[BAEL-6371_char_frequence] Find the Most Frequent Characters in a String (#13877)

core-java-modules/core-java-string-algorithms-3/pom.xml

@@ -39,8 +39,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>${maven-compiler-plugin.version}</version>
                 <configuration>
-                    <source>${java.version}</source>
-                    <target>${java.version}</target>
+                    <source>${maven.compiler.source}</source>
+                    <target>${maven.compiler.target}</target>
                     <compilerArgument>-parameters</compilerArgument>
                 </configuration>
             </plugin>

core-java-modules/core-java-string-algorithms-3/src/main/java/com/baeldung/charfreq/CharacterWithHighestFrequency.java

@@ -0,0 +1,58 @@
+package com.baeldung.charfreq;
+
+import static java.util.Map.Entry.comparingByValue;
+import static java.util.stream.Collectors.counting;
+import static java.util.stream.Collectors.groupingBy;
+import static java.util.stream.Collectors.toSet;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+public class CharacterWithHighestFrequency {
+    public static Character byStream(String input) {
+        return input.chars()
+          .mapToObj(x -> (char) x)
+          .collect(groupingBy(x -> x, counting()))
+          .entrySet()
+          .stream()
+          .max(comparingByValue())
+          .get()
+          .getKey();
+    }
+
+    public static Set<Character> byMap(String input) {
+        Map<Character, Integer> map = new HashMap<>();
+        for (char c : input.toCharArray()) {
+            map.compute(c, (character, count) -> count == null ? 1 : ++count);
+        }
+        int maxCount = map.values()
+          .stream()
+          .mapToInt(Integer::intValue)
+          .max()
+          .getAsInt();
+
+        return map.keySet()
+          .stream()
+          .filter(c -> map.get(c) == maxCount)
+          .collect(toSet());
+    }
+
+    public static Set<Character> byBucket(String input) {
+        int[] buckets = new int[128];
+
+        int maxCount = 0;
+        for (char c : input.toCharArray()) {
+            buckets[c]++;
+            maxCount = Math.max(buckets[c], maxCount);
+        }
+
+        int finalMaxCount = maxCount;
+        return IntStream.range(0, 128)
+          .filter(c -> buckets[c] == finalMaxCount)
+          .mapToObj(i -> (char) i)
+          .collect(Collectors.toSet());
+    }
+}

core-java-modules/core-java-string-algorithms-3/src/test/java/com/baeldung/charfreq/CharacterWithHighestFrequencyUnitTest.java

@@ -0,0 +1,43 @@
+package com.baeldung.charfreq;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Collections;
+import java.util.Set;
+
+import org.junit.jupiter.api.Test;
+
+import com.google.common.collect.ImmutableSet;
+
+class CharacterWithHighestFrequencyUnitTest {
+    private static final String INPUT1 = "aaaaaaaaaa(10) bbbbbbb ccccc dddd eee ff";
+    private static final Set<Character> EXPECTED1 = Collections.singleton('a');
+
+    private static final String INPUT2 = "YYYYYYY(7) bbbbb -------(7) dddd eee kkkkkkk(7) ff";
+    private static final Set<Character> EXPECTED2 = ImmutableSet.of('Y', '-', 'k');
+
+    @Test
+    void whenGettingSingleCharWithHighestFrequencyByStream_shouldSuccess() {
+        char result1 = CharacterWithHighestFrequency.byStream(INPUT1);
+        assertEquals('a', result1);
+    }
+
+    @Test
+    void whenGettingCharWithHighestFrequencyByMap_shouldSuccess() {
+        Set<Character> result1 = CharacterWithHighestFrequency.byMap(INPUT1);
+        assertEquals(EXPECTED1, result1);
+
+        Set<Character> result2 = CharacterWithHighestFrequency.byMap(INPUT2);
+        assertEquals(EXPECTED2, result2);
+
+    }
+
+    @Test
+    void whenGettingCharWithHighestFrequencyByBucket_shouldSuccess() {
+        Set<Character> result1 = CharacterWithHighestFrequency.byBucket(INPUT1);
+        assertEquals(EXPECTED1, result1);
+
+        Set<Character> result2 = CharacterWithHighestFrequency.byBucket(INPUT2);
+        assertEquals(EXPECTED2, result2);
+    }
+}