what is fail-safe & fail-fast Iterators in java & how they are implemented - Stack Overflow

"Fail safe" means: it won't fail.
"Fail fast" means: it may fail ... and the failure condition is checked aggressively so that the failure condition is detected before damage can be done.

The fail-fast iterators are typically implemented using a volatile counter on the list object.

• When the list is updated, the counter is incremented.
• When an Iterator is created, the current value of the counter is embedded in the Iterator object.
• When an Iterator operation is performed, the method compares the two counter values and throws a CME if they are different.

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