Added the code of Replace char in a string at specific index (#5869)

* Added the code of Replace char in a string at specific index

* added test cases

* Renamed the Test class name end with UnitTest

* Renamed the Test class name end with UnitTest

* Renamed the Test class name end with UnitTest

* Replaced README with the upstream repo

java-strings/src/main/java/com/baeldung/string/ReplaceCharacterInString.java

@@ -0,0 +1,21 @@
+package com.baeldung.string;
+
+public class ReplaceCharacterInString {
+    public  String replaceCharSubstring(String str, char ch, int index) {
+        String myString = str.substring(0, index) + ch + str.substring(index+1);
+        return myString;
+    }
+
+    public  String replaceCharStringBuilder(String str, char ch, int index) {
+        StringBuilder myString = new StringBuilder(str);
+        myString.setCharAt(index, ch);
+        return myString.toString();
+    }
+
+    public String replaceCharStringBuffer(String str, char ch, int index) {
+        StringBuffer myString = new StringBuffer(str);
+        myString.setCharAt(index, ch);
+        return myString.toString();
+    }
+
+}