Bael 7510 (#15776)

* BAEL-7490 read write file in separate thread

* Change the to try resources

* Update the code to sync with article

* First draft

* Change module

* Update the method return type to Integer and return null if not found

* Remove public as using junit 5

---------

Co-authored-by: Wynn Teo <wynnteo@Wynns-MacBook-Pro.local>

algorithms-modules/algorithms-searching/src/main/java/com/baeldung/algorithms/firstnonrepeating/FirstNonRepeatingElement.java

@@ -0,0 +1,61 @@
+package com.baeldung.algorithms.firstnonrepeating;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class FirstNonRepeatingElement {
+    public static Integer findFirstNonRepeatingUsingForLoop(List<Integer> list) {
+        for (int i = 0; i < list.size(); i++) {
+            int current = list.get(i);
+            boolean isRepeating = false;
+            for (int j = 0; j < list.size(); j++) {
+                if (i != j && current == list.get(j)) {
+                    isRepeating = true;
+                    break;
+                }
+            }
+            if (!isRepeating) {
+                return current;
+            }
+        }
+        return null;
+    }
+
+    public static Integer findFirstNonRepeatedElementUsingIndex(List<Integer> list) {
+        for (int i = 0; i < list.size(); i++) {
+            if (list.indexOf(list.get(i)) == list.lastIndexOf(list.get(i))) {
+                return list.get(i);
+            }
+        }
+        return null;
+    }
+
+    public static Integer findFirstNonRepeatingUsingHashMap(List<Integer> list) {
+        Map<Integer, Integer> counts = new HashMap<>();
+        for (int num : list) {
+            counts.put(num, counts.getOrDefault(num, 0) + 1);
+        }
+        for (int num : list) {
+            if (counts.get(num) == 1) {
+                return num;
+            }
+        }
+        return null;
+    }
+
+    public static Integer findFirstNonRepeatingUsingArray(List<Integer> list) {
+        int maxElement = Collections.max(list);
+        int[] frequency = new int[maxElement + 1];
+        for (int num : list) {
+            frequency[num]++;
+        }
+        for (int num : list) {
+            if (frequency[num] == 1) {
+                return num;
+            }
+        }
+        return null;
+    }
+}

algorithms-modules/algorithms-searching/src/test/java/com/baeldung/algorithms/firstnonrepeating/FirstNonRepeatingElementUnitTest.java

@@ -0,0 +1,43 @@
+package com.baeldung.algorithms.firstnonrepeating;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class FirstNonRepeatingElementUnitTest {
+
+    private List<Integer> list;
+
+    @BeforeEach
+    void setUp() {
+        list = Arrays.asList(1, 2, 3, 2, 1, 4, 5, 4);
+    }
+
+    @Test
+    void whenUsingForLoop_thenReturnFirstNonRepeatingElement() {
+        int result = FirstNonRepeatingElement.findFirstNonRepeatingUsingForLoop(list);
+        assertEquals(3, result);
+    }
+
+    @Test
+    void whenUsingIndexOf_thenReturnFirstNonRepeatingElement() {
+        int result = FirstNonRepeatingElement.findFirstNonRepeatedElementUsingIndex(list);
+        assertEquals(3, result);
+    }
+
+    @Test
+    void whenUsingHashMap_thenReturnFirstNonRepeatingElement() {
+        int result = FirstNonRepeatingElement.findFirstNonRepeatingUsingHashMap(list);
+        assertEquals(3, result);
+    }
+
+    @Test
+    void whenUsingArray_thenReturnFirstNonRepeatingElement() {
+        int result = FirstNonRepeatingElement.findFirstNonRepeatingUsingArray(list);
+        assertEquals(3, result);
+    }
+}