java 的基本数据类型关系图

1、取值范围

类型	默认值	占用存储空间/字节	范围
byte	0	1	-127~128(-2的7次方到2的7次方-1)
short	0	2	-32768~32767(-2的15次方到2的15次方-1)
int	0	4	-2147483648~2147483647(-2的31次方到2的31次方-1)
long	0	8	-9223372036854774808~9223372036854774807(-2的63次方到2的63次方-1)
float	0.0	4	3.402823e+38 ~ 1.401298e-45（e+38表示是乘以10的38次方，同样 ，e-45表示乘以10的负45次方）
double	0.0	8	1.797693e+308~ 4.9000000e-324
char	空	2	\u0000~\uFFFF
boolean	false	---------------	false、true

2、从小到大的相关关系图

图中从左向右的转换都是隐式转换，无需再代码中进行强制转换 

byte i = 12;
System.out.println("byte:"+i);
short i2 = i;
System.out.println("short:"+i2);
int i3 = i;
System.out.println("int:"+i3);
long i4 = i;
System.out.println("long:"+i4);
float i5 = i;
System.out.println("float:"+i5);
double i6 = i;
System.out.println("double:"+i6);char j = '²';
System.out.println("char:"+j);
int j3 = j;
System.out.println("int:"+j3);
long j4 = j;
System.out.println("long:"+j4);
float j5 = j;
System.out.println("float:"+j5);
double j6 = j;
System.out.println("double:"+j6);

输出：

byte:12
short:12
int:12
long:12
float:12.0
double:12.0
char:²
int:178
long:178
float:178.0
double:178.0

从右向左均要进行强制类型转换，才能通过编译。强制转换会丢失精度

double i = 178.33;
System.out.println("double:"+i);
float i1 = (float) i;
System.out.println("float:"+i1);
long i2 = (long) i;
System.out.println("long:"+i2);
int i3 = (int) i;
System.out.println("int:"+i3);
short i4 = (short) i;
System.out.println("short:"+i4);
byte i5 = (byte) i;
System.out.println("byte:"+i5);
char i6 = (char) i;
System.out.println("char:"+i6);

输出：

double:178.33
float:178.33
long:178
int:178
short:178
byte:-78
char:²