Optional.orElse() vs Optional.orElseGet()

Recently, I upgraded my project to Java 8 and enjoy to explorer new syntax more like Ruby or Scala.
Today, there was annoyed bug found in my code which is subtle to understand in documentation.

See following codes

@Test
public void jdk8OptionalTest() {
    Optional<String> s = Optional.of("Value's supposed to be");
    String ss = s.orElse(wontRunThis());
    System.out.println("optional returns " + ss);
}
 
private static String wontRunThis() {
    System.out.println("won't run this");
    return "something";
}

As expected ss="Value's supposed to be" as a result of the run.
The problem is that wontRunThis() was still executed although the Optional<String> wasn't empty.
It seems like JDK 8 treats wontRunThis() as literal and evaluates it regardless the condition.

A little bit wondering around this problem, I could fix the problem by using Optional.orElseGet()

@Test
public void jdk8OptionalTest2() {
    Optional<String> s = Optional.of("Value's supposed to be");
    String ss = s.orElseGet(() -> wontRunThis());
    System.out.println("optional returns " + ss);
}

So, the difference is that orElseGet() would execute the function only if Optional was empty.

Lesson learned. Optional.orElse() can be used to assigning literal value but we shouldn't expect a control flow with it.

Java 8 helps to reduce boilerplates but it, sometimes, behaves to different way from my expectation.
I'm still getting used with this new version of Java language.