Is switch operator atomic?

Question

In documentation it is said you could equally use if-else multiple times or switch-case:

int condition;

setCondition(int condition) {
    this.condition = condition;
}

Either switch-case

switch (condition) {
  case 1: print("one"); break;
  case 2: print("two"); break;

or

if (condition == 1) { print("one"); }
else if (condition == 2) { print("two"); }

Next, conditionis declared volatile and method setCondition() is called from multiple threads. If-else is not atomic and volatile variable write is a synchronizing action. So both "one" and "two" string could be printed in the last code.

It could be avoided if some method local variable with initial value was used:

int localCondition = condition;
if (local condition == ..) ..

Does switch-case operator hold some initial copy of variable? How are cross threads operations implemented with it?

Answer 1

From the Java specification on switch statements:

When the switch statement is executed, first the Expression is evaluated. [...]

This suggests that the expression is evaluated once and that the result is temporarily kept somewhere else, and so no race-conditions are possible.

I can't find a definite answer anywhere though.

---

A quick test shows this is indeed the case:

public class Main {
  private static int i = 0;

  public static void main(String[] args) {
    switch(sideEffect()) {
      case 0:
        System.out.println("0");
        break;
      case 1:
        System.out.println("1");
        break;
      default:
        System.out.println("something else");
    }

    System.out.println(i); //this prints 1
  }

  private static int sideEffect() {
    return i++;
  }
}

And indeed, sideEffect() is only called once.