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

The difference is pretty subtle and if you dont pay much attention then you will keep it using in a wrong way.

Best way to understand the difference between orElse() and orElseGet() is that orElse() will always be executed if the Optional<T> is null or not, But orElseGet() will only be executed when Optional<T> is null.

The dictionary meaning of orElse is :- execute the part when something is not present, but here it contradicts, see the below example:

    Optional<String> nonEmptyOptional = Optional.of("Vishwa Ratna");
    String value = nonEmptyOptional.orElse(iAmStillExecuted());

    public static String iAmStillExecuted(){
    System.out.println("nonEmptyOptional is not NULL,still I am being executed");
    return "I got executed";
    }

Output: nonEmptyOptional is not NULL,still I am being executed


    Optional<String> emptyOptional = Optional.ofNullable(null);
    String value = emptyOptional.orElse(iAmStillExecuted());
    public static String iAmStillExecuted(){
    System.out.println("emptyOptional is NULL, I am being executed, it is normal as 
    per dictionary");
    return "I got executed";
    }

Output: emptyOptional is NULL, I am being executed, it is normal as per dictionary

For orElseGet() , The method goes as per dictionary meaning, The orElseGet() part will be executed only when the Optional is null.

Benchmarks:

+--------------------+------+-----+------------+-------------+-------+
| Benchmark          | Mode | Cnt | Score      | Error       | Units |
+--------------------+------+-----+------------+-------------+-------+
| orElseBenchmark    | avgt | 20  | 60934.425  | ± 15115.599 | ns/op |
+--------------------+------+-----+------------+-------------+-------+
| orElseGetBenchmark | avgt | 20  | 3.798      | ± 0.030     | ns/op |
+--------------------+------+-----+------------+-------------+-------+

Remarks: orElseGet() has clearly outperformed orElse() for our particular example.

Hope it clears the doubts of people like me who wants the very basic ground example :)

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