Enhance Upper.java – Optimized, Safe, and Clean Implementation of String Uppercase Conversion

src/main/java/com/thealgorithms/strings/Upper.java

@@ -1,6 +1,7 @@
 package com.thealgorithms.strings;
 
 public final class Upper {
+
     private Upper() {
     }
 
@@ -15,25 +16,42 @@ public static void main(String[] args) {
     }
 
     /**
-     * Converts all the characters in this {@code String} to upper case
+     * Converts all the characters in this {@code String} to upper case.
      *
      * @param s the string to convert
      * @return the {@code String}, converted to uppercase.
+     * @throws IllegalArgumentException if {@code s} is null
      */
     public static String toUpperCase(String s) {
         if (s == null) {
-            throw new IllegalArgumentException("Input string connot be null");
+            throw new IllegalArgumentException("Input string cannot be null");
         }
         if (s.isEmpty()) {
             return s;
         }
-        StringBuilder result = new StringBuilder(s);
-        for (int i = 0; i < result.length(); ++i) {
-            char currentChar = result.charAt(i);
-            if (Character.isLetter(currentChar) && Character.isLowerCase(currentChar)) {
-                result.setCharAt(i, Character.toUpperCase(currentChar));
+
+        // Check if any lowercase letter exists before creating a new String
+        boolean hasLower = false;
+        for (int i = 0; i < s.length(); i++) {
+            if (Character.isLowerCase(s.charAt(i))) {
+                hasLower = true;
+                break;
             }
         }
-        return result.toString();
+
+        // If no lowercase characters, return the same string
+        if (!hasLower) {
+            return s;
+        }
+
+        // Convert lowercase letters to uppercase
+        char[] chars = s.toCharArray();
+        for (int i = 0; i < chars.length; i++) {
+            if (Character.isLowerCase(chars[i])) {
+                chars[i] = Character.toUpperCase(chars[i]);
+            }
+        }
+
+        return new String(chars);
     }
 }