Threads - Beginning Java - page 789

The class implements Callable<int[]> because a task returns an array of values of type int . The con-
structor expects four arguments: the type of transactions to be generated, DEBIT or CREDIT , the maximum
number of transactions to be created, a reference to a Vector<Account> containing the accounts, and a
reference to a Vector<Clerk> containing the clerks. The constructor just stores these in the object and
creates an array of the appropriate size to store the total value of the transactions created for each ac-
count.
The call() method returns a reference of type int[] , which is a reference to the array of totals. The for
loop generates the transactions in much the same way as the previous example. Having created a transac-
tion, the method tries to assign the transaction to a clerk in an indefinite while loop. It searches through
the clerks vector to find a clerk that can accept a transaction; you implement the doTransaction()
method for a Clerk object to return true if the transaction can be accepted and false otherwise. If a
clerk accepts the transaction, the indefinite while loop is terminated and the outer for loop continues.
If no clerk is free, the method calls sleep() to wait a while before trying again to see if a clerk has be-
come free. This continues until a clerk does become free or until the thread is interrupted.
On each iteration of the outer for loop that generates transactions, you call the static interrupted()
method for the Thread class to allow the possibility for the thread to be canceled externally. If you don't
do this, the only possibility for canceling the thread is while it is sleeping. When the call() method
ends, it returns a reference to the totals array.
Defining a Clerk
Each clerk also runs as a separate thread, but because a clerk does not need to return a value, you can
define it as a Runnable class:

import java.util.List;
import java.util.Collections;
import java.util.LinkedList;

public class Clerk implements Runnable {
// Constructor
public Clerk(int ID, Bank theBank) {
this.ID = ID;
this.theBank = theBank; // Who the clerk works for

}

// Receive a transaction
synchronized public boolean doTransaction(Transaction transaction) {
if(inTray.size() >= maxTransactions)
return false;
inTray.add(transaction); // Add transaction to the list

return true;

}

// The working clerk...
public void run() {
while(true) {
while(inTray.size() == 0) { // No transaction waiting?