Java Learning notes-chapter III identifiers, keywords, and data types

Chapter III identifiers, keywords, and data types

1. Java Identifiers and Keywords: in programming, a notation is usually used to identify variables, constants, methods, and classes, which are called identifiers . The Java language itself uses some identifiers, called Java keywords, that users should avoid using when naming identifiers, which would otherwise prevent the program from compiling.

The keywords in Java are as follows:

null void TD valign= "Center" align= "middle" > TD valign= "Center" align= "middle" >

abstract	abstract	assert	assert	boolean	Boolean
break	interrupt	byte		catch	capture
char	character	class		continue	continue
default	default	do	do		dual precision
else	otherwise	enum	enumeration	extends	inherit
< Span style= "font-size:14px" >false	false	final	final	finally	final
float	single precision floating point	for	when ... Time	if
implements	implement	import	introduce	instanceof
int	int	interface	interface	long	long
native	local	new	new	empty
package	package	private	private	protected	protected
public	public	return	return	short	short integer
static	static	STRICTFP	precision floating point	super	super
switch	convert	synchronized	sync	this	this
throw	throw	throws	throw	transient	temporary
true	true	try	try	empty
volatile	variable	while	when ... Time	byvalue	by value
case	case	const	constant	future	future
generic		goto	jump to	inner	internal
Outer	External	operator	Operator	Rest	Remainder
Var	Variable

The red part of the table above is reserved in Java but is temporarily unused, called a reserved word, and the user cannot use it as an identifier.

2. Naming rules for Java identifiers

(1) identifiers consist of letters, numbers, underscores (_), Dollar signs ($), and no length limitation;

(2) The first letter of the identifier must be a letter, an underscore (_), or a dollar sign ($);

(3) Letters in identifiers are case-sensitive , for example, dog and dog are two different identifiers;

(4) Constants are usually named in uppercase letters in English, and variables are usually named in lowercase English words. If a variable consists of multiple words, the first word is lowercase, the second word is capitalized, such as hotdog;

(5) The class identifier usually begins with an uppercase English letter;

(6) You cannot use Java keywords, reserved words as custom identifiers.

3. Java data type:Java data types are divided into simple data types and composite data types.

(1) Simple data type: numeric type (integer (Byte, short, int, long), float (float, double)),

Character type (char), Boolean type (Boolean)

Data type	Key words	The number of occupied positions	Default value	Range of values
BYTE type	Byte	8	0	-2 of 7 power to 7 power-1 ( -128~127)
Short-integer	Short	16	0	-2 of 15 power to 15 power-1 ( -32768~32767)
Integral type	Int	32	0	-2 of 31 power to 31 power-1 ( -2147483648~2147483647)
long	long	64	0	-2 63 Power 63 Power-1
< Span style= "font-size:14px" > single-precision	float	32	0.0	&NBSP;
Double precision	Double	64	0.0
Character type	Char	16	' \u0000 '	' \u0000 ' ~ ' \uffff '
Boolean type	Boolean	8	False	True, False

Explanatory notes:

A. Different types of data, which occupy a different memory space.

B. Different types of data that can perform different operations. such as integer and floating-point data can be arithmetic operations, and Boolean data is not.

C.byte, short, int, long are all used to represent integers, except that the range of values is different. There are three representations of integer data: decimal notation (consisting of positive, negative, and number 0, the number part cannot start with 0 ), octal representation (consisting of positive, negative, and number 0, the number part starts with 0 ), hexadecimal representation (by positive, negative sign, number 0, letter a~f or a~f (representing a value of~15), the number of parts beginning with 0 x or 0x ) The long integer data must be followed by an uppercase "L" or a lowercase "l" after the value.

Both D.float and double represent floating-point data, except that the range and precision of the values are different. The double type has a greater range of values and higher precision. floating-point data can be represented in two ways: decimal notation (e.g. 4.0,5.32), scientific notation (after decimal notation with "E" or "E" and exponential parts, note: "E" or "E" must be preceded by a number, the exponent part can be negative , but it must be an integer. e.g. 4.5E-5)

E. Character types are used to represent symbols in the Unicode character set, such as control characters, letters, numbers, and Chinese characters. The Unicode character set uses hexadecimal numbers to denote a character, such as ' \u0061 ' for lowercase letter A. ordinary character notation is a character enclosed in single quotation marks, and case-sensitive, such as ' a ' and ' a ' are two characters, while ' VA ' is illegal. The escape character notation format is ' \ character ', which is used primarily to denote characters that cannot be displayed, such as carriage returns, newline characters, tabs, and so on.

F. The Boolean type has only "true" and "false" values of two. It is not possible to represent true and False in Java with 1 and%, so Boolean variables cannot participate in various mathematical operations.

priority for simple data types: (Byte, short, char) →int→long→float→double

Mutual conversion of data types: low priority → high priority (auto-conversion) priority High → low priority (CAST)

For example int x=10;float y;  Y=x; Automatic conversion

float X=20.5f;int y;  y= (int) x; Forced conversions

(2) composite data type: The composite data types in Java include classes (class), interfaces (interface), and arrays.

Java Learning notes-chapter III identifiers, keywords, and data types