Final answer:
The Java method call s.addAll(t) is equivalent to the union of sets S and T in mathematics, but it modifies the set s, unlike the mathematical operation that creates a new set.
Step-by-step explanation:
When you have sets of integers S and T, and corresponding Java Set objects named s and t, the mathematical operation S ∪ T (the union of S and T) is conceptually equivalent to the Java method call s.addAll(t). However, there is an important detail to consider: The addAll method modifies the set s by adding all elements from set t that are not already present in s. The result is that the contents of s after the method call will represent the union of the two sets, but the original set s is changed in the process. If we compare this to the mathematical operation, the union creates a new set without modifying the original sets. Therefore, for them to be perfectly equivalent, we must not mind altering the contents of set s.