Classy Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 147

public class Client {
public static void main(String[] args) {
System.out.println(Cache.getSum());
}
}
Solution 52: Sum Fun
On cursory inspection, you might think that this program adds the numbers from 1 to 100, but it
doesn't. Take a closer look at the loop. It is the typical half-open loop, so it goes from 0 to 99. With
that in mind, you might think that the program prints the sum of the numbers from 0 to 99. Using
the formula from the hint, this sum is 99 x 100 / 2, or 4,950. The program, however, thinks
otherwise. It prints 9900 , fully twice this value. What accounts for its enthusiasm?
The author of the program obviously went to a lot of trouble to make sure that sum was initialized
before use. The program combines lazy and eager initialization and even uses synchronization to
make sure that the cache works in the presence of multiple threads. It seems that this program has
all the bases covered, yet it doesn't work. What's the matter with it?
Like the program in Puzzle 49 , this program suffers from a class initialization ordering problem. To
understand its behavior, let's trace its execution. Before it can invoke Client.main , the VM must
initialize the class Client . This initialization is so simple that it isn't worth talking about. The
Client.main method invokes Cache.getSum . Before the getSum method can be executed, the VM
must initialize the class Cache .
Recall that class initialization executes static initializers in the order they appear in the source. The
Cache class has two static initializers: the static block at the top of the class and the initialization
of the static field initialized . The block appears first. It invokes the method
initializeIfNecessary , which tests the field initialized . Because no value has been assigned to
this field, it has the default boolean value of false . Similarly, sum has the default int value of 0 .
Therefore, the initializeIfNecessary method does what you'd expect, adding 4,950 to sum and
setting initialized to true .
After the static block executes, the static initializer for the initialized field sets it back to false ,
completing the class initialization of Cache . Unfortunately, sum now contains the correct cached
value, but initialized contains false : The Cache class's two pieces of critical state are out of
sync.
The main method in the Client class then invokes Cache.getSum , which in turn invokes
initializeIfNecessary . Because the initialized flag is false , the initializeIfNecessary
method enters its loop, which adds another 4,950 to the value of sum , increasing its value to 9,900.
The getSum method returns this value, and the program prints it.