JAVA002---java basic syntax

Keywords and reserved words

• Key words
  • Words that are given special meaning by the Java language
  • All made up of lowercase letters
• Reserved words
  • Reserved by Java, not used in the current version, words that may be keywords in later versions
  • For example:goto,const

Identifier

is the custom name in the program, variable name, method name, class name, interface name, etc.

Naming rules

• A. consist of letters, numbers, _ and $;

B. Cannot begin with a number;

C. Inability to use keywords and reserved words;

D. No duplicate name and case sensitivity within the same scope;

Naming rules

Package Name	All lowercase	Beanutil
Variable name, method name	Hump rule + First Letter lowercase	Beanutil
Class name, interface name	Hump Rule + First letter capitalization	Beanutil
Constant name	All caps + underline segmentation	Bean_util

Comments

It is used for the annotation program, which improves the readability of the code and facilitates later maintenance.

① Single-line comment :

Format://...

The end of the line from//To this line is a comment.

② Multi-line comments

Format:/* ... */

The comments are between/* and the next */. (so multi-line annotations cannot be nested )

③ Document Comments

Format:/** ... */

Is a Java-specific comment that generates the API documentation for the code . (same cannot be nested)

Variable 1) data type

Java is a strongly typed language , and each variable must declare its type before it is used, and the system allocates a different size of memory space for different types of variables.

Integral type	Byte	1 bytes	-128~127	byte B = 100;
	Short	2 bytes	-215~215-1	Short S = 5;
	Int	4 bytes	-231~231-1	int i = 1;
	Long	8 bytes	-263~263-1	Long large = 12345L;
Floating point Type	Float	4 bytes	Single precision	float F = 3.14F;
	Double	8 bytes	Double precision	Double d = 3.14;
Character type	Char	2 bytes	0~65535	Char C1 = ' a ', c2 = ' \ n ', C3 = ' Medium ', C4 = 65535, C5 = ' \u0041 ';
Boolean type	Boolean	1 guests	true | False	Boolean B1 = True, B2 = false;

Note :

• Integer literals are type int by default, and floating-point literals are by default double type;
• Both float and double are imprecise, so try to avoid using floating-point types in comparisons;
• A long literal is appended with an L suffix; the literal of type float is added with an F suffix;
• Boolean has only two values of true and false, cannot be 0 1 or null;
• The char type occupies 2 bytes, representing the characters in the Unicode character set, which can be numbers, letters, symbols, Chinese, Japanese, and Korean, and there are three ways to assign them:
  • Use single quotation marks to contain a single character or escape character: ' A ', ' \ n '
  • Use single quotation marks to include 16 decimal numbers: ' \u0041 '
  • Number of direct uses between 0~65535:65

2) variables

• Concept
  • is a storage area in memory;
  • The region has its own type (data type) and name (variable name);
  • The value of the region can only change within the data type range;
• Significance
  • Used to store constants of the same type and can be reused;
• Attention
  • The scope of the variable;
  • Declared first, then initialized before it can be used;
• Use formatting
  • Variable name of data type = initialization value;

JAVA002---java basic syntax