Создание объектов и хранение

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.

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

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.

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.