Java中易混淆

1，instance variance vs. class variance , constant vs. static

local 

instance var: each object of that class has that a different version of that variable. eg: count1, count2, each have their own variables, their own box

class var: a variable shared by all objects in a class  eg: there's one variable counter , that all objects of the type of the class are referring to the one single box

specify:

instance var: private/ public and give a name

class variable: add "static", a static variable-----a static variable, eg: constant. PI should be shared as a class variable among all circles. We just want one it share, so we call static

constant is static

but static is not necessarily constant, they don't have to be final

eg:

/* create a class variable */

private static int COUNTER;

instance variable有很多copy
class variable只有一个copy 被share
区别在于有static
并且可以通过类名访问
constant实际上都是class vvativariable，除了static之外，还需要加上final 关键字。但是也可以定义constant instance variable和constant local variable