BAEL-5922 User for Optional in Java (#13051)

* repositories. model class. initial tests

* tests declarative api

* removing from module java 11

* copy to optional module

* fix assertEquals import

core-java-modules/core-java-optional/src/main/java/com/baeldung/optionaluses/User.java

@@ -0,0 +1,21 @@
+package com.baeldung.optionaluses;
+
+public class User {
+
+    public User(String id, String name) {
+        this.id = id;
+        this.name = name;
+    }
+
+    private String id;
+
+    private String name;
+
+    public String getName() {
+        return name;
+    }
+
+    public String getId() {
+        return id;
+    }
+}

core-java-modules/core-java-optional/src/main/java/com/baeldung/optionaluses/UserRepositoryWithNull.java

@@ -0,0 +1,20 @@
+package com.baeldung.optionaluses;
+
+import java.util.Arrays;
+import java.util.List;
+
+public class UserRepositoryWithNull {
+
+    private final List<User> dbUsers = Arrays.asList(new User("1", "John"), new User("2", "Maria"), new User("3", "Daniel"));
+
+    public User findById(String id) {
+
+        for (User u : dbUsers) {
+            if (u.getId().equals(id)) {
+                return u;
+            }
+        }
+
+        return null;
+    }
+}

core-java-modules/core-java-optional/src/main/java/com/baeldung/optionaluses/UserRepositoryWithOptional.java

@@ -0,0 +1,21 @@
+package com.baeldung.optionaluses;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Optional;
+
+public class UserRepositoryWithOptional {
+
+    private final List<User> dbUsers = Arrays.asList(new User("1", "John"), new User("2", "Maria"), new User("3", "Daniel"));
+
+    public Optional<User> findById(String id) {
+
+        for (User u : dbUsers) {
+            if (u.getId().equals(id)) {
+                return Optional.of(u);
+            }
+        }
+
+        return Optional.empty();
+    }
+}

core-java-modules/core-java-optional/src/test/java/com/baeldung/optionaluses/UsesForOptionalUnitTest.java

@@ -0,0 +1,64 @@
+package com.baeldung.optionaluses;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class UsesForOptionalUnitTest {
+
+    @Test
+    public void givenNonExistentUserId_whenSearchForUser_andNoNullCheck_thenThrowException() {
+
+        UserRepositoryWithNull userRepositoryWithNull = new UserRepositoryWithNull();
+        String nonExistentUserId = "4";
+
+        assertThrows(NullPointerException.class, () -> {
+            System.out.println("User name: " + userRepositoryWithNull.findById(nonExistentUserId)
+              .getName());
+        });
+    }
+
+    @Test
+    public void givenNonExistentUserId_whenSearchForUser_thenOptionalShouldBeTreatedProperly() {
+
+        UserRepositoryWithOptional userRepositoryWithOptional = new UserRepositoryWithOptional();
+        String nonExistentUserId = "4";
+
+        String userName = userRepositoryWithOptional.findById(nonExistentUserId)
+          .orElse(new User("0", "admin"))
+          .getName();
+
+        assertEquals("admin", userName);
+    }
+
+    @Test
+    public void givenExistentUserId_whenFoundUserWithNameStartingWithMInRepositoryUsingNull_thenNameShouldBeUpperCased() {
+
+        UserRepositoryWithNull userRepositoryWithNull = new UserRepositoryWithNull();
+
+        User user = userRepositoryWithNull.findById("2");
+        String upperCasedName = "";
+
+        if (user != null) {
+            if (user.getName().startsWith("M")) {
+                upperCasedName = user.getName().toUpperCase();
+            }
+        }
+
+        assertEquals("MARIA", upperCasedName);
+    }
+
+    @Test
+    public void givenExistentUserId_whenFoundUserWithNameStartingWithMInRepositoryUsingOptional_thenNameShouldBeUpperCased() {
+
+        UserRepositoryWithOptional userRepositoryWithOptional = new UserRepositoryWithOptional();
+
+        String upperCasedName = userRepositoryWithOptional.findById("2")
+          .filter(u -> u.getName().startsWith("M"))
+          .map(u -> u.getName().toUpperCase())
+          .orElse("");
+
+        assertEquals("MARIA", upperCasedName);
+    }
+}