01-03 Java keywords, identifiers, annotations, constants and binary questions, variables, and data types

1: Keywords

    (1) the words    (2) that are given a specific meaning by the Java languagefeature:        all lowercase.    (3) Precautions:        A:goto and const exist as reserved words.        B:advanced Notepad similar to Notepad + + has a special color tag for the keyword

2: identifier

    (1) is a sequence    of characters that give names to classes, interfaces, methods, variables, etc. (2) Composition rule:        A: English case letter        B: Digital        c:$ and _    (3) Note:        A: Cannot start with a number        B: cannot be a keyword in Java        C: Case Sensitive    (4) Common naming rules (see name)        A: The package    is actually a folder, used to distinguish the same class name  all lowercase            single-level packages: Lowercase                example: liuyi,com            Multi-level package: lowercase, and use. Separate                example: Cn.itcast,com.baidu                        B: Class or interface            one word: First capitalization                example: Student,demo            multiple words: Capitalize each word first letter                Example: Helloworld,studentname        C: Method or Variable            a word: First letter lowercase                example: name,main            multiple words: Starting with the second word, capitalize each word with an                example: Studentage,showallnames ()        D: Constant            all uppercase            one word: Uppercase                Example: PI            multiple words: Uppercase and separated by _                example: Student_max_age

3: Notes

    (1) The text that explains the program    (2) Category:        A: Single-line comment     //        B: Multiline comment/    * */        /**/
Note: Multiple lines cannot be nested, and a single row is OK
    (3) wrote a annotated version of the HelloWorld case.        We're going to write a procedure in the back.        Requirements:        Analysis:        implementation:        code embodiment:    (4) The role of annotations        A: Explain the program, improve the reading of the code.        B: Can help us debug the program.            later, we will explain a more advanced debugging tool

Example of writing a comment

/*Requirements: I am going to write a Java program, the "HelloWorld" This sentence output in the console analysis: A: To write a Java program, you must define class B: The data can be output, indicating that our program can be run independently, and the program to be alone Run, you must define the Main method C: Output The data in the console, you must use the OUTPUT statement implementation: A:java language provides a keyword: class is used to define classes, followed by the class name B:main method format is fixed : public static void Main (string[] args) {} C: The format of the output statement is fixed: System.ou            T.println ("HelloWorld"); "HelloWorld" is something that can be changed.*///This is my HelloWorld case.class HelloWorld {/*in order for the program to run independently, the main method is defined as the entry of the program is automatically called by the JVM*/Public StaticvoidMain (string[] args) {//in order to display the data in the console, we used the output statementSystem.out.println ("HelloWorld"); }}

4: constant

    (1In the course of program execution, the amount of its value does not change (2) Category: A: Literal constants B: Custom Constants (later) (3) literal constant A: string constant"Hello"B: Integer Constants12,23C: decimal constant12.345D: Character Constants' A ', ' a ', ' 0 'E: Boolean Constantstrue,falseF: Empty constantNULL( later) (4) provides four representations in Java for integer constants A: binary consists of 0, 1.        Start with 0b. B: Octal by 0,1,... 7 composition.        Start with 0 (0) . C: Decimal by 0,1,... 9 composition.        Integers are decimal by default. D: Hexadecimal by 0,1, ... 9,a,b,c,d,e,f (both uppercase and lowercase) are composed. Start with 0x.

/*constant: The amount at which the value does not change during program execution. Category: A: Literal constants B: Custom Constants (later) literal constants A: String constants are enclosed in double quotation marks.            Example: "Hello", "World", "HelloWorld" B: integer constant All integers example: 100,200 C: decimal constant all decimals    Example: 10.23,110.11 D: an example of a character constant quoted in single quotes : ' A ', ' a ', ' 0 ' ERROR: ' AB ' E: boolean constant More special examples: True,false F: null constants followed by example: null*/class Constantdemo {public staticvoidMain (string[] args) {//output of string constantsSystem.out.println ("Hello"); //output of integer constantsSYSTEM.OUT.PRINTLN (100); //output of decimal constantsSystem.out.println (100.10); //output of character constantsSystem.out.println (' a '); System.out.println (A); System.out.println (' 0 '); //This is problematic, character constants have single quotes, string constants with double quotes        //System.out.println (' ab ');                //output of Boolean constantsSystem.out.println (true); System.out.println (false); }}

5: Binary Conversion

    (1) Other decimal        coefficients: is the numeric cardinality of each bit: X-        cardinality is the X-        right: the data on each digit, from the right, and starting from 0, the corresponding number is the right of the data.                results: The sum of the coefficients * cardinality ^ power power.    (2) decimal to other binary in        addition to the base, until the quotient is 0, the remainder reversal.    (3) Fast conversion method        of the binary conversion A: The conversion between decimal and binary is            8421 yards.        B: Binary to octal, hexadecimal conversion

Binary overview and binary, octal, hex plots:

Different binary data performance:

 /*   Different binary data representation: binary: Consists of 0, 1.        Start with 0b. Octal: by 0,1,... 7 composition.        Start with 0. Decimal: By 0,1,... 9 composition.        The default integer is decimal. Hex: by 0,1,... 9,a,b,c,d,e,f (both uppercase and lowercase) are composed. Start with 0x.  */ class jinzhidemo {public static  Span style= "COLOR: #0000ff" >void   main (string[] args) {System.out.println (100); //  decimal   System.out.println (0b100);  //         binary  System.out.println (0100); //         octal  System.out.println (0x100); //  hex  }}  

Conversion of the binary

Other binary to Decimal

Decimal to other binary:

Fast Conversion Method:

Master Law

01-03 Java keywords, identifiers, annotations, constants and binary questions, variables, and data types