定义：被java语言赋予特殊的含义 ，用作专门用途的字符串
特点：全部关键字都是小写字母
官方地址：https://docs.oracle.com/javase/tutorial/java/nutsandbolts/_keywords.html
?详细内容：
*?not used **?added in 1.2***?added in 1.4****?added in 5.0
abstract
continue
for
new
switch
assert***
default
goto
package
synchronized
boolean
do
if
private
this
break
double
implements
protected
throw
byte
else?
import?
public?
throws?
case
enum****
instanceof
return
transient
catch
extends
int
short
try
char
final
interface
static
void
class
finally
long
strictfp**
volatile
const*
float
native
super
while?