Threads - Beginning Java 8 Language Features - page 181

Another thread got CPU time to print from 1 to 26 before it was preempted. The second time, the first thread (the thread
that started printing integers first) got the CPU time and it printed only two integers, 6 and 7, and so on. You can see
that both threads got CPU time. However, the amount of CPU time and the sequence in which they got the CPU time are
unpredictable. Each time you run this program, you may get a different output. The only guarantee that you get from
this program is that all integers between 1 and 500 will be printed twice in some order.
Issues in Using Multiple Threads
Some issues are involved when you use multiple threads in a program. You need to consider these issues only if
multiple threads have to coordinate based on some conditions or some shared resources.
In the previous sections, the examples involving threads were trivial. They simply printed some integers on the
standard output. Let's have a different kind of example that uses multiple threads, which access and modify the value
of a variable. Listing 6-4 has the code for a BalanceUpdate class. Note that all methods defined in the BalanceUpdate
class are static .
Listing 6-4. Multiple Threads Modifying the Same Variable
// BalanceUpdate.java
package com.jdojo.threads;
public class BalanceUpdate {
// Initialize balance to 100
private static int balance = 100;
public static void main(String[] args) {
startBalanceUpdateThread(); // Thread to update the balance value
startBalanceMonitorThread(); // Thread to monitor the balance value
}
public static void updateBalance() {
// Add 10 to balance and subtract 10 from balance
balance = balance + 10;
balance = balance - 10;
}
public static void monitorBalance() {
int b = balance;
if (b != 100) {
System.out.println("Balance changed: " + b);
System.exit(1); // Exit the program
}
}
public static void startBalanceUpdateThread() {
// Start a new thread that calls the updateBalance() method in an infinite loop
Thread t = new Thread(() -> {
while (true) {
updateBalance();
}
});
t.start();
}