Variables

Terminology

“field” and “variable”

4 kinds of variables:

Instance Variables (Non-Static Fields)（实例变量
Class Variables (Static Fields)（类/静态变量
Local Variables（本地/局部变量
Parameters （参数

If we are talking about “fields in general” (excluding local variables and parameters), we may simply say “fields”. If the discussion applies to “all of the above”, we may simply say “variables”. If the context calls for a distinction, we will use specific terms (static field, local variables, etc.) as appropriate. You may also occasionally see the term “member” used as well. A type’s fields, methods, and nested types are collectively called its members.

域(fields)不包括局部变量和参数，变量(variables)包括全部(fields, local variables, parameters)

Primitive Data Type

Data Type	Default Value(for fields)	Size	Min	Max
byte	0	8-bit	-128	127
short	0	16-bit	-32,768	32767
int	0	32-bit	-2^31	-2^31 -1
long	0L	64-bit	-2^63	-2^63 -1
float	0.0f	32-bit	link	link
double	0.0d	64-bit	above link	above link
boolean	false	not precisely defined.	false	true
char	‘\u0000’	16-bit	‘\u0000’	‘\uffff’

在数字字面值中使用下划线

In Java SE 7 and later, any number of underscore characters (_) can appear anywhere between digits in a numerical literal. This feature enables you, for example. to separate groups of digits in numeric literals, which can improve the readability of your code.

For instance, if your code contains numbers with many digits, you can use an underscore character to separate digits in groups of three, similar to how you would use a punctuation mark like a comma, or a space, as a separator.

The following example shows other ways you can use the underscore in numeric literals:

long creditCardNumber = 1234_5678_9012_3456L;
long socialSecurityNumber = 999_99_9999L;
float pi =  3.14_15F;
long hexBytes = 0xFF_EC_DE_5E;
long hexWords = 0xCAFE_BABE;
long maxLong = 0x7fff_ffff_ffff_ffffL;
byte nybbles = 0b0010_0101;
long bytes = 0b11010010_01101001_10010100_10010010;

You can place underscores only between digits; you cannot place underscores in the following places:

At the beginning or end of a number

Adjacent to a decimal point in a floating point literal

Prior to an F or L suffix

In positions where a string of digits is expected

The following examples demonstrate valid and invalid underscore placements (which are highlighted) in numeric literals: