A Programmer[ap]s Guide to Java Certification

Given the following program, which statements are guaranteed to be true?

public class ThreadedPrint { static Thread makeThread(final String id, boolean daemon) { Thread t = new Thread(id) { public void run() { System.out.println(id); } }; t.setDaemon(daemon); t.start(); return t; } public static void main(String[] args) { Thread a = makeThread("A", false); Thread b = makeThread("B", true); System.out.print("End\n"); } }

Select the two correct answers.

1. The letter A is always printed.

2. The letter B is always printed.

3. The letter A is never printed after End .

4. The letter B is never printed after End .

5. The program might print B , End and A, in that order.

Which statement is true?

Select the one correct answer.

1. No two threads can concurrently execute synchronized methods on the same object.

2. Methods declared synchronized should not be recursive, since the object lock will not allow new invocations of the method.

3. Synchronized methods can only call other synchronized methods directly.

4. Inside a synchronized method, one can assume that no other threads are currently executing any other methods in the same class.

Given the following program, which statement is true?

public class MyClass extends Thread { static Object lock1 = new Object(); static Object lock2 = new Object(); static volatile int i1, i2, j1, j2, k1, k2; public void run() { while (true) { doit(); check(); } } void doit() { synchronized(lock1) { i1++; } j1++; synchronized(lock2) { k1++; k2++; } j2++; synchronized(lock1) { i2++; } } void check() { if (i1 != i2) System.out.println("i"); if (j1 != j2) System.out.println("j"); if (k1 != k2) System.out.println("k"); } public static void main(String[] args) { new MyClass().start(); new MyClass().start(); } }

Select the one correct answer.

1. The program will fail to compile.

2. One cannot be certain whether any of the letters i , j , and k will be printed during execution.

3. One can be certain that none of the letters i , j , and k will ever be printed during execution.

4. One can be certain that the letters i and k will never be printed during execution.

5. One can be certain that the letter k will never be printed during execution.