How can you generate your own events?

[Anonymous]

Hi all ,
I'm trying to generate a custom event, like your acount balance goes below certain limit, then, it will generate an event.
I'll just paste the code below, I guess what i did is quite fair, but couldnt work out.
If anyone can help me ?
///// INTERFACE BALANCELISTENER.JAVA
package event;
import java.util.*;
import event.BankEvent;
public interface BalanceListener extends EventListener
{
public void lowFundEvent(BankEvent be);
}

///// BANKEVENT.JAVA
package event;
import java.util.*;
import event.Account;
public class BankEvent extends EventObject
{
public BankEvent(Account acc)
{
super(acc);
}
}
//// CLASS ACCOUNT.JAVA
package event;
import event.BalanceListener;
import event.BankEvent;
public class Account
{
private int acctId;
private int balance;

public Account(int acctId,int balance)
{
this.acctId = acctId;
this.balance = balance;
}
public void addBalanceListener(BalanceListener bl)
{
if(this.balance < 5000)
{
bl.lowFundEvent(new BankEvent(this));
}
}
public void deductAmount(int amount)
{
balance = balance - amount;
System.out.println("Account : deductAmount : "+balance);
}
}
////// TRANSACTION.JAVA
package event;
import java.util.*;
import event.*;
public class Transaction implements BalanceListener
{
private Account ac1;

public Transaction()
{
ac1 = new Account(11,10000);
ac1.deductAmount(6000);
ac1.addBalanceListener(this);
}
public static void main(String[] args)
{
Transaction tr = new Transaction();
}
public void lowFundEvent(BankEvent be)
{
System.out.println(be.getSource().getClass().getName());
System.out.println("Event is raised");
}
}

[Manfred Leonhardt]

Hi Vikram!
You are on the right track. Your interface seems ok. However your other three classes need some changes to work better.
Lets start with the BankEvent class. You need to include more information into this class because you have not named it lowBalanceEvent. Therefore, the easiest thing to do is to include a String explaining what in the bank went wrong.

///// BANKEVENT.JAVA
public class BankEvent extends EventObject
{
  String msg = null;// Message so target knows what happened.
  //
  // Constructor for class
  //
  public BankEvent( Account acc, String msg )
  {
    super(acc);
    this.msg = msg;
  }
  //
  // Return meaningful message.
  //
  public String getMessage()
  {
    return( msg );
  }
}

Next we turn to your Account class. This one needs a few more methods and the balance check should be moved to the deduct method because that is where it is important. The methods addBalanceListener and removeBalanceListener should not really do any work (as you have tried to do in your addBalanceListener). They should just add or remove from the listener vector member variable so we can keep track of who is listening.

//// CLASS ACCOUNT.JAVA
public class Account
{
  private int acctId;
  private int balance;
  private Vector listeners = new Vector();
  //
  // Constructor for class
  //
  public Account( int acctId, int balance )
  {
    this.acctId = acctId;
    this.balance = balance;
  }
  //
  // Add balance listeners
  // 
  public void addBalanceListener(BalanceListener bl)
  {
    if( !listeners.contains( bl ) )
    {
      listeners.addElement( bl );
    }
  }
  //
  // Remove balance listeners
  //
  public void removeBalanceListener( BalanceListener bl )
  {
    if( listeners.contains( bl ) )
    {
      listeners.removeElement( bl );
    }
  }
  //
  // Fire bank events for problem balances
  //
  public void notifyBalanceListeners()
  {
    Vector balListeners = (Vector)listeners.clone();
    String s = "Low Balance: $" + balance + " found in account: " + acctId;
    BankEvent be = new BankEvent( this, s );
    for( int i = 0; i < balListeners.size(); ++i )
    {
      BalanceListener bl = null;
      bl = (BalanceListener)balListeners.elementAt( i );
      bl.lowFundEvent( be );
    }
  }
  //
  // Deduct amount from account. Check balance once completed.
  //
  public void deductAmount(int amount)
  {
    balance = balance - amount;
    System.out.println( "Account - deductAmount = " + balance );
    //
    // Call balance listeners if balance is to low.
    //
    if( balance < 4000 )
    {
      notifyBalanceListeners();
    }
  }
}

And finally the Transaction class which sets up the account and drains it to a low level. You should add the listener before we make any withdrawals since that is the purpose. Also the lowFundEvent method gets simpler because of the message String that BankEvent now contains.

////// TRANSACTION.JAVA
public class Transaction implements BalanceListener
{
  private Account ac1;
  //
  // Constructor method.
  //
  public Transaction()
  {
    System.out.println( "Starting Account with $10K" );
    ac1 = new Account( 11, 10000 );
    //
    // Adding transaction as balance listener
    //
    ac1.addBalanceListener(this);
    System.out.println( "Removing $6001 from account..." );
    ac1.deductAmount( 6001 );
  }
  //
  // Interface method implemented
  //
  public void lowFundEvent( BankEvent be )
  {
    System.out.println( be.getMessage() );
  }
  //
  // Main program here!
  //
  public static void main( String[] args ) 
  {
    Transaction tr = new Transaction();
  }
}

With the changes made above the app works pretty well!
Good Luck,
Manfred.