Either usage example.

demo/03/src/main/java/pl/amu/edu/demo/either/EitherExample.java

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+package pl.amu.edu.demo.either;
+
+import io.vavr.control.Either;
+import pl.amu.edu.demo.data.Person;
+
+import java.time.LocalDate;
+
+import static java.util.Objects.isNull;
+
+public class EitherExample {
+
+    public static void main(String[] args) {
+        new EitherExample().run();
+    }
+
+    private void run() {
+        var balbina = Person.builder()
+                .displayName("Gąska Balbinka").birthDate(LocalDate.ofYearDay(1959, 1)).build();
+        var poziomka = Person.builder()
+                .displayName("magic Poziomka").build();
+        var maybeBalbinasAge = getAge(balbina);
+        var maybePoziomkasAge = getAge(poziomka);
+        System.out.printf("Is Balbinas age empty? %s\n", maybeBalbinasAge.isEmpty());
+        System.out.printf("Is Poziomkas age empty? %s\n", maybePoziomkasAge.isEmpty());
+        System.out.printf("Balbinas age: %d\n", maybeBalbinasAge.get());
+        System.out.printf("Poziomkas error: %s\n", maybePoziomkasAge.getLeft());
+    }
+
+    private Either<ProcessingError, Integer> getAge(Person person) {
+        if (isNull(person.birthDate)) {
+            // konwencja: błędy po lewej
+            return Either.left(ProcessingError.of("No birthdate!"));
+        }
+        return Either.right(person.getAge());
+    }
+
+}