diff --git a/.idea/misc.xml b/.idea/misc.xml
index 4258c62..f1bf852 100644
--- a/.idea/misc.xml
+++ b/.idea/misc.xml
@@ -8,7 +8,7 @@
       </list>
     </option>
   </component>
-  <component name="ProjectRootManager" version="2" languageLevel="JDK_18" default="true" project-jdk-name="18" project-jdk-type="JavaSDK">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_17" project-jdk-name="18" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/out" />
   </component>
 </project>
\ No newline at end of file
diff --git a/.idea/workspace.xml b/.idea/workspace.xml
index 76e3a07..2ae6fd6 100644
--- a/.idea/workspace.xml
+++ b/.idea/workspace.xml
@@ -5,7 +5,10 @@
   </component>
   <component name="ChangeListManager">
     <list default="true" id="b3bac6d6-f8b1-474b-a5e1-72bf67a3867c" name="Changes" comment="add sample input and sample output in the README.md">
-      <change beforePath="$PROJECT_DIR$/README.md" beforeDir="false" afterPath="$PROJECT_DIR$/README.md" afterDir="false" />
+      <change beforePath="$PROJECT_DIR$/.idea/misc.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/misc.xml" afterDir="false" />
+      <change beforePath="$PROJECT_DIR$/.idea/workspace.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/workspace.xml" afterDir="false" />
+      <change beforePath="$PROJECT_DIR$/pom.xml" beforeDir="false" afterPath="$PROJECT_DIR$/pom.xml" afterDir="false" />
+      <change beforePath="$PROJECT_DIR$/src/main/java/io/github/dbc/DuplicateCharactersCounter.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/io/github/dbc/DuplicateCharactersCounter.java" afterDir="false" />
     </list>
     <option name="SHOW_DIALOG" value="false" />
     <option name="HIGHLIGHT_CONFLICTS" value="true" />
@@ -40,17 +43,17 @@
     <option name="hideEmptyMiddlePackages" value="true" />
     <option name="showLibraryContents" value="true" />
   </component>
-  <component name="PropertiesComponent"><![CDATA[{
-  "keyToString": {
-    "RunOnceActivity.OpenProjectViewOnStart": "true",
-    "RunOnceActivity.ShowReadmeOnStart": "true",
-    "SONARLINT_PRECOMMIT_ANALYSIS": "true",
-    "com.intellij.testIntegration.createTest.CreateTestDialog.defaultLibrary": "JUnit5",
-    "com.intellij.testIntegration.createTest.CreateTestDialog.defaultLibrarySuperClass.JUnit5": "",
-    "full.screen.before.presentation.mode": "true",
-    "settings.editor.selected.configurable": "editing.templates"
+  <component name="PropertiesComponent">{
+  &quot;keyToString&quot;: {
+    &quot;RunOnceActivity.OpenProjectViewOnStart&quot;: &quot;true&quot;,
+    &quot;RunOnceActivity.ShowReadmeOnStart&quot;: &quot;true&quot;,
+    &quot;SONARLINT_PRECOMMIT_ANALYSIS&quot;: &quot;true&quot;,
+    &quot;com.intellij.testIntegration.createTest.CreateTestDialog.defaultLibrary&quot;: &quot;JUnit5&quot;,
+    &quot;com.intellij.testIntegration.createTest.CreateTestDialog.defaultLibrarySuperClass.JUnit5&quot;: &quot;&quot;,
+    &quot;full.screen.before.presentation.mode&quot;: &quot;true&quot;,
+    &quot;settings.editor.selected.configurable&quot;: &quot;editing.templates&quot;
   }
-}]]></component>
+}</component>
   <component name="RecentsManager">
     <key name="CreateTestDialog.Recents.Supers">
       <recent name="" />
@@ -59,7 +62,7 @@
       <recent name="io.github.dbc" />
     </key>
   </component>
-  <component name="RunManager" selected="Java Scratch.Scratch">
+  <component name="RunManager" selected="JUnit.DuplicateCharactersCounterTest">
     <configuration name="DuplicateCharactersCounterTest" type="JUnit" factoryName="JUnit" temporary="true" nameIsGenerated="true">
       <module name="JCP_C1_P1" />
       <extension name="coverage">
@@ -132,8 +135,8 @@
     </configuration>
     <recent_temporary>
       <list>
-        <item itemvalue="Java Scratch.Scratch" />
         <item itemvalue="JUnit.DuplicateCharactersCounterTest" />
+        <item itemvalue="Java Scratch.Scratch" />
         <item itemvalue="JUnit.DuplicateCharactersCounterTest.shouldReturnMapOfCorrectSize" />
         <item itemvalue="JUnit.DuplicateCharactersCounterTest.shouldThrowAnIllegalArgumentExceptionForNullInput" />
         <item itemvalue="JUnit.DuplicateCharactersCounterTest.shouldReturnCount1ForNonDuplicat1Characters" />
diff --git a/pom.xml b/pom.xml
index 1149d30..d234004 100644
--- a/pom.xml
+++ b/pom.xml
@@ -7,6 +7,18 @@
     <groupId>io.github.dbc</groupId>
     <artifactId>JCP_C1_P1</artifactId>
     <version>1.0-SNAPSHOT</version>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>17</source>
+                    <target>17</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
     <dependencies>
         <dependency>
             <groupId>org.junit.jupiter</groupId>
diff --git a/src/main/java/io/github/dbc/DuplicateCharactersCounter.java b/src/main/java/io/github/dbc/DuplicateCharactersCounter.java
index 9020649..8ef83a1 100644
--- a/src/main/java/io/github/dbc/DuplicateCharactersCounter.java
+++ b/src/main/java/io/github/dbc/DuplicateCharactersCounter.java
@@ -1,5 +1,6 @@
 package io.github.dbc;
 
+import java.util.HashMap;
 import java.util.Map;
 
 public class DuplicateCharactersCounter {
@@ -10,6 +11,19 @@ public class DuplicateCharactersCounter {
      * @return Map with characters and their count.
      */
     public Map<Character, Integer> countDuplicateCharacters(String string) {
-        return Map.of();
+        Map<Character, Integer> frequencies = new HashMap<>();
+        if (string == null) {
+            throw new IllegalArgumentException("string is null");
+
+        }
+        for (Character character : string.toCharArray()) {
+
+            if (frequencies.containsKey(character)) {
+                frequencies.put(character, frequencies.get(character) + 1);
+            } else {
+                frequencies.put(character, 1);
+            }
+        }
+        return frequencies;
     }
 }