BAEL-9498

core-java-modules/core-java-collections-7/src/main/java/com/baeldung/entityCollection/EntityUtils.java

@@ -0,0 +1,23 @@
+package com.baeldung.entityCollection;
+
+import java.util.Collection;
+import java.util.List;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+public class EntityUtils {
+
+    public static <T, R> List<R> extractField(Collection<T> entities, Function<T, R> extractor) {
+        return entities.stream()
+                .map(extractor)
+                .collect(Collectors.toList());
+    }
+
+    public static <T, R> Set<R> extractFieldAsSet(Collection<T> entities, Function<T, R> extractor) {
+        return entities.stream()
+                .map(extractor)
+                .collect(Collectors.toSet());
+    }
+}
+

core-java-modules/core-java-collections-7/src/main/java/com/baeldung/entityCollection/IdExtractor.java

@@ -0,0 +1,27 @@
+package com.baeldung.entityCollection;
+
+import java.util.*;
+import java.util.stream.Collectors;
+
+public class IdExtractor {
+
+    public static List<Long> extractIdsClassic(List<User> users) {
+        List<Long> ids = new ArrayList<>();
+        for (User user : users) {
+            ids.add(user.getId());
+        }
+        return ids;
+    }
+
+    public static List<Long> extractIdsStream(List<User> users) {
+        return users.stream()
+                .map(User::getId)
+                .collect(Collectors.toList());
+    }
+
+    public static Set<Long> extractUniqueIds(List<User> users) {
+        return users.stream()
+                .map(User::getId)
+                .collect(Collectors.toSet());
+    }
+}

core-java-modules/core-java-collections-7/src/main/java/com/baeldung/entityCollection/User.java

@@ -0,0 +1,15 @@
+package com.baeldung.entityCollection;
+
+public class User {
+    private final Long id;
+    private final String name;
+
+    public User(Long id, String name) {
+        this.id = id;
+        this.name = name;
+    }
+
+    public Long getId() {
+        return id;
+    }
+}

core-java-modules/core-java-collections-7/src/test/java/com/baeldung/entityCollection/IdExtractionUnitTest.java

@@ -0,0 +1,116 @@
+package com.baeldung.entityCollection;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class IdExtractionUnitTest {
+
+    @Test
+    void givenListOfUsers_whenUsingClassicLoop_thenReturnListOfIds() {
+        List<User> users = Arrays.asList(
+                new User(1L, "A"),
+                new User(2L, "B"),
+                new User(3L, "C")
+        );
+
+        List<Long> ids = IdExtractor.extractIdsClassic(users);
+
+        assertEquals(Arrays.asList(1L, 2L, 3L), ids);
+    }
+
+    @Test
+    void givenEmptyUsersList_whenUsingClassicLoop_thenReturnEmptyList() {
+        List<User> users = List.of();
+        List<Long> ids = IdExtractor.extractIdsClassic(users);
+        assertTrue(ids.isEmpty());
+    }
+
+    @Test
+    void givenUsersList_whenUsingStream_thenReturnListOfIds() {
+        List<User> users = Arrays.asList(
+                new User(10L, "A"),
+                new User(20L, "B")
+        );
+
+        List<Long> ids = IdExtractor.extractIdsStream(users);
+        assertEquals(Arrays.asList(10L, 20L), ids);
+    }
+
+    @Test
+    void givenEmptyUsersList_whenUsingStream_thenReturnEmptyList() {
+        List<User> users = List.of();
+        List<Long> ids = IdExtractor.extractIdsStream(users);
+        assertTrue(ids.isEmpty());
+    }
+
+    @Test
+    void givenUsersWithNullIds_whenUsingStream_thenAllowNullValuesInList() {
+        List<User> users = Arrays.asList(
+                new User(null, "A"),
+                new User(5L, "B")
+        );
+
+        List<Long> ids = IdExtractor.extractIdsStream(users);
+        assertEquals(Arrays.asList(null, 5L), ids);
+    }
+
+    @Test
+    void givenUsersWithDuplicateIds_whenUsingUniqueIdExtractor_thenReturnUniqueSet() {
+        List<User> users = Arrays.asList(
+                new User(1L, "A"),
+                new User(1L, "B"),
+                new User(2L, "C")
+        );
+
+        Set<Long> ids = IdExtractor.extractUniqueIds(users);
+        assertEquals(Set.of(1L, 2L), ids);
+    }
+
+    @Test
+    void givenEmptyUsersList_whenUsingUniqueIdExtractor_thenReturnEmptySet() {
+        List<User> users = List.of();
+        Set<Long> ids = IdExtractor.extractUniqueIds(users);
+        assertTrue(ids.isEmpty());
+    }
+
+    @Test
+    void givenListOfUsers_whenUsingUtilityExtractor_thenReturnListOfIds() {
+        List<User> users = Arrays.asList(
+                new User(100L, "A"),
+                new User(200L, "B")
+        );
+
+        List<Long> ids = EntityUtils.extractField(users, User::getId);
+        assertEquals(Arrays.asList(100L, 200L), ids);
+    }
+
+    @Test
+    void givenEmptyList_whenUsingUtilityExtractor_thenReturnEmptyList() {
+        List<User> users = List.of();
+        List<Long> ids = EntityUtils.extractField(users, User::getId);
+        assertTrue(ids.isEmpty());
+    }
+
+    @Test
+    void givenUsersWithDuplicates_whenUsingUtilityExtractorAsSet_thenReturnUniqueIds() {
+        List<User> users = Arrays.asList(
+                new User(5L, "A"),
+                new User(5L, "B"),
+                new User(6L, "C")
+        );
+
+        Set<Long> ids = EntityUtils.extractFieldAsSet(users, User::getId);
+        assertEquals(Set.of(5L, 6L), ids);
+    }
+
+    @Test
+    void givenEmptyList_whenUsingUtilityExtractorAsSet_thenReturnEmptySet() {
+        List<User> users = List.of();
+        Set<Long> ids = EntityUtils.extractFieldAsSet(users, User::getId);
+        assertTrue(ids.isEmpty());
+    }
+}
+