Java Language Foundation II

1 Keyword 1.1 keyword overview

• Words that are given special meaning by the Java language.

1.2 Features of keywords

• The letters that make up the keywords are all lowercase.

1.3 Keyword Considerations

• ①goto and const exist as reserved words (which may be promoted to keywords in the new version of the JDK) and do not currently apply.
• ② classes such as notepad++, a high-level notepad, for the keyword has a distinctive color tag, very intuitive.
• ③main is not a keyword, but it cannot be used when defining identifiers.

2 Identifier 2.1 Identifier overview

• A sequence of characters that name a class, interface, method, variable, etc.

2.2 Composition Rules

• English uppercase and lowercase letters
• numeric characters
• $ and _

2.3 Precautions

• Cannot start with a number
• Cannot be a keyword in Java
• Case sensitive

2.4 Common naming conventions in identifiers

• Package: is actually a folder, used to solve the problem of the same name, named All lowercase.
  • Single-stage: Xuweiwei
  • Multilevel: Com.xuweiwei
• Class or interface
  • One word: The first letter of the word must be capitalized. such as Hello.
  • Multiple words: The first letter of each word must be capitalized. such as HelloWorld.
• Methods and variables
  • One word: The first letter of the word is lowercase. such as show.
  • Multiple words: Starting with the second word, capitalize the first letter of each word. such as Showallnames.
• Constant
  • One word: all uppercase. such as Pi.
  • Multiple words: Each letter is capitalized, separated by _. such as Student_max_age.

3 Notes 3.1 Comments Overview

• The text used to interpret the description program.

3.2 The classification format of annotations in Java

• Single-line Comment

// Comment Text

• Multi-line comments

/* * Comment Text */

• Document comments

/** * Documentation Comments */

3.3 Annotated version of HelloWorld

 PackageCom.xuweiwei;/*** Requirements: Write a Java program, the "HelloWorld" This sentence output to the console * Analysis: *① Write a Java program, you must first define a class *② the data can be output, indicating that our program can run independently, and the program to run independently, you must define the M Ain method *③ Data output to the console, you must use the OUTPUT statement * implementation: The *①java language provides a keyword: class, used to define classes, followed by the class name *②main method format is fixed: * Public s The format of the tatic void Main (string[] args) {} *③ output statement is fixed: * SYSTEM.OUT.PRINTLN ("Hello world!!"); *//*** This is my HelloWorld case*/ Public classHelloWorld {/*** In order for the program to run independently, the main method must be defined * The Main method is the program's entry * is automatically recognized and invoked by the JVM *@paramargs*/     Public Static voidMain (string[] args) {//in order to display the data in the console, we used the output statementSystem.out.println ("Hello world!!"); }}

4 Constants and Variables 4.1 constants 4.1.1 Overview of constants

• Its value cannot be changed during program execution

4.1.2 Classification of constants in Java

• Literal constants
• Custom Constants

4.1.3 Literal constants

• String constants
  • Content enclosed in double quotation marks
  • such as: "Hello ah"
• Integer constants
  • All integers
  • such as: 1, 2, 3 ...
• Decimal constants
  • All decimals
  • Example: 13.15
• Character constants
  • Content enclosed in single quotation marks
  • such as: ' A '
• Boolean Constants
  • More special, only true and false
• Empty constants
  • Null

• Example:

 PackageCom.xuweiwei;/*** Constant: The amount of value that cannot be changed during the execution of a program.  * Categories of constants: * Literal Constants * Custom Constants * Literal constants: *① string constants are enclosed in double quotes * such as: "Helloworld" *② integer constant all integers * such as: 100,200 *③ decimal constant All decimals * such as: 10.23 *④ character constants are enclosed in single quotes * For example: ' A ' *⑤ Boolean constant is more special * such as: TRUE or false *⑥ NULL constant * such as: null * * 
    */ Public classConstantdemo { Public Static voidMain (string[] args) {//output of string constantsSystem.out.println ("Who Are You?")); //output of integer constantsSYSTEM.OUT.PRINTLN (12); //Decimal ConstantsSystem.out.println (12.3); //Character ConstantsSystem.out.println (' 1 '); //Boolean ConstantsSystem.out.println (true); //Empty ConstantsString str =NULL;    System.out.println (str); }}

4.2 An overview of variable 4.2.1 variables

• The amount at which the value can change within a range during program execution.

4.2.2 The definition format of a variable

Variable name of data type = initialization value;

• In essence, a variable is actually a small area of memory, using variable names to access the area, so each variable must be declared before it is used, and then assigned before it can be used.
• Why do you define variables, which are mainly used to store constant constants that hold uniform types, and can be reused.

4.2.3 Data types

• The Java language is a strongly typed language that defines specific data types for each data and allocates different sizes of memory space in memory.

• Data type
  • Basic data types
    • Numeric type
      • Integer type (byte, short, int, long)
      • Floating-point type (float, double)
    • Boolean Type (Boolean)
    • Character type (char)
  • Reference data type
    • Class
    • Array
    • Interface

4.2.4 issues to be aware of using variables

• ① Scope
  • The variable is defined within the curly brace, which is valid within the curly brace.
  • Also, you cannot define two identical variables within the same curly brace.
• ② Initialization value
  • No initialization values can be used directly.
• ③ on a single line, it is recommended to define only one variable
  • You can define multiple, but not recommended.

4.2.5 Data type Conversions

• The Boolean type cannot be converted to another data type
• Automatic type conversion
  • Byte, short, char-->int-->long-->float-->double
  • Byte, short, and Char are converted to the int type at the time of calculation.
• Forcing type conversions
  • Target type variable name = (target type) (data to be converted);

• Example:

 PackageCom.xuweiwei; Public classDataType { Public Static voidMain (string[] args) {/*** Default type conversion*/        intA = 3; byteb = 4; intc = A +b;        System.out.println (c); /*** Forced type conversion*/        byteD = (byte) (A +b);    System.out.println (d); }}

Java Language Foundation II