parent child relation checked exceptions issues in overriding

//In Exception Hierarchy IOException super class,FileNotFoundException subclass .
/* While overriding the size of the CheckedException 
should not increase */
DEMO 1:
package oops;

import java.io.FileNotFoundException;
import java.io.IOException;

class Parentclass
{
    public void sm()throws IOException
    {
    System.out.println("super class");
    }
}  
public class OREx extends Parentclass{
    
    public void sm()throws FileNotFoundException  // no error
    {    
        System.out.println("sub class");
    }
    public static void main(String a[])throws Exception
       {    
        OREx s=new OREx();
        s.sm();      // sub class
       }
}

Demo 2:
package oops;

import java.io.FileNotFoundException;
import java.io.IOException;

class Parentclass
{
    public void sm()throws FileNotFoundException
    {
    System.out.println("super class");
    }
}  
public class OREx extends Parentclass{
    
    public void sm()throws IOException // compile time error
    // Exception IOException is not compatible with throws clause
    {    
        System.out.println("sub class");
    }
    public static void main(String a[])throws Exception
       {    
        OREx s=new OREx();
        s.sm();      
       }
}

DEMO 3:
package oops;

import java.io.FileNotFoundException;
import java.io.IOException;

class Parentclass
{
    public void sm()throws FileNotFoundException
    {
    System.out.println("super class");
    }
}  
public class OREx extends Parentclass{
    
    public void sm()throws RuntimeException // no error
    {    
        System.out.println("sub class");
    }
    public static void main(String a[])throws Exception
       {    
        OREx s=new OREx();
        s.sm();      // sub class
       }
}