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Note that if Foo was mutable, the Helper field would need to be declared volatile as shown in CON00-J. Declare status flags as volatileEnsure visibility of atomically modifiable shared variables. Also, the method getHelper() is an instance method and the accessibility of the helper field is private. This allows safe publication of the Helper object, in that, a thread cannot observe a partially initialized Foo object (CON26-J. Do not publish partially-constructed objects).
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