How to Write Userdefined Exceptions

Code For Balance Class
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//  Demo class for User-Defined Exceptions To Ensure The
//  Validity of Data Contained In A Class

import java.io.*;

public class Balance
{
   int bal;
   public Balance(){
   }

   public void setBalance( int amt ){
      this.bal = amt;
   }

   public void withdraw( int amt )
    throws NoSufficientFundException
   {
      if (bal >= amt) {
         bal = bal - amt;
      }
      else  {
         throw new NoSufficientFundException("Balance is less than what u
expect..some error

message..");
      }
   }

   public void getBalance()
   {
       return bal;
   }
}

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Code For NoSufficientFundException
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//  Exception Class For No Sufficient Fund(userdefined one)

public class NoSufficientFundException extends Exception
{
    String strValue;

	public PositionException( String value) {
         this.strValue = value;
	}

	public String toString() {
	   return "Exception occurred.. " + strValue;
        }
}

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Code For ( Test)Main Program
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// Test Program For Class Balance
// Dealing With The Exceptions That It Throws

import java.io.*;

public class TestBalance
{
   static public void main( String[] args ) {
      Balance obj = new Balance();

      try {
         obj.setBalance(100);

         //some code....

         obj.withdraw(150); //here the amount we specified is greater than
the actual balance

         obj.getBalance();

      }
      catch( NoSufficientFundException e )  {
	     System.out.println(e);
      } //end of catch
   } //end of main
}//end of TestBalance