The compiler should be able to optimize all of them to the same machine code.

1. This is already good.
2. Easily optimized by constant folding.
3. ~~This one depends on the semantics of signed underflow, so it may not do what you want.~~
4. ~~The loop can only exit if x==10, so as long as the nextInt() method doesn't have side effects, the loop should be eliminated. But, again, language semantics can affect this.~~

Edit: Very wrong for 3 & 4, see replies.