Translation program: refers to the process of translating high-level language into machine language.

Preliminary knowledge:
The computer's hierarchical structure can be roughly divided into five layers: high-level language, assembly language, operating system, machine language, and micro instruction system.

1. Translation programs can be divided into two situations: compiler and interpreter
   

2. Compiler
   The so-called compiling program refers to compiling the "source program" into "machine language" at one time, that is, "executable file". After the compilation is complete, execute the file. For example, C language programs are all compiled into machine language before execution. (See the compilation process of C language: preprocessing-compiling-assembly-link-loading)

3. Explain execution
   The so-called interpretation and execution means that for the "source program", a sentence is translated into machine language, and the sentence is executed immediately. After the execution is completed, the next sentence is translated and executed. Sequentially: Translate a statement and execute a statement immediately. For example Python

4. Special case: JAVA, compile once, execute everywhere
   JAVA is not exactly the same as the above two situations. Java introduces the concept of bytecode files (.class) and JAVA virtual machine (JVM). In detail, it is:Java is similar to the C language that compiles the source program at one time, but the C program directly compiles the source program into machine language, and the java program is compiled into a bytecode file (.class). Subsequently, the java bytecode file is interpreted as machine language on the machine loaded with JVM, and then executed.
   

The Java compiler compiles the .java file into bytecode (.class file). The code in the class file is an intermediate code between the source program and the machine code. The bytecode cannot be anything except JVM. The platform understands that when the JVM receives the bytecode, it will identify the platform it is working on, and then convert the bytecode into native machine code.

To sum up: the compilation process of java is equivalent to adding a link in the compilation process of c, that is, from "source program -> machine language" to "source program -> bytecode file -> machine language", The bytecode file can only be recognized and understood by the JVM. Therefore, this new process makes java not as fast as C/C++, but the new "Cross-platform", as long as there is a jvm machine can execute bytecode files, that is, "Compile once, execute everywhere”。