What am i doing wrong in this Java optional – orElse statement

August 23, 2022

 List<String> list = 
  Arrays.asList("TEST1", "TEST2", "TEST3");

    final String finalStr = 
Optional.ofNullable(list.stream()
            .filter(s -> s.contains("pattern:a:b:"))
            .map(str -> str.substring(str.lastIndexOf(':') + 1))
            .findFirst())
            .orElse(list.stream()
                    .filter(s -> !s.contains("pattern:c:d:"))
                    .findFirst())
            .orElseThrow();

It will work if the list contains pattern:a:b: but for above example always throw an exception if that pattern is not available. In the orElse part it doesnt even seem to be executing.

My result is
[java] => java.util.NoSuchElementException: No value present when i am epecting to return me the first element.

Edit: I wish to first find if any string with pattern:a:b exists in the list, if it does not, i wish it to return me the first string that does not contain the pattern:c:d For example

"pattern:a:b:TEST1", "pattern:c:d:TEST2", "TEST3" should always return TEST1.

"pattern:c:d:TEST1", "TEST2", "TEST3" should always return TEST2.

"TEST1", "TEST2", "TEST3" should always return TEST1.

>Solution :

This is a very nice question.

Let’s follow the evaluation. Since the inner part of the Optional.ofNullable(...) results in Optional<String>, the entire chain wraps it in Optional<Optional<String>>.

After evaluation of the parameter in Optional.ofNullable(...), the result is roughly this:

Optional.ofNullable(Optional.<String>empty())      // Optional<Optional<String>>
                .orElse(list.stream().findFirst()) // Optional<String>
                .orElseThrow();                    // String

The orElse is not called as long as the Optional.empty() is not null, hence the outer Optional is not empty. The flow continues with the inner Optional.enpty on which the orElseThrow is called and always fails by throwing java.util.NoSuchElementException.

What you want to do is this:

final String finalStr = list.stream()                        // Stream<String>
        .filter(s -> s.contains("pattern:a:b:"))             // Stream<String>
        .map(str -> str.substring(str.lastIndexOf(':') + 1)) // Stream<String>
        .findFirst()                                         // Optional<String>
        .or(() -> list.stream().findFirst())                 // Optional<String>
        .orElseThrow();                                      // String

The or method is the key one as on empty Optional<T> it supplies with a different one without losing the Optional<T> context.