Road Java based learning, - notes day1

Road JavaJ based learning, day1

A .java language background
1. In 1995 sun has introduced computer language, in 2009 was acquired by ORACLE Oracle Corporation.
Two, JRE, JDK, JVM
developed in three steps:
1. Write code
2. Compile the code javac file name .java
3. Run the code java file name

1.JRE: JRE is java runtime environment (Java Runtime Environment)
contains JVM java virtual machine and core class libraries.
Java core libraries: very central code repository
class: a = a java file class
library: store multiple java file repository
2.JDK (Java Develop Kit) in Java, software development tools, including the compiler and runtime tools to .
3.JVM is an acronym for Java Virtual Machine (Java virtual machine), JVM itself does not allow cross-platform, cross-platform that allows Java programs.
Cross-platform Java language is because with a different versions on different operating systems Java virtual machine, the virtual machine can recognize the Java bytecode.

Three, Path environment variable configuration

Computer - Right - Properties - Advanced System Settings - environment variables
in the system variable, select New

configuration% JAVA_HOME% \ bin in your path; the best place it on the front, to avoid the emergence of javac and java version different issues

Fourth, programming

//class定义一个类，后面跟类名，通过public对类名进行限制，限制类名和文件名必须保持一致
public class HelloWorld{
//main作为程序的入口点，main方法，也叫主方法，如果没有编写主方法，程序就找不到入口点，编译会出现错误
	public static void main(String[] args){
	//可以在控制台打印双引号包裹的内容，要打印多行内容，就编写多条语句
		System.out.println("HelloWorld");
	}
		
}

System.out.println("");双引号中间可以为空；	System.out.println(‘’);单引号中间不能为空报错；
System.out.println();具有换行的效果

public class ConstantDemo{
	/*
		常量：在程序执行过程中是不会发生变化的量
		常量分类：
			字符串常量：被双引号包裹的内容都是字符串常量
			字符常量：被单引号包裹的内容，注意：单引号只能存放一个字符
			整数常量：所以整数
			小数常量：所以小数
			布尔常量：true false
			空常量：null
			
	
	*/
		public static void main(String[] args){
			//字符串常量
			System.out.println("HelloWorld!!");
			System.out.println("---------");
			
			//整数常量
			System.out.println(678);
			System.out.println("----------");
			
			//小数常量
			System.out.println(1.154);
			System.out.println("---------");
			
			//字符常量
			System.out.println('A');
			System.out.println("--------");
			
			//布尔常量
			System.out.println(true);
			System.out.println(false);
			
			//空常量 是不能直接输出的
			//Syestem,out.println(null)
		}