Principle of Java language cross platform

JVM

• The Java code we write runs on the JVM virtual machine. The Java virtual machine itself does not have cross platform function, but there are different versions of virtual machines under each operating system, so as to realize cross platform
  

JRE

• The runtime environment of Java programs, including the JVM and the core class libraries required by the runtime

JDK

• Java program development kit, including tools used by JRE and developers
  

JDK download and installation

Download address

• https://www.jdkdownload.com/

Configure environment variables

• Properties - > advanced system settings - > environment variables - > system variables

• New JAVA_HOME

• Add% java to PATH_ HOME%\bin

• If the display is as follows, the configuration is successful

  

Helloworld

Write HelloWorld java

public class HelloWorld {
  	public static void main(String[] args) {
    	System.out.println("HelloWorld");
  	}
}

Compile to helloworld.com Class source file

javac helloworld.java

Run program

java helloworld

View effect

Frequently asked questions about coding

• Misspelled words
• Not strictly case sensitive
• Punctuation marks must be in Chinese
• Parentheses must appear in pairs
• Character encoding problem

Do you want the source file and class name to be consistent?

• If the class is not public, the source file name can be inconsistent with the class name
• If the class is public, the source file name must be consistent with the class name

Can there be multiple classes in the source file?

• There can be multiple classes in a source file, which will be generated after compilation class bytecode file
• However, a source file can only have one public class

Must main be in the public class?

• No, but it is customary to write in public

notes

Single-Line Comments

// Note Content 

multiline comment

/*
	Note Content 
*/

keyword

• There are 50 keywords in total, of which const and go are reserved words

• true, false and null are not keywords

  

constant

definition

• The amount whose value cannot be changed during program execution

Example

public class ConstantDemo {
	public static void main(String[] args) {
		//string constant 
		System.out.println("HelloWorld");
		
		//integer constant 
		System.out.println(12);
		System.out.println(-23);
		
		//Decimal constant
		System.out.println(12.34);
		
		//character constants 
		System.out.println('a');
		System.out.println('0');
        System.out.println('sand');
		
		//Boolean Literals 
		System.out.println(true);
		System.out.println(false);
	}
}

identifier

Naming rules

• Only 26 English letters, uppercase and lowercase, 0-9 numbers, underscores, Dollar sign$
• Java keywords (including reserved words) and special values cannot be used
• The number cannot begin
• Cannot contain spaces
• Strictly case sensitive

Naming conventions

• See the name and know the meaning

• Class name, interface name, etc.: the first letter of each word is capitalized

• Variable, method name, etc.: start from the second word with the first letter in uppercase and the rest in lowercase

• Package name, etc.: each word is lowercase, and points are used between words division

• Constant name, etc.: each word is capitalized and underlined between words_ division

Basic data type

data type

• Basic data type

  Integer, floating point, character, Boolean

• Reference data type

  Class, array, interface

Four categories and eight data types

variable

definition

• The amount whose value can change during program execution

Precautions when using variables

• Declaration before use
• Must be initialized before use
• Variable has scope
• Variables in the same scope cannot have the same name

Example

public class VariableDemo {
	public static void main(String[] args){
        // Define byte type variables
        byte b = 100;
        System.out.println(b);
        // Define short integer variables
        short s = 1000;
        System.out.println(s);
        // Defining integer variables
        int i = 123456;
        System.out.println(i);
        // Define long integer variables
        long l = 12345678900L;
        System.out.println(l);
        // Define single precision floating point variables
        float f = 5.5F;
        System.out.println(f);
        // Define double precision floating point variables
        double d = 8.5;
        System.out.println(d);
        // Defining Boolean variables
        boolean bool = false;
        System.out.println(bool);
        // Define character variables
        char c = 'A';
        System.out.println(c);
        
        // Define string variables
        String s = "HelloWorld";
        System.out.println(s);
	}
}