Question

Given:

public class TestSeven extends Thread {

private static int x;

public synchronized void doThings() {
    int current = x;
    current++;
    x = current;
}

public void run() {
    doThings();
  }
}

Which statement is true?

A. Compilation fails.

B. An exception is thrown at runtime.

C. Synchronizing the run() method would make the class thread-safe.

D. The data in variable “x” are protected from concurrent access problems.

E. Declaring the doThings() method as static would make the class thread-safe.

F. Wrapping the statements within doThings() in a synchronized(new Object()) { } block would make the class thread-safe.

isn’t it enough to mark doThings() as synchronized in order to make that class Thread-safe ? i see that the correct answer is D but the Model answer of this question is E, But i don’t understand why?

Answer 1

E. Declaring the doThings() method as static would make the class thread-safe.

That is kind of a tricky answer. The method is already synchronized, but on the instance, whereas the state is in a static field, i.e. on the class. Making it static synchronized is indeed the correct answer, because then it synchronizes on the class, not on a (meaningless) instance.

D. The data in variable “x” are protected from concurrent access problems.

private static int x;

This is a static variable. It is shared by all instances of the class, so synchronizing on individual instances is not helpful, in the same way as F would not be helpful, which synchronizes on a complete throw-away dummy object.