public class ThreadDemo {
synchronized void a() { actBusy(); }
static synchronized void b() { actBusy(); }
static void actBusy() {
try { Thread.sleep(1000); }
catch (InterruptedException e) {}
}
public static void main(String[] args) {
final ThreadDemo x = new ThreadDemo();
final ThreadDemo y = new ThreadDemo();
Runnable runnable = new Runnable() {
public void run() {
int option = (int) (Math.random() * 4);
switch (option) {
case 0: x.a(); break;
case 1: x.b(); break;
case 2: y.a(); break;
case 3: y.b(); break;
} }
};
Thread thread1 = new Thread(runnable);
Thread thread2 = new Thread(runnable);
thread1.start();
thread2.start();
} }
Self
Test Answers 753
754 Chapter 9: Threads
Which of the following pairs of method invocations could NEVER be executing at the same
time? (Choose all that apply.)
A. x.a() in thread1, and x.a() in thread2
B. x.a() in thread1, and x.b() in thread2
C. x.a() in thread1, and y.a() in thread2
D. x.a() in thread1, and y.b() in thread2
E. x.b() in thread1, and x.a() in thread2
F. x.b() in thread1, and x.b() in thread2
G. x.b() in thread1, and y.a() in thread2
H. x.b() in thread1, and y.b() in thread2
Answer:
A , F and H. A is incorrect because synchronized instance methods called on the same
instance, block each other. F and H could not happen because synchronized static
methods in the same class block each other, regardless of which instance was used to call
the methods. (An instance is not required to call static methods; only the class.)
�˚ C could happen because synchronized instance methods called on different instances
do not block each other. B, D, E, and G could all happen because instance methods and
static methods lock on different objects, and do not block each other.
Please read this care fully, this example is from the K & B book, chapter-9, example-15. I think it will help you.
