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The root of the problem is that the thread that calls setFirst() and the thread that calls getFirst() lack a happens-before relationship. A happens-before relationship exists between a thread that writes to a volatile variable and a thread that subsequently reads it. However, setFirst() and getFirst() read only read from a volatile variable—the volatile reference to the array; neither method writes to the volatile variable.
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To ensure visibility, accessor methods may synchronize access while performing operations on nonvolatile elements of an array, whether it the array is referred to by a volatile or a nonvolatile reference. Note that the code is thread-safe even though the array reference is not volatile.
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This noncompliant code example declares the Map instance field volatile. The instance of the Map object is mutable , because of its put() method.
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