the reference (no variable holds it) and the object becomes unreachable.

English

Two variables, one object

Assigning one object variable to another copies the reference, not the object.
Scanner sc2 = sc; — now sc and sc2 point to the same Scanner.
A change through sc2 is visible through sc — it's one object.
This reference-copy behavior is a defining feature (and trap) of objects.

English

Watch out

new creates ONE object and hands back a reference to it. Assigning that variable to another (b = a;) copies the reference, so both names point to the same object — not a second copy. Contrast primitives (int y = x; copies the value). Forgetting this leads to "I changed b, why did a change too?" surprises.

English

Example

Instantiation and aliasing:

Random r = new Random(); → r refers to a new Random object.
Random r2 = r; → r2 points at the same object (no new object made).
Both r and r2 now share one generator's state.

English

Key idea

Create an object with new plus a constructor (instantiation): new Scanner(System.in). new returns a reference you store in a variable. Assigning one object variable to another copies the reference — both then refer to the same object (unlike primitives, which copy the value).

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