[java] Difference between `Optional.orElse()` and `Optional.orElseGet()`

The following example should demonstrate the difference:

String destroyTheWorld() {
  // destroy the world logic
  return "successfully destroyed the world";
}

Optional<String> opt = Optional.of("Save the world");

// we're dead
opt.orElse(destroyTheWorld());

// we're safe    
opt.orElseGet(() -> destroyTheWorld());

The answer appears in the docs as well.

public T orElseGet(Supplier<? extends T> other):

Return the value if present, otherwise invoke other and return the result of that invocation.

The Supplier won't be invoked if the Optional presents. whereas,

public T orElse(T other):

Return the value if present, otherwise return other.

If other is a method that returns a string, it will be invoked, but it's value won't be returned in case the Optional exists.

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