Principle
The name of the System class comes from the concept in the operating system, which represents the environment and resources of the entire computer system. The System class in Java provides methods to access and operate system-related functions and properties.

Java's System class is provided by the Java core library, which uses native code to implement the functions of interacting with the underlying operating system. These native methods call specific functions of the underlying operating system through the Java Native Interface (JNI).

The principles of the System class can be divided into the following aspects:

Standard input, output, and error streams: The System class provides access to and operations on the standard input stream (System.in), standard output stream (System.out), and standard error stream (System.err). These streams are associated with the input and output streams of the underlying operating system.
System properties: The System class provides methods for getting and setting system properties, such as getProperty() and setProperty(). System properties are configuration information in the form of key-value pairs, which can be used to get and set system-related information, such as operating system type, file path separator, etc.
Local library loading and mapping: The System class provides methods for loading local libraries and mapping local libraries, such as load() and loadLibrary(). Native libraries are dynamic link libraries (DLLs or shared libraries) written in other programming languages (such as C, C++), and functions in them can be called in Java.
Garbage collector and memory management: The System class provides a method gc() to run the garbage collector, which is used to try to recycle unused objects to free up memory. In addition, the System class also provides some methods related to memory management, such as arraycopy() and identityHashCode().
Control of the Java virtual machine: The System class provides some methods related to the control of the Java virtual machine, such as exit() and runFinalization(). These methods allow the program to terminate the virtual machine or perform some necessary cleanup operations under certain conditions.

By calling the static methods of the System class and accessing its constants, Java programs can directly interact and configure the underlying operating system. This allows developers to operate and control the execution environment of the program more flexibly.

In short, the System class is a key class in the Java core library, providing functions and properties related to system operation and configuration. It implements interaction with the underlying operating system through native code and provides developers with a convenient interface through static methods and constants.

Example

Here is an example use of a common method of the System class:

1. Standard input, output, and error streams:

// Read user input from standard input
Scanner scanner = new Scanner(System.in);
System.out.print(" Please enter your name: ");
String name = scanner.nextLine();
System.out.println(" Hello, "+ name);

// Print information to standard output
System.out.println(" This is an output message ");

// Prints an error message to the standard error stream
System.err.println(" An error has occurred ");

2.System properties:

// Gets the value of the specified system property
String javaVersion = System.getProperty("java.version");
System.out.println("Java version: "+ javaVersion);

// Set the value of the system property
System.setProperty("user.dir", "/path/to/directory");

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