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Semantics of volatile do not guarantee the atomicity of complex operations that involve read-modify-write sequences such as incrementing a value. See CON01-J. Ensure that composite compound operations on shared variables are atomic for more information.
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CON25-EX1: If all reads and writes of 64 bit long and double values occur within a synchronized method call, the atomicity of the read/write is guaranteed. This requires that no unsynchronized methods in the class expose the value and that the value is inaccessible (directly or indirectly) from other code. (CON01-J. Ensure that composite compound operations on shared variables are atomic)
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