Learn java together [3] --- all preparations in the Early Stage

Java SE :( java Standard Edition)
Java ME; (java mobile Edition)
Java EE; (java Enterprise Edition)
This is three versions of java, or three systems called java;
Note that the first version is the most basic version. No matter which path you take, you will learn the first version, after learning the first version, you can select either the m version or the E version.
 
Java was launched by Sun, and the founder of java was the old man named "high commander". At the beginning of this year, Sun was acquired by Oracle. The purchase price is USD 7.4 billion;
 
The three versions are J2SE at the beginning, and there are two in them;
 
JDK: Java Development Kit (java Development Kit;) [This is a required thing ;]
JRE: Java Runtime Environment (java execution Environment or java Runtime Environment );
 
JDK contains JRE. (The java program must be able to compile after it runs. Therefore, it is not only JRE, but JDK .)
 
Platform: English translation as a platform;
 
Java SE has several versions: jdk1.4 and jdk1.5 (5.0). [these two versions mean one.] And jdk1.6 (6.0). The current maximum version is 7.0;
 
Aliases of two versions:
Jdk1.5 (5.0): Tiger, Tiger;
Jdk1.6 (6.0): mustang, mustang;
 
Javac.exe: compile;
Java.exe: execution;
 
The next step is to modify the path variable value and copy the bin path of the jdk we installed to the path to ensure that our java program files can be compiled and executed on the computer.
 
Download jDK, install jdK, and directly install JDK. Set the environment variables (User variables or system variables) and point to the bin directory in the jDK installation directory, enter cmd, open the command line window, enter java-version, and display the java version information. If it is in place, the preparation is complete, then you can write a java program.
 
You can use NotePad to write java programs, advanced text editing tools like Editplus and ultraEdit to write java programs, and professional IDE (Integrated Development Environment.
 
Finally, we use NotePad to implement a Hello World java program.
 
All java code and suffix names end with java.
 
The class file is a bytecode file, and the program will eventually execute this bytecode file (bytecode)
 
Compiled command line program: javac Test. java;
After compilation is successful, a class file is automatically generated;
Then execute: java Test (note that there is no suffix later .)
HelloWorld is output in the command line.
 
The compiled class file name is consistent with the class name defined in java.
 
Java is a cross-platform language. It does not actually execute binary code, but bytecode.
 
JVM (Java Virtual Machine) is not cross-platform and is written in C language. The reason why java can achieve cross-platform is that JVM is not cross-platform.
Author: "Li Yuan Cao"