Optional demo.

demo/03/src/main/java/pl/amu/edu/demo/data/Address.java

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+package pl.amu.edu.demo.data;
+
+import lombok.AccessLevel;
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.experimental.FieldDefaults;
+
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@FieldDefaults(level = AccessLevel.PUBLIC, makeFinal = true)
+@Builder(toBuilder = true)
+public class Address {
+    String addressLine1 = "Northern Bridge";
+    String addressLine2 = null;
+    String zipCode = "01-868";
+}

demo/03/src/main/java/pl/amu/edu/demo/data/Housing.java

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+package pl.amu.edu.demo.data;
+
+import lombok.AccessLevel;
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Getter;
+import lombok.experimental.FieldDefaults;
+
+import java.util.Optional;
+
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@FieldDefaults(level = AccessLevel.PUBLIC, makeFinal = true)
+@Getter
+@Builder
+public class Housing {
+    Address address;
+    boolean isApartment;
+    Double area;
+
+    public Optional<String> zipCode() {
+        return Optional.ofNullable(address).map(address -> address.zipCode);
+    }
+
+}

demo/03/src/main/java/pl/amu/edu/demo/data/Person.java

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+package pl.amu.edu.demo.data;
+
+import lombok.AccessLevel;
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Getter;
+import lombok.experimental.FieldDefaults;
+
+import java.time.LocalDate;
+import java.util.Objects;
+
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@FieldDefaults(level = AccessLevel.PUBLIC, makeFinal = true)
+@Getter
+@Builder
+public class Person {
+    String displayName;
+    String firstName;
+    String lastName;
+    LocalDate birthDate;
+    Housing housing;
+    boolean isMarried;
+
+    public boolean isAdult() {
+        return Objects.nonNull(birthDate) &&
+                LocalDate.now().minusYears(18).isAfter(birthDate);
+    }
+
+}

demo/03/src/main/java/pl/amu/edu/demo/maybe/OptionalExample.java

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+package pl.amu.edu.demo.maybe;
+
+import pl.amu.edu.demo.data.Address;
+import pl.amu.edu.demo.data.Housing;
+import pl.amu.edu.demo.data.Person;
+
+import java.time.LocalDate;
+import java.util.Optional;
+
+public class OptionalExample {
+
+    public static void main(String[] args) {
+        new OptionalExample().run();
+    }
+
+    private void run() {
+        var person = Person.builder()
+                .displayName("Gąska Balbinka").birthDate(LocalDate.ofYearDay(1959, 1)).build();
+        var address = Address.builder().build();
+        var underTheBridge = Housing.builder().area(0.2).address(address).build();
+        var maybeAdult = Optional.ofNullable(person)
+                .filter(Person::isAdult);
+        var housing = maybeAdult
+                .map(Person::getHousing)
+                .orElse(underTheBridge);
+        var zipCode = Optional.of(housing)
+                .flatMap(Housing::zipCode)
+                .orElseThrow();
+        maybeAdult
+                .map(Person::getBirthDate)
+                .map(birthDate -> {
+                    var name = person.displayName;
+                    return String.format("%s: born %s lives at %s area.", name, birthDate, zipCode);
+                })
+                .ifPresent(System.out::println);
+    }
+
+}