java-Basics-Variables-keywords-four classes eight data types-data conversion

　　　　Java Overview

public class Demo {//define a class

public static void Main (string[] args) {//Main method, starting point of all Programs

/* Print output on the screen */

System.out.println ("Hello world!");

}

}

(1) public class definition:

The class name must be consistent with the file name, or the program will not compile in a *. There can be only one public class in Java;

(2) Main method main ():

The Main method represents the beginning of a program, and all the program code is executed sequentially, and the main method in Java is also placed in a class.

(3) System output SYSTEM.OUT.PRINTLN ():

The function of this statement is to display the output information directly on the screen. Each complete statement code requires the use of ";" To the end.

(4) Note:

Represents a single-line comment, which is a double slash (//) before the content of the comment, and the Java compiler ignores this part of the message.

/*...*/represents a multiline comment, a multiline comment, which begins with a single slash plus a star marker (/*) in front of the comment content and ends with a star-shaped marker plus a single slash (*/) at the end of the comment content. This method is generally used when the comment content is more than one line.

identifiers, Keywords

in the program used to define the name of the identifier , such as the file name, class name, method name or variable name, etc., in the

The definition of identifiers in Java is formatted by letters , numbers , _ ( underscores ), and can be made up of $ , which cannot be duplicated, cannot start with a number, and cannot be a keyword in java. And every identifier has its own meaning.

Note : identifiers are case-sensitive.

There are case differences when defining identifiers, such as oracle,oracle,oracle, and so on, although the words are the same, but all belong to different identifiers.

hint : A simple suggestion for writing identifiers.

Some readers who are just in touch with programming languages may find it cumbersome to remember the rules above, so remind readers here that identifiers should always start with letters and try not to include other symbols.

Java keywords in :

abstract abstract	assert	boolean Boolean	break  jump out of the loop	byte   byte	CASE 	catch
Char character class	class	Continue jump out of the current loop	Const Constants	Default Object Methods	Do and Loop	Double floating-point data type
else judge	extends inherit	enum	< Span style= "FONT-SIZE:16PX; Color: #ff0000 ">final cannot be inherited, can inherit	< P align= "left" >finally	float double floating-point	for Span style= "COLOR: #0000ff" > loop
Goto	If judgment	Implements implementation Interface	Import Guide Package	instanceof Determine whether an object belongs to a data type	int integral type	Interface defining interface keywords
Long Integer type	Native	New instantiates an object	Package Packages	Private privatization modifier	Protected	Public Common
return return value	short short	static modifier	synchronized	super call parent class	STRICTFP	this Represents itself
Throw	Throws	Transient	Try	void	Volatile	While Loop

Basic types of 4 classes 8 kinds

four class	eight types	Number of bytes	Data Representation range
Integral type	Byte	1	-128~127
	Short	2	-32768~32767
	Int	4	-2147483648~2147483648
	Long	8	-263~263-1
Floating point Type	Float	4	-3.403e38~3.403e38
	Double	8	-1.798e308~1.798e308
Character type	Char	2	Represents a character, such as (' A ', ' a ', ' 0 ', ' home ')
Boolean type	Boolean	1	Only two values true and False

Note: The default integer type in Java is type int

The default floating-point type in Java is double type

　　　　　　　　　

　　　　　　　　to define the syntax format for a variable :

Data type variable name = data value;

int a = 100;

　　　　　　 A variable can be assigned without assigning a value after it is defined. No assignment can be used . variables have scope restrictions when used, variables cannot be defined repeatedly.

Data type conversions:

Automatic type conversion

A data type that represents a small range is converted to a large range of data types, which is called automatic type conversion

Automatic type conversion format:

Data type variable with large range = data type value with a small range;

Such as:

Double d = 1000;

Or

int i = 100;

Double D2 = i;

Forcing type conversions

A data type that represents a large range is converted to a small range of data types, which is called coercion type conversion

Forced type conversion Format:

Range small Data type variable = (range small data type) range large data type value;

Such as:

int i = (int) 6.718; The value of I is 6

Or

Double d = 3.14;

int i2 = (int) d; The value of I2 is 3

java-Basics-Variables-keywords-four classes eight data types-data conversion