程序名：MyFirstJavaProgram.java 

//package com.example;

public class MyFirstJavaProgram {
public static void main(String[] args){
System.out.println("Hello World!");
}
}

cmd下编译运行

D:\javaprogram>javac MyFirstJavaProgram.java

D:\javaprogram>java MyFirstJavaProgram
Hello World!

第一个java小程序