Java Specification of basic grammar, identifiers, keywords, constants and variables
Java writing specifications
- End "}" must begin "{" longitudinally aligned with the first character of the row.
- Beginning "{" on the upstream end of the code, and before adding the spaces.
- Up the "{", the downlink downlink code indentation horizontal tab.
-
No uplink "{", the first letter of the downlink and the uplink code is aligned
Identifier
Outline
In Java variables, constants, functions, and statements are all block names, their names are collectively referred to as Java identifiers. For the classes, objects, methods, variables, data types, and custom interfaces named.
composition
Identifiers are numbers, letters, underscores (_) and dollar sign ($) symbol or RMB (¥) components.
Naming Convention
- There can be letters, numbers, underscores, dollar signs composed, can not start with a number.
- Strictly case-sensitive.
- It is not Java keywords and reserved words.
- Meaningful, usually nouns.
E.g
Legal name: Hello, hello_CSDN, _hello, abc $ 123,
Illegal name: class, 123abc, abc, 123
Keyword
Java keywords have special meaning to the Java compiler string, and also a convention programmers, programmers use a keyword to tell the compiler what I do.
Keyword
meaning
abstract
Members of the class or method show abstract properties
assert
Assertions, used for debugging
boolean
One of the basic data types, declare Boolean types of keywords
break
Advance out of a block
byte
One of the basic data types, Type Byte
case
Used in the switch represents a branch which is among the statements,
catch
In exception processing for capturing an abnormal
char
One of the basic data types, character types
class
Declare a class
const
Reserved keywords, there is no specific meaning
continue
Back to the beginning of a block
default
By default, for example, used in a switch statement, indicating that a default branch. Java8 , also acts on the default implementation of the interface functions declared
do
Used in the do-while loop structure
double
One of the basic data types, double precision floating point type
else
Used in a conditional statement, indicating that the time when the branch condition is not satisfied
enum
enumerate
extends
It indicates that a type is a subtype of another type. For a class, or it may be another class abstract class; For an interface, the interface may be another
final
Used to describe the final attribute that identifies a class can not derive subclasses or members of the method can not be covered, or the value of the member of the domain can not be changed, the constants used to define
finally
Statement block for processing exceptions, to declare a substantially certain to be performed to
float
One of the basic data types, single precision floating point type
for
An endless guide word structure
goto
Reserved keywords, there is no specific meaning
if
Guide word conditional statement
implements
Showed that a class implements the given interface
import
To access the show specified class or package
instanceof
It used to test whether an object instance of the object type is designated
int
One of the basic data types, integer type
interface
interface
long
One of the basic data types, long integer type
native
The method is used to declare a computer-related language (e.g., C / C ++ / FORTRAN implementation language)
new
To create a new instance of an object
package
package
private
An access control mode: the private mode
protected
An access control method: Protected Mode
public
An access control method: a common mode
return
Return data from member methods
short
One of the basic data types , short integer type
static
Show a static properties
strictfp
用来声明FP_strict（单精度或双精度浮点数）表达式遵循IEEE、754算术规范
super
表明当前对象的父类型的引用或者父类型的构造方法
switch
分支语句结构的引导词
synchronized
表明一段代码需要同步执行
this
指向当前实例对象的引用
throw
抛出一个异常
throws
声明在当前定义的成员方法中所有需要抛出的异常
transient
声明不用序列化的成员域
try
尝试一个可能抛出异常的程序块
void
声明当前成员方法没有返回值
volatile
表明两个或者多个变量必须同步地发生变化
while
用在循环结构中
可以按照关键字作用划分为四组：
- 用于数据类型：boolean、byte、char、double、false、float、int、long、new、short、true、void、instanceof、
- 用于语句：break、case、catch、continue、default、do、else、for、if、return、switch、try、while、finally、throw、this、super
- 用于修饰：abstract、final、native、private、protected、public、static、synchronized、transent、volatile
-
用于方、类、接口、包和异常：class、extends、implements、interface、package、import、throws
没有几个被Java保留的没有意义的关键字：cat、uture、generic、innerr、operator、outer、rest、var
还有三个保留字他们不是关键字而是文字包含了Java定义的值，他们不能用作标识符：
true、false、null
变量
什么是变量
变量是内存地址的别名，因为内存地址不好记忆。是为了方便存取内存地址中的数据。
变量三元素：变量类型、变量名、变量值
变量名的命名约定
- 满足标识符命名约定
- 符合驼峰法命名约定
- 见名如意
- 变量名的长度没有限制
数据类型
Demo
变量声明：
变量类型 变量名；
int n;//声明一个整型变量名为n
赋值：
变量名=变量值；
n=3;//给变量n赋值为3.
边声明边赋值：
int n = 3;
特点：
- 先声明，在赋值，后使用
- 局部或全局变量不能重复定义
转义字符
类型转换
自动类型转换
Demo
int d= 123;
doublef = d;
自动类型转换是一个小数据范围转大的数据范围，不会丢失数据细节。那么double的值就是123.00。
强制类型转换
如果a类型的数据表示范围比b类型大，a赋值给b则需要强制类型转换。
Demo
double d = 123.45;
float f = (float)d;
这是一个简单的强制类型转换。因为double类型是双精度而float是单精度，double类型的数据表示范围要比float大，所以需要强制类型转换，会丢失数据细节。
也就是说double类型强制转换为float类型会丢掉一个精度，那么float的值就是123.4。就像我们要把大象放在冰箱里需要把它切开丢掉一些不需要的东西，是同样的道理。
常量
关键字：final
是用final定义后的变量不能再次进行更改操作。
Demo
fnal int num = 5;
这时候这个名为num的常量变量就不能再次更改了，如果对他进行更改了系统会划红线报错，不能执行必须改正。
Demo
class Demo1{ public static void main(String[] args){ int a = 1; int b = 2; int c; System.out.printle("变量a的值=" + a + "/n变量b的值=" + b); c = a; a = b; b = c; System.out.printle("变量a的值=" + a + "/n变量b的值=" + b); } }这是比较简单的a、b互换数字的逻辑代码。今天就到这里啦，我们下期再见！
会长时间不间断跟新我所学到的知识。
错误希望指正。
我们会变得更好，加油！
我们评论区里见！