[BAEL-7993] Count the number of occurrences in an array in Java (#16714)

* feat: count occurences in array

* fix: review

* fix: review

Author: lucaCambi77

algorithms-modules/algorithms-miscellaneous-9/src/main/java/com/baledung/algorithms/countoccurence/CountOccurrence.java

@@ -0,0 +1,37 @@
+package com.baledung.algorithms.countoccurence;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+public class CountOccurrence {
+
+    public static int[] countOccurrencesWithCounter(int[] elements, int n) {
+        int[] counter = new int[n];
+
+        for (int element : elements) {
+            counter[element]++;
+        }
+
+        return counter;
+    }
+
+    public static <T> Map<T, Integer> countOccurrencesWithMap(T[] elements) {
+
+        Map<T, Integer> counter = new HashMap<>();
+
+        for (T element : elements) {
+            counter.merge(element, 1, Integer::sum);
+        }
+
+        return counter;
+    }
+
+    public static <T> Map<T, Long> countOccurrencesWithStream(T[] elements) {
+
+        return Arrays.stream(elements)
+            .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
+    }
+}

algorithms-modules/algorithms-miscellaneous-9/src/test/java/com/baeldung/algorithms/countoccurence/CountOccurrenceUnitTest.java

@@ -0,0 +1,43 @@
+package com.baeldung.algorithms.countoccurence;
+
+import static com.baledung.algorithms.countoccurence.CountOccurrence.countOccurrencesWithCounter;
+import static com.baledung.algorithms.countoccurence.CountOccurrence.countOccurrencesWithMap;
+import static com.baledung.algorithms.countoccurence.CountOccurrence.countOccurrencesWithStream;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+public class CountOccurrenceUnitTest {
+
+    @Test
+    void givenArrayOfIntegers_whenCountOccurrenceWithCounter_thenCounterIndexIsOccurrence() {
+        int[] counter = countOccurrencesWithCounter(new int[] { 2, 3, 1, 1, 3, 4, 5, 6, 7, 8 }, 10);
+        assertEquals(2, counter[3]);
+    }
+
+    @Test
+    void givenArrayOfIntegers_whenCountOccurrenceWithMap_thenMapValueIsOccurrence() {
+        Map<Integer, Integer> counter = countOccurrencesWithMap(new Integer[] { 2, 3, 1, -1, 3, 4, 5, 6, 7, 8, -1 });
+        assertEquals(2, counter.get(-1));
+    }
+
+    @Test
+    void givenArrayOfString_whenCountOccurrenceWithMap_thenMapValueIsOccurrence() {
+        Map<String, Integer> counter = countOccurrencesWithMap(new String[] { "apple", "orange", "banana", "apple" });
+        assertEquals(2, counter.get("apple"));
+    }
+
+    @Test
+    void givenArrayOfString_whenCountOccurrenceWithStream_thenMapValueIsOccurrence() {
+        Map<String, Long> counter = countOccurrencesWithStream(new String[] { "apple", "orange", "banana", "apple" });
+        assertEquals(2, counter.get("apple"));
+    }
+
+    @Test
+    void givenArrayOfIntegers_whenCountOccurrenceWithStream_thenMapValueIsOccurrence() {
+        Map<Integer, Long> counter = countOccurrencesWithStream(new Integer[] { 2, 3, 1, -1, 3, 4, 5, 6, 7, 8, -1 });
+        assertEquals(2, counter.get(-1));
+    }
+}

algorithms-modules/algorithms-miscellaneous-9/src/test/java/com/baeldung/algorithms/permutation/StringPermutationUnitTest.java

@@ -1,7 +1,5 @@
 package com.baeldung.algorithms.permutation;
 
-import org.junit.jupiter.api.Test;
-
 import static com.baeldung.algorithms.permutation.StringPermutation.isPermutationInclusion;
 import static com.baeldung.algorithms.permutation.StringPermutation.isPermutationWithMap;
 import static com.baeldung.algorithms.permutation.StringPermutation.isPermutationWithOneCounter;
@@ -10,6 +8,8 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import org.junit.jupiter.api.Test;
+
 public class StringPermutationUnitTest {
 
     @Test