Why RuntimeException Legal here? :   
     import java.io.*;
     import java.sql.*;
     import java.io.*;
     import java.sql.*;
     class Vehicle{
	    public void park()throws IOException{//compiletime checked
		
	    }
     }
     class Car extends Vehicle{
	     public void park()throws RuntimeException{//compile time unchecked
		     //
	     }	
     }
  	 
why compile time unchecked exceptions legal here.sdfsdfsdfdsfddsfdsfds