通常在一个class中，我们会构造变量variables。

variables

By kind

• Instance variable
  -declared inside a class.(but not in a method.)
  -they represent the "field" that each individual object has.
  -live inside the objects they belong to.

当我们谈到class时，class不是object——class更像一个模板template，用来建立object。通过调用该class的构造函数constructor，我们可以得到objects。

public class Duck{
  int size;
}
// Here, "size" is an instance variable; 
// Every Duck has a "size" instance variable.

// Constructor

• Local variable
  -declared inside of a method, including method parameters.
  -只在method运行时存在。they are temporary, and live as long as the method is on the stack(not reaching the closing curly brace.)

public void foo(int x) {   // parameter "x"
  int i = x + 3;
  boolean b = true;   // i, b are local variables
}

By type
object reference / primitive reference

需要重点注意的是：

• 我们定义在一个class内部定义的都是reference variables, 是指向objects的遥控器，而不是objects。

Stack: live - Method invocations, Local variables (local variables). (比如go(), main())
Heap: live - ALL Objects live.