Java Runtime class

Java Runtime class is used to interact with java runtime environment. Java Runtime class provides methods to execute a process, invoke GC, get total and free memory etc. There is only one instance of java.lang.Runtime class is available for one java application.
The Runtime.getRuntime() method returns the singleton instance of Runtime class.

How to shutdown system in Java

You can use shutdown -s command to shutdown system. For windows OS, you need to provide full path of shutdown command e.g. c:\\Windows\\System32\\shutdown.
Here you can use -s switch to shutdown system, -r switch to restart system and -t switch to specify time delay.

public class Runtime2{  
 public static void main(String args[])throws Exception{  
  Runtime.getRuntime().exec("shutdown -s -t 0");  
 }  
}  

How to shutdown windows system in Java

public class Runtime2{  
 public static void main(String args[])throws Exception{  
  Runtime.getRuntime().exec("c:\\Windows\\System32\\shutdown -s -t 0");  
 }  
}  

How to restart system in Java

public class Runtime3{  
 public static void main(String args[])throws Exception{  
  Runtime.getRuntime().exec("shutdown -r -t 0");  
 }  
}  

@reference_1_javatpoint.com
Java Runtime class