Local variables and thread safety

In Java if you have the following method:

public int foo(int n) {
    int i=n;
    i = i+1; i = i-1;
    return i;
}

So in a sequential program the return value will always be the same as the input.

ie: j == foo(j)

However if you have multiple threads call foo, can you guarantee that j==foo(j)?

I would say yes that it is guaranteed, because i is a local variable, and each thread has its own stack so i will be a different memory location for each thread.

I would say you couldn't guarantee that j==foo(j) if i is a instance variable:

private int i;
public int foo(int n) {
    i=n;
    i = i+1; i = i-1;
    return i;
}

Because the threads can interleave and the value of i can change half way through a thread executing the method, or one thread can increment i, but before it gets chance to decrement it, another thread returns with it's input incremented twice and decremented only once.