Self-study notes-Introduction to Java 1

Computer architecture

Computer = Hardware + Software = Hardware + [ System Software + Application Software ]

System software = Kernel + shell

Java language overview

After Java1.2, it is divided into 3 versions:

• JavaSE : We often say that the foundation of Java, including the operation of the infrastructure required environment Java core libraries +
• JavaEE : Built on top of JavaSE, used to build enterprise-level applications
• JavaME : With the popularity of the Android platform, JavaME has been eliminated

Development environment

1. Download and install. Here we install Java11 (LTS)

   

2. Directory structure of Java 11:

   

3. The difference between JDK, JRE, and JVM:

   JDK: Java development kit for development

   JRE: Java operating environment

   JVM: Java virtual machine, used to interpret and execute bytecode files (class files). Moreover, the cross-platform features of Java are inseparable from the JVM (each system has a corresponding JVM)

   

   JRE = Java class library + Java virtual machine

   JDK = JRE + compiler and other development tools

4. Javac.exe Japanese Java.exe

   javac.exe: Compiler , translate Java source code into bytecode file (.class)

   java.exe: Interpreter , mainly used to start JVM to interpret, interpret and execute bytecode files

The cross-platform principle of Java

What is cross-platform? It means that the same bytecode file can be run on multiple different systems such as Windows and Linux.

The cross-platform nature of Java is inseparable from the existence of the Java virtual machine.

The cross-platform of Java does not mean that the Java language has cross-platform functions, but that there are different JVMs for different platforms to run.

The va language has cross-platform functions, but for different platforms there are different JVMs to let it run.