First knowledge of Java

The first part of the Java Foundation

I. Knowledge of Java

1. What is Java?

Java is an object-oriented programming language that can compose cross-platform applications, and is a generic term for the Java programming language and Java platform (ie, javase, EE, javame), introduced by Sun Microsystems Corporation in May 1995.

2. Build a Java Project

① Create a new java-project:file---new---java-project, creating PROJECT;SRC Storage package and source files;

Project name: named English, letters, numbers, underscores, cannot start with a number, and each word is capitalized in the first letter.

Example: Helloworld_1

　　　　

　　　　

② new Package in SRC

Name: Letters, numbers, underscores, cannot start with a number; The URL is inverted +project name (lowercase).

Example: Com.jredu100.helloworld

　　　　

　　　　　　

③ new Java class,name and Class name are the same in the package.

Example: HelloWorld

　　　　

Second, Java basic knowledge

1. How to name identifiers　

① Pascal nomenclature: Capitalize the first letter, followed by the first letter of all the words are capitalized;

Classes, interfaces, enumerations, etc. are used in Pascal's nomenclature.

Hump rule: First letter lowercase, all subsequent words capitalized.

The method name, variable name and so on use the hump rule.

When the ② identifier is named, it begins with a letter, underscore, and $ symbol, and the remainder can be letters, underscores

and numbers.

2. Basic data types in Java

①int (integral type)

②short (short integer type)

③long (Long integer type)

④byte (bytes)

⑤float (single-precision floating-point type)

⑥double (double-precision floating-point type)

⑦char (character type): Char c= ' a '; or char c= ' word ';

⑧boolean (Boolean): Only true and false two values

3. Data type Transfer

① Basic data type: pass by value, pass the copy, modify the copy without affecting the original data;

② Reference Data type: passed by reference, passing a pointer, if the modification affects the original data.

4. Constant value

(1) constant type (package change)
Boolean constant: (True, False)
Character Constants
string Constants
NULL constant: Indicates that the reference variable is saved with an empty address and has not pointed to any objects.
Integral type constant
floating-point constants
(2) Use of constants
declaring constants needs to be decorated with final　　　　
//constant name All letters are capitalized (pi=3.14f)
//constants cannot be modified during program operation
final float pi=3.14f;　　　　
//pi=3.1415f;
float Area=10*10*pi;　　　　
System.out.println (area);

5. Basic Data type Conversion
(1) automatic type conversion rules
Wasting memory space
A, Rule 1: Arithmetic operations
Low---> High level
Short s_int=100;
int n=s_int;
System.out.println (n);
(2) Forced type conversion
forced type conversions (high-level---> Low-level) are required when assigning a high number of types to a low-level type
int turns to short:
　　 int n3=100;
Short s_int2= (short) N3;
System.out.println (s_int2);
* from high to low turn, loss of data accuracy;
For example: float f3=7.999f;
int n4= (int) F3;
System.out.println (N4);
6. Operators
(1) operator type
arithmetic operations: + (plus)-(minus) * (multiply)/(except)% (take surplus)
relational operations: equals sign: = = not equals symbol:! = greater than symbol:> less than symbol:< greater than equals symbol: >= less than equals symbol: <=
bitwise operator:& | ~; logical operator:&& | |!
Assignment Operation: = + = = *=/=%=
single-Mesh operation: + (take positive)-(minus) + + (self-increment)--(self-reduction)
Multi-mesh operation (ternary operator): A>b?true:false
shift operator:<< left shift, >> right SHIFT, >>> unsigned right shift
(2) operator precedence and binding
Priority: Non-> and > or
other best-in-Class () control, highest priority
(3) Scanner
//1. Get your membership card number 4 digits
System.out.println ("Please enter the membership card number (4-bit integer)");
//instantiation of a Scanner object SC: Object name NEW: New
//system.in Standard input stream, console input
Scanner sc=new Scanner (system.in);
//sc.nextint () Gets the value of an int from the standard input stream (input decimal, missed match, error)
int cardid=sc.nextint ();//get an int data value
System.out.println (CardID);

　　

First knowledge of Java