[Java] Object,System,Runtime, Process

1. Package
   java.lang.
   
2. Object
   It is the root base class of java classes hierarchy.
   
3. Class
   It is in-memory representation of Java class. This representation serves as the starting point for querying class-felated information (such as modifier(static, public\private\protected, abstract, final), field, method).
   
4. System
   
   • Operate input stream, output stream, error output stream
   • Get environment
   • Get/Set system properties
     The system properties are key-value pairs. The default properties provided ty VM are:
     

             java.vendor.url
             java.class.path
             user.home
             java.class.version
             os.version
             java.vendor
             user.dir
             user.timezone
             path.separator
             os.name
             os.arch
             line.separator
             file.separator
             user.name
             java.version
             java.home

     
     
   • Load library
   • Copy array
   • Get current time
   • Inform VM to run garbage collection at some good time
   • Inform to exit VM and stop program
     
5. Runtime
   
   • Add VM shutdown hook
   • Load library
   • Memory
     
     1. Get maximum memory
     2. Get free/total memory available for the running program
   • Execute the specified program in the seperate process
   • Inform to stop VM
   • Inform to start garbage collection
     
6. xxx