in case of system defined exceptions,if any violation occured while executing the code,it will create a exception object and throw that object.

in catch block u catch it and give appropriate message.

userdefined exception:in case of user defined exceptions,create a class as a subclass of java.lang.Throwable and implement   toString() method in your class.

Example:

public class UserException extends java.lang.Throwable{

private String ex;

public UserException()

{

}

public UserException(String s)

{
    ex=s;

}

public String toString()

{

return ex;

}

}

public class App{

public static void main(String args[])

{

//take the value from key board   if it is equal to exception generate user defined exception

InputStreamReader isr=new InputStreamReader(System.in);

BufferedReader br=new BufferedReader(isr);

try{

if(br.readLine()=="exception"){

    UserExceptin ue=new User Exception("this is the user defined exception");

throw ue;

}

}

catch(Exception e)

{
System.out.println(e);

}

}