BAEL-6764: Collect stream of entrySet() to LinkedHashMap (#15806)

* entryset to linkedhashmap unit tests

* use set over list for groupingBy unit test

* add clearer unit test method names

* remove public modifier from unit tests

* rename unit test as per pr comment

core-java-modules/core-java-collections-maps-7/src/test/java/com/baeldung/map/entrysettolinkedhashmap/EntrySetToLinkedHashMapUnitTest.java

@@ -0,0 +1,111 @@
+package com.baeldung.map.entrysettolinkedhashmap;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.entry;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class EntrySetToLinkedHashMapUnitTest {
+
+    private Map<Integer, String> map;
+
+    @Test
+    void givenMap_whenUsingCollectorsGroupingBy_thenCollectToLinkedHashMap() {
+        Map<String, Set<String>> countryToCities = Map.of("Paris", "France", "Nice", "France", "Madrid", "Spain")
+            .entrySet()
+            .stream()
+            .collect(Collectors.groupingBy(Map.Entry::getValue, LinkedHashMap::new, Collectors.mapping(Map.Entry::getKey, Collectors.toSet())));
+
+        assertThat(countryToCities).isExactlyInstanceOf(LinkedHashMap.class)
+            .containsOnly(entry("France", Set.of("Paris", "Nice")), entry("Spain", Set.of("Madrid")));
+    }
+
+    @Test
+    void givenMap_whenUsingCollectorsToMap_thenCollectAndConvertToLinkedHashMap() {
+        Map<Integer, String> result = new LinkedHashMap<>(map.entrySet()
+            .stream()
+            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)));
+
+        assertThat(result).isExactlyInstanceOf(LinkedHashMap.class)
+            .containsOnly(entry(1, "value 1"), entry(2, "value 2"));
+    }
+
+    @Test
+    void givenMap_whenUsingCollectorsToMap_thenCollectToLinkedHashMap() {
+        Map<Integer, String> result = map.entrySet()
+            .stream()
+            .collect(Collectors.toMap(
+                Map.Entry::getKey, Map.Entry::getValue,
+                (e1, e2) -> {
+                    throw new RuntimeException();
+                }, 
+                LinkedHashMap::new));
+
+        assertThat(result).isExactlyInstanceOf(LinkedHashMap.class)
+            .containsOnly(entry(1, "value 1"), entry(2, "value 2"));
+    }
+
+    @Test
+    void givenMap_whenUsingLinkedHashMapConstructor_thenObtainLinkedHashMap() {
+        Map<Integer, String> result = new LinkedHashMap<>(map);
+
+        assertThat(result).isExactlyInstanceOf(LinkedHashMap.class)
+            .containsOnly(entry(1, "value 1"), entry(2, "value 2"));
+    }
+
+    @Test
+    void givenMap_whenUsingPutWithForLoop_thenInsertIntoLinkedHashMap() {
+        Map<Integer, String> result = new LinkedHashMap<>();
+
+        for (Map.Entry<Integer, String> entry : map.entrySet()) {
+            result.put(entry.getKey(), entry.getValue());
+        }
+
+        assertThat(result).isExactlyInstanceOf(LinkedHashMap.class)
+            .containsOnly(entry(1, "value 1"), entry(2, "value 2"));
+    }
+
+    @Test
+    void givenMap_whenUsingPutWithMapForEach_thenInsertIntoLinkedHashMap() {
+        Map<Integer, String> result = new LinkedHashMap<>();
+
+        map.forEach((k, v) -> result.put(k, v));
+
+        assertThat(result).isExactlyInstanceOf(LinkedHashMap.class)
+            .containsOnly(entry(1, "value 1"), entry(2, "value 2"));
+    }
+
+    @Test
+    void givenMap_whenUsingPutWithSetForEach_thenInsertIntoLinkedHashMap() {
+        Map<Integer, String> result = new LinkedHashMap<>();
+
+        map.entrySet()
+            .forEach(entry -> result.put(entry.getKey(), entry.getValue()));
+
+        assertThat(result).isExactlyInstanceOf(LinkedHashMap.class)
+            .containsOnly(entry(1, "value 1"), entry(2, "value 2"));
+    }
+
+    @Test
+    void givenMap_whenUsingPutWithStreamForEach_thenInsertIntoLinkedHashMapp() {
+        Map<Integer, String> result = new LinkedHashMap<>();
+
+        map.entrySet()
+            .stream()
+            .forEach(entry -> result.put(entry.getKey(), entry.getValue()));
+
+        assertThat(result).isExactlyInstanceOf(LinkedHashMap.class)
+            .containsOnly(entry(1, "value 1"), entry(2, "value 2"));
+    }
+
+    @BeforeEach
+    void init() {
+        map = Map.of(1, "value 1", 2, "value 2");
+    }
+}