[linkedhashmap-first-last] get first and last entry from LinkedHashMap (#15066)

* [linkedhashmap-first-last] get first and last entry from LinkedHashMap

* [linkedhashmap-first-last] add the stream approach

core-java-modules/core-java-collections-maps-7/pom.xml

@@ -79,5 +79,18 @@
             <version>1.5</version>
         </dependency>
     </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <configuration>
+                    <argLine>
+                        --add-opens java.base/java.util=ALL-UNNAMED
+                    </argLine>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
 
 </project>

core-java-modules/core-java-collections-maps-7/src/test/java/com/baeldung/map/linkedhashmapfirstandlastentry/GetFirstAndLastEntryFromLinkedHashMapUnitTest.java

@@ -0,0 +1,85 @@
+package com.baeldung.map.linkedhashmapfirstandlastentry;
+
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Field;
+import java.util.Iterator;
+import java.util.LinkedHashMap;
+import java.util.Map.Entry;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+public class GetFirstAndLastEntryFromLinkedHashMapUnitTest {
+    private static final LinkedHashMap<String, String> THE_MAP = new LinkedHashMap<>();
+
+    static {
+        THE_MAP.put("key one", "a1 b1 c1");
+        THE_MAP.put("key two", "a2 b2 c2");
+        THE_MAP.put("key three", "a3 b3 c3");
+        THE_MAP.put("key four", "a4 b4 c4");
+    }
+
+    @Test
+    void whenIteratingEntrySet_thenGetExpectedResult() {
+        Entry<String, String> firstEntry = THE_MAP.entrySet().iterator().next();
+        assertEquals("key one", firstEntry.getKey());
+        assertEquals("a1 b1 c1", firstEntry.getValue());
+
+        Entry<String, String> lastEntry = null;
+        Iterator<Entry<String, String>> it = THE_MAP.entrySet().iterator();
+        while (it.hasNext()) {
+            lastEntry = it.next();
+        }
+
+        assertNotNull(lastEntry);
+        assertEquals("key four", lastEntry.getKey());
+        assertEquals("a4 b4 c4", lastEntry.getValue());
+
+    }
+
+    @Test
+    void whenConvertingEntriesToArray_thenGetExpectedResult() {
+
+        Entry<String, String>[] theArray = new Entry[THE_MAP.size()];
+        THE_MAP.entrySet().toArray(theArray);
+
+        Entry<String, String> firstEntry = theArray[0];
+        assertEquals("key one", firstEntry.getKey());
+        assertEquals("a1 b1 c1", firstEntry.getValue());
+
+        Entry<String, String> lastEntry = theArray[THE_MAP.size() - 1];
+        assertEquals("key four", lastEntry.getKey());
+        assertEquals("a4 b4 c4", lastEntry.getValue());
+    }
+
+    @Test
+    void whenUsingStreamAPI_thenGetExpectedResult() {
+        Entry<String, String> firstEntry = THE_MAP.entrySet().stream().findFirst().get();
+        assertEquals("key one", firstEntry.getKey());
+        assertEquals("a1 b1 c1", firstEntry.getValue());
+
+        Entry<String, String> lastEntry = THE_MAP.entrySet().stream().skip(THE_MAP.size() - 1).findFirst().get();
+
+        assertNotNull(lastEntry);
+        assertEquals("key four", lastEntry.getKey());
+        assertEquals("a4 b4 c4", lastEntry.getValue());
+    }
+
+    @Test
+    void whenUsingReflection_thenGetExpectedResult() throws NoSuchFieldException, IllegalAccessException {
+        Field head = THE_MAP.getClass().getDeclaredField("head");
+        head.setAccessible(true);
+        Entry<String, String> firstEntry = (Entry<String, String>) head.get(THE_MAP);
+        assertEquals("key one", firstEntry.getKey());
+        assertEquals("a1 b1 c1", firstEntry.getValue());
+
+        Field tail = THE_MAP.getClass().getDeclaredField("tail");
+        tail.setAccessible(true);
+        Entry<String, String> lastEntry = (Entry<String, String>) tail.get(THE_MAP);
+        assertEquals("key four", lastEntry.getKey());
+        assertEquals("a4 b4 c4", lastEntry.getValue());
+    }
+
+
+}