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A common misconception is that shared references to immutable objects are immediately visible across multiple threads as soon as they are updated. For example, a developer can mistakenly believe that a class containing fields that refer only to immutable objects is itself immutable and consequently threa

Section 14.10.2, "Final Fields and Security" of Java Programming Language, Fourth Edition [JPL 2006] states:

The problem is that, while the shared object is immutable, the reference used to access the shared object is itself shared and often mutable. Consequently, a correctly synchronized program must synchronize access to that shared reference, but often programs do not do this, because programmers do not recognize the need to do it.

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The getHelper() method publishes the mutable helper field. Because the Helper class is immutable, it cannot be changed after it is initialized

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Bibliography

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