Primitive Variable / Reference Variable

Primitive Variable

Primitive Type	Size	Minimum Value	Maximum Value	Wrapper Type
char	16-bit	Unicode 0	Unicode 216-1	Character
byte	8-bit	-128	+127	Byte
short	16-bit	-215 (-32,768)	+215-1 (32,767)	Short
int	32-bit	-231 (-2,147,483,648)	+231-1 (2,147,483,647)	Integer
long	64-bit	-263 (-9,223,372,036,854,775,808)	+263-1 (9,223,372,036,854,775,807)	Long
float	32-bit	单精度 float ff = 1.2344355f;		Float
double	64-bit	双精度 double dd = 1.2344355;		Double
boolean	1-bit	true or false		Boolean
void	-----	-----	-----	Void

Use a byte if either you specifically need to manipulate a signed byte (rare!), or when you need to move around a block of bytes;
整数：
万：short
万亿：int
巨大的整数用long  

浮点数：
单精度：float
双精度：double

Reference Variable - "remote control"
" It is important to understand that it is the type of reference variable - not the type of object that it refers to - that determines what members can be accessed. "

suppose you have:

Object x = "hello";

The type of the variable is Object, but the type of the object it refers to is String. it's the variable type which determines what you can do though - so you can't call

// Invalid
String y = x.toUpperCase();

The compiler only knows that you're calling a method on Object, which doesn't includetoUpperCase. Similarly, overloaded methods are only resolved against the ones you know about:

public class Superclass
{
    public void foo(Object x) {}
}

public class Subclass extends Superclass
{
    public void foo(String y) {}
}
...
Subclass x = new Subclass();
Superclass y = x;

x.foo("hello"); // Calls Subclass.foo(String)
y.foo("hello"); // Calls Superclass.foo(Object)