[Java] some good conventions

1 prefer returning  empty collections but not null :  can avoid many if null tests

2 String operation in loop : try to use StringBuilder and StringBuffer because String is immutable

3  try your best to avoid create unused object ---- creating object is the most expensive

4 when size is fixed, try to use array but  not  ArrayList --- array is more fast than arrayList

5 exit() is priory than finally : when exit() is called, finally() will not be reached

6 is odd number(奇数) ?
               return num%2 == 1 --> what about negative numbers ?
so better solution is  : return (nums&1) == 1

7 three tips for memory : a. use finally b. release connection once after inquire (查询) DB
                                   c.remember to release instance of static members

8 give attention to avoid deadlock : sometimes it's just a order of calling two methods

9 want to calculate the running time ?
 System.currentTimeMillis() : is related to a absolute number (1970/01/01...), faster than nanoTime.
 System.nanoTime() : not related to any time point, more accurate(精确 millis vs nano)\

10 float or double ?
float : 4 bytes : 7 significant digits(有效数字)
double : 8 bytes : 14

11 Exponentiation (幂运算) a^N
Java use xor to calculate Exponentiation
there are two ways : 
multiply:
res = a*a;
res*=a;
.....

Math.pow(a,N) ---> too slow, try to use multiply 

12 JSON is easy to use : 
JSONbject obj;
obj.put("key","value")//{key : value}

array = new JSONArray()
 array.add(value1);
array.add(value2)
array.add(value3)
...
 obj.put("arrayKey", array)//{key : [value1,v2,v3...]}
============================================ 
FileReader reader = ....
JSONParser  jsonParser = new ...
JSONObject obj  = (JSONObject)jsonParser.parse(FileReader)

12 search sub string : String.indexOf() :  -1 for not found

13 when we known not found returns -1, when using if , it's better to use 
   if(i<0) rather than if(i == -1) --> who knows it will not return -2 ?!

14 SOLID : one class just do one thing!

15 less than 10 classes in a package

16 less than 50 methods in a class

17 less than 200 lines in a file

18 use equals() rather than ==

19 in "if" put const before the variable if("const".equals(s))

20 try not to use float and double as possible

21 use final for all methods, parameters of method and variables you created !

22 always add default in switch 

23 use {}, even one line, particularly for switch case