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Java的传参都是值传递，对于引用类型会传递引用的值，但是子函数中得到的是独立的复制的值。
String str = "abc";
change(str);
public static void change(String s){
s = null;
}
上述代码s也指向abc，但是s和str 是独立的，改变s只会让s指向空，而不会改变str以及str的内容。另外String, Integer, StringBuffer等等都是final,他们指向的内容初始化后不会再改变,但是可以使指向他们的引用指向其他内容。

Java 的变量存储：
Stack: 一般变量及对象的引用。
Heap: new 生成的对象 (Includes static variables and method in PermGen section in HEAP, things changed in Java8)
Constant Pool: independent from stack & heap. store string constant per class file

Memory Layout for C:

Address: High Command Line & Environment Variables
Stack

                        Heap

                        Data Segment (global and static variables):
                                                 BSS (Uninitialized Segment, would be initialized to 0 by loader)

                                                 Initialized Segment (includes read only constant strings)

                        Text Segment (binary code)

Static & Extern
Static is used before local, global variables and functions to make them only visible in the same source file. static local variable also stores in data segments other than stack and life cycle is longer than function. Extern declare the variable is defined in another compile unit. function is by default decorated by extern;

inline & define
inline function tells the compiler to replace the function body with function calling other than switching context to save statck. inline is similar to normal function calling, it will check the parameters and easy
to debug. While macro does not check parameters and is potential unsafe, unable to debug. Thus,
not recommend to define complex functions.

Volatile
in C, this keyword tells compiler that the variable could be modified by other threads anytime, do not optimize its access (e.g save it in cache or register). Volatile has similar function in Java, however it does not guarantee atom operations, You should use synchronized keyword.

attribute
attribute((function/variable/type)) 放在函数申明的分号；之前，让编译器检查优化函数。
例如add(int a, int b)attribute((const)) 多次调用同一参数时会优化只调用一次。
int a__attribute__((aligned 8)) = 10; 8字节对齐分配

Function Pointers
int (_pointer_name) (int a, int b); a pointer point to function address just like variable or array.
int (_test) (int a, int b) = add;
test (2, 3) is equal to add(2, 3);
typedef int (myfunc) (int, int) is define a type myfunc as int () (int, int)
myfunc f = func1; use it as f(2, 3) = func1(2, 3)